Protist

{{Short description|Eukaryotes other than animals, plants or fungi}} {{For|the journal|Protist (journal){{!}}''Protist'' (journal)}} {{distinguish|Protest}} {{good article}} {{pp-move}} {{Paraphyletic group | name = Protists | fossil_range = {{Long fossil range|1600|0|Paleoproterozoic–present<ref name="Strassert-2021">{{cite journal|first1=Jürgen F. H.|last1=Strassert|first2=Iker|last2=Irisarri|first3=Tom A.|last3=Williams|first4=Fabien|last4=Burki|title=A molecular timescale for eukaryote evolution with implications for the origin of red algal-derived plastids|journal=Nature Communications|date=25 March 2021|volume=12|issue=1|page=1879|doi=10.1038/s41467-021-22044-z|pmid=33767194|pmc=7994803|bibcode=2021NatCo..12.1879S }}</ref>|earliest=2386}} | image = Protist collage 2.jpg | image_alt = | image_caption = Examples of protists. Clockwise from top left: red algae, kelp, ciliate, golden algae, dinoflagellate, metamonad, amoeba, slime mold. | auto = yes | parent = Eukaryota | subdivision_ranks = Major subdivisions  | subdivision = * Amorphea ** Amoebozoa ** Obazoa (including animals and fungi) * Ancyromonadida * CRuMs * Discoba * Malawimonadida * Metamonada * Diaphoretickes ** Archaeplastida (including land plants) ** Pancryptista ** Haptista ** Telonemia **SAR *** Stramenopiles *** Alveolata *** Rhizaria **Disparia ***Provora ***Membrifera | excludes = * Animalia * Fungi * Embryophyta (land plants) | excludes_text = Eukaryotes that are not considered protists }}

A '''protist''' ({{IPAc-en|ˈ|p|r|oʊ|t|ᵻ|s|t}} {{respell|PROH|tist}}) or '''protoctist''' is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic group encompassing the entire eukaryote tree of life, from which land plants, animals, and fungi evolved. They are primarily single-celled, exhibiting a wide range of forms such as amoebae, ciliates, thick-walled microalgae and, more commonly, flagellates. Several transitions to multicellularity have occurred among protists, from colonies with alternating cell types to giant slime molds, fungus-like organisms, and seaweeds with differentiated tissues.

Protists were historically regarded as a separate taxonomic kingdom known as '''Protista''' or '''Protoctista''', or were lumped together as part of the traditional plant and animal kingdoms as algae and protozoa, respectively. With the advent of molecular phylogenetics and electron microscopy studies, some protists were shown to be more closely related to animals or plants than to other protists, and algae were found to be intermixed with protozoa. The classification suffered major revisions, as seemingly unrelated forms were found to be evolutionarily related, and vice versa.

In modern classifications, protists are spread across several large clades known as supergroups, many of them containing disparate forms. For example, the Archaeplastida includes mostly phototrophs like red and green algae, from which land plants evolved. Opisthokonta groups fungi, animals, and their single-celled relatives. Amoebozoa and Rhizaria harbor the majority of amoeboid organisms, such as testate amoebae, foraminifers and radiolarians. Stramenopiles and Alveolata are diverse groups of flagellates, many of which have evolved into major parasites (e.g., oomycetes, apicomplexans) or phototrophs (diatoms, brown algae, dinoflagellates). The earliest diverging groups, collectively known as Excavata (e.g., euglenids, metamonads), are flagellates that represent the ancestral traits of the last eukaryotic common ancestor (LECA). Despite the comparatively low number of described species, protists compose the majority of eukaryotic diversity as indicated by environmental DNA studies. Most protists are yet undescribed.

Protists encompass almost all of the biological traits seen in eukaryotes, and many exhibit unique adaptations. These include a range of nutritional modes through specialized feeding structures (phagotrophy, osmotrophy, myzocytosis) or chloroplasts (phototrophy), often mixing both as mixotrophy. Cellular respiration also varies due to modifications of their mitochondria. Almost all protists have a complex cytoskeleton composed of relatively conserved structures across evolution, namely a flagellar apparatus with basal bodies from which microtubules emerge and support the remaining cellular structures. Many protists have unique organelles that serve other functions, such as contractile vacuoles for homeostasis, or eyespots for light perception. Protist cells tend to host symbionts such as bacteria and archaea, usually to support their metabolism and nutrition. Although traditionally presumed to be asexual, protists are capable of sexual reproduction, and can exhibit diverse and complex life cycles with different generations and life stages.

Protists are abundantly present in all ecosystems, including extreme habitats, as important components of the biogeochemical cycles and trophic webs. As producers, they are responsible for a large portion of global primary production and carbon fixation. As consumers and decomposers, they regulate fungal and bacterial populations, and release nutrients to other trophic levels. Some form mutualistic relationships with other protists or animals such as corals and termites. Others are important parasites. Pathogenic protists cause many well-known human and animal diseases such as malaria and toxoplasmosis, or significant plant diseases like clubroot and potato blight. Free-living protists can also negatively impact aquatic life as harmful algal blooms.

The early evolution of protists corresponds with the evolution of eukaryotes, which split from archaea around 3 billion years ago and eventually gave rise to a common ancestor (LECA) with essential traits such as mitochondria and a complex endomembrane system, some time during the Paleo- or Mesoproterozoic eras. In the gap between these two events, fossils are often interpreted as stem-group eukaryotes, with intermediate traits. Following the appearance of LECA, its descendants (crown-group eukaryotes) experienced a rapid diversification in the span of 300 million years that originated the modern supergroups. Still, their abundance in the fossil record remained low until the Neoproterozoic, when the first fossils of opisthokonts, amoebae, and multicellular algae appear. Throughout the Phanerozoic, protists evolved into the forms that dominate ecosystems today, leaving an extensive fossil record of primarily siliceous and calcareous shells.

== Definition == [[File:Tree of Living Organisms 2.png|thumb|left|The tree of life showing the position of protists, from which all other eukaryotes evolved.]] Protists are defined as all eukaryotes that are not animals, land plants or fungi, the three traditional "higher" kingdoms of eukaryotes. Because of this definition by exclusion, protists compose a paraphyletic group from which the ancestors of those three kingdoms evolved.<ref name="O'Malley-2012">{{cite journal | last1=O’Malley | first1=Maureen A. | last2=Simpson | first2=Alastair G. B. | last3=Roger | first3=Andrew J. | doi = 10.1007/s10539-012-9354-y | title = The other eukaryotes in light of evolutionary protistology | journal = Biology & Philosophy| volume = 28 | issue = 2 | pages = 299–330 | year = 2012 | s2cid = 85406712 }}</ref><ref name="Sebé-Pedrós-2017">{{cite journal|last1=Sebé-Pedrós|first1=Arnau|last2=Degnan|first2=Bernard M.|last3=Ruiz-Trillo|first3=Iñaki|title=The origin of Metazoa: a unicellular perspective|journal=Nature Reviews Genetics|volume=18|pages=498–512|date=8 May 2017|doi=10.1038/nrg.2017.21|doi-access=free|issue=8|pmid=28479598}}</ref> As such, there is no unique trait that unifies all protists yet excludes non-protists. Still, together they exhibit a remarkable diversity of life cycles, trophic levels, modes of locomotion, and cellular structures that dwarfs those seen in "higher" eukaryotes.<ref name="Burki-2021" /> A less popular view is that protists are defined as exclusively single-celled eukaryotes,<ref name="Harper-2009">{{cite book | chapter = Protists | pages = 204–233 | last = Harper | first = David | name-list-style = vanc | author-link = David Harper (palaeontologist) | author2 = Benton, Michael | author-link2 = Michael Benton | title = Introduction to Paleobiology and the Fossil Record | url = https://www.a-centauri.com/archivio/geo/Paleontologia/Benton_Introduction%20to%20Paleobiology%20and%20the%20Fossil%20Record.pdf | publisher = Wiley-Blackwell | year = 2009 | isbn = 978-1-4051-4157-4}}</ref> but this disregards the various transitions to multicellularity among protists.<ref name="O'Malley-2012" />

The distinction between protists and other kingdoms was blurry before genetic analysis. Organisms that are unquestionably known as protists include a wide range of photosynthetic species, known as "algae", including various types of macroalgae that have a similar complexity to plants. Other protists have a fungus-like nutrition or appearance, such as the oomycetes. The remaining heterotrophic protists are often called "protozoa".<ref name="O'Malley-2012" /><ref name="Scamardella-1999" /> Some minuscule animals (the myxozoans)<ref name="Levine-1980" /> and the "lower" fungi (namely the aphelids, rozellids and microsporidians, collectively the opisthosporidians)<ref>{{cite journal |last1=Karpov |first1=Sergey A. |last2=Mamkaeva |first2=Maria A. |last3=Aleoshin |first3=Vladimir V. |last4=Nassonova |first4=Elena |last5=Lilje |first5=Osu |last6=Gleason |first6=Frank H. |date=2014 |title=Morphology, phylogeny, and ecology of the aphelids (Aphelidea, Opisthokonta) and proposal for the new superphylum Opisthosporidia |journal=Frontiers in Microbiology |volume=5 |page=112 |bibcode=2014FrMic...500112K |doi=10.3389/fmicb.2014.00112 |pmc=3975115 |pmid=24734027 |doi-access=free}}</ref> were traditionally classified as protists,<ref>{{cite journal|title=Haeckel's Kingdom Protista and Current Concepts in Systematic Protistology|first=J. O.|last=Corliss|date=1998|journal=Stapfia|volume=56|pages=85–104|url=https://www.zobodat.at/pdf/STAPFIA_0056_0085-0104.pdf}}</ref><ref>{{cite book|first=John O.|last=Corliss|chapter=Protist systematics|publisher=John Wiley & Sons|title=Encyclopedia of Life Sciences|date=19 April 2001|doi=10.1038/npg.els.0003153|isbn=978-0-470-01590-2}}</ref> and some algae (particularly red and green algae) were lumped with plants.<ref name="Cavalier-Smith-1998">{{cite journal | first=Thomas | last=Cavalier-Smith |date=August 1998 |title=A revised six-kingdom system of life |journal=Biological Reviews of the Cambridge Philosophical Society |volume=73 |issue=3 |pages=203–266 |bibcode=1998BioRv..73..203C |doi=10.1111/j.1469-185X.1998.tb00030.x |pmid=9809012}}</ref>

According to the current consensus, the label 'protist' specifically excludes animals, embryophytes (land plants) —meaning that all eukaryotic algae fall under this label— and all fungi. Opisthosporidians are considered part of a larger fungal kingdom, although they are studied by protistologists and mycologists alike.<ref name="Adl-2019"/><ref>{{citation |last1=Tedersoo |first1=Leho |title=High-level classification of the Fungi and a tool for evolutionary ecological analyses |date=2018 |journal=Fungal Diversity |volume=90 |pages=135–159 |doi=10.1007/s13225-018-0401-0 |hdl=10138/238983 |s2cid=21714270 |display-authors=5 |last2=Sánchez-Ramírez |first2=Santiago |last3=Kõljalg |first3=Urmas |last4=Bahram |first4=Mohammad |last5=Döring |first5=Markus |last6=Schigel |first6=Dmitry |last7=May |first7=Tom |last8=Ryberg |first8=Martin |last9=Abarenkov |first9=Kessy |hdl-access=free}}</ref><ref name="Wijayawardene-2022">{{cite journal |last1=Wijayawardene |first1=N.N. |last2=Hyde |first2=K.D. |last3=Dai |first3=D.Q. |last4=Sánchez-García |first4=M. |last5=Goto |first5=B.T. |last6=Saxena |first6=R.K. |last7=Erdoğdu |first7=M. |last8=Selçuk |first8=F. |last9=Rajeshkumar |first9=K.C. |last10=Aptroot |first10=A. |last11=Błaszkowski |first11=J. |last12=Boonyuen |first12=N. |last13=da Silva |first13=G. |last14=de Souza |first14=F.A. |last15=Dong |first15=W. |display-authors=5 |year=2022 |title=Outline of Fungi and fungus-like taxa – 2021 |url=https://www.researchgate.net/publication/358798332 |journal=Mycosphere |volume=13 |issue=1 |pages=53–453 |doi=10.5943/mycosphere/13/1/2 |hdl=10481/76378 |s2cid=249054641 |doi-access=free |hdl-access=free |last16=Ertz |first16=D. |last17=Haelewaters |first17=D. |last18=Jones |first18=E.B. |last19=Karunarathna |first19=S.C. |last20=Kirk |first20=P.M. |last21=Kukwa |first21=M. |last22=Kumla |first22=J. |last23=Leontyev |first23=D.V. |last24=Lumbsch |first24=H.T. |last25=Maharachchikumbura |first25=S.S.N. |last26=Marguno |first26=F. |last27=Martínez-Rodríguez |first27=P. |last28=Mešić |first28=A. |last29=Monteiro |first29=J.S. |last30=Oehl |first30=F. |last31=Pawłowska |first31=J. |last32=Pem |first32=D. |last33=Pfliegler |first33=W.P. |last34=Phillips |first34=A.J.L. |last35=Pošta |first35=A. |last36=He |first36=M.Q. |last37=Li |first37=J.X. |last38=Raza |first38=M. |last39=Sruthi |first39=O.P. |last40=Suetrong |first40=S. |last41=Suwannarach |first41=N. |last42=Tedersoo |first42=L. |last43=Thiyagaraja |first43=V. |last44=Tibpromma |first44=S. |last45=Tkalčec |first45=Z. |last46=Tokarev |first46=Y.S. |last47=Wanasinghe |first47=D.N. |last48=Wijesundara |first48=D.S.A. |last49=Wimalaseana |first49=S.D.M.K. |last50=Madrid |first50=H. |last51=Zhang |first51=G.Q. |last52=Gao |first52=Y. |last53=Sánchez-Castro |first53=I. |last54=Tang |first54=L.Z. |last55=Stadler |first55=M. |last56=Yurkov |first56=A. |last57=Thines |first57=M.}}</ref>

== Morphology ==

Protists exist in diverse shapes and sizes.<ref name="Hausmann-1996">{{cite book|chapter=Cellular Organization of Protists|pages=13–29|title=Protistology|edition=3rd|publisher=E. Schweizerbart'sche Verlagsbuchhandlung|location=Berlin|first1=Klaus|last1=Hausmann|first2=Norbert|last2=Hülsmann|first3=Renate|last3=Radek|date=1996|isbn=3-510-65208-8|url=https://archive.org/details/protistology0000haus|url-access=registration}}</ref>{{rp|p=27}} While most are single-celled, others have evolved various forms of multicellularity, ranging from simple colonies of cells to large, complex organisms like giant kelp and slime molds.<ref name="Lamża-2023">{{cite journal|first1=Łukasz|last1=Lamża|title=Diversity of 'simple' multicellular eukaryotes: 45 independent cases and six types of multicellularity|journal=Biological Reviews|date=December 2023|volume=98|pages=2188–2209|doi=10.1111/brv.13001|pmid=37475165|issue=6|doi-access=free}}</ref> Single-celled protists are traditionally grouped by their shape and how they move, into four broad categories.<ref name="Sleigh-1989-1">{{cite book|last=Sleigh|first=Michael A.|chapter=Protozoa as members of the Protista|pages=1–12|title=Protozoa and other protists|date=1989|location=New York|publisher=E. Arnold|isbn=0-7131-2943-3|url=https://archive.org/details/lbg.593.2571|url-access=registration }}</ref>{{rp|p=5}}

{{multiple image | perrow = 3 | total_width = 350 | title = Examples of unicellular protists | image1 = 41598_2017_Article_BFsrep39892_Fig1a_HTML.png | caption1 = ''Prymnesium'', a flagellated alga | image2 = Cladococcus_abietinus.jpg | caption2 = ''Cladococcus'', a radiolarian | image6 = Gephyrocapsa_oceanica.jpg | caption6 = ''Gephyrocapsa'', a coccoid alga | caption5 = ''Leptophryx'', a filose amoeba | image4 = Falciparum gametocyte.jpg | caption4 = ''Plasmodium'', a sporozoan | image3 = Naegleria_lustrarea_jeu.13031_fig1D.jpg | caption3 = ''Naegleria'', an amoeboflagellate | image7 = Stephanopogon_sp.jpg | caption7 = ''Stephanopogon'', a multiflagellate | image8 = Stentor coeruleus mouth.jpg | caption8 = Closeup of the ciliate ''Stentor'' | image9 = Opalina_ranarum_Protsville.jpg | caption9 = ''Opalina'', an opaline | image12 = XenophyophoreNOAA.jpg | caption12 = A 20 cm-wide deep-sea xenophyophore }}

* Flagellates are the most common protists, and very likely the most abundant eukaryotes on Earth.<ref name="Jeuck-2013">{{cite journal|last1=Jeuck|first1=Alexandra|last2=Arndt|first2=Hartmut|title=A Short Guide to Common Heterotrophic Flagellates of Freshwater Habitats Based on the Morphology of Living Organisms|journal=Protist|volume=164|issue=6|date=2013|doi=10.1016/j.protis.2013.08.003|pages=842–860|doi-access=free}}</ref> They move using one or more whip-like structures called flagella.{{efn|name=cilia|Eukaryotic flagella are interchangeable with 'cilia' from a biological perspective. The usage of these two names depends on the author: some prefer to reserve cilia for shorter appendages and flagella for longer ones, while others prefer cilia for eukaryotes and flagella for prokaryotes. The term 'undulipodium' was proposed to unify the two concepts, as it refers specifically to the homologous microtubular structure found in both, but not found in prokaryotic flagella.<ref>{{cite journal|title=Lynn Margulis replies|journal=BioScience|volume=36|issue=5|date=1986|pages=293–294|doi=10.1093/bioscience/36.5.293-a}}</ref><ref>{{cite journal|title=Undulipodia, flagella and cilia|first1=Lynn|last1=Margulis|author-link=Lynn Margulis|volume=12|issue=1–2|pages=105–108|journal=Biosystems|date=1980|doi=10.1016/0303-2647(80)90041-6|pmid=7378551 |bibcode=1980BiSys..12..105M }}</ref><ref>{{cite journal|title=Terminology and nomenclature of the cytoskeletal elements associated with the flagellar/ciliary apparatus in protists|first1=R. A.|last1=Andersen|first2=D. J. S.|last2=Barr|first3=D. H.|last3=Lynn|first4=M.|last4=Melkonian|first5=Ø.|last5=Moestrup|first6=M. A.|last6=Sleigh|journal=Protoplasma|date=1991|doi=10.1007/BF01320809|volume=164|issue=1–3 |pages=1–8|bibcode=1991Prpls.164....1A }}</ref>}} Most are heterotrophic (known as zooflagellates), feeding on bacteria or other organisms, ranging from filter feeders like choanoflagellates to active predators like provorans. Many are photo- or mixotrophic (known as phytoflagellates) and are studied as algae, like the dinoflagellates.<ref>{{cite book|last=Taylor|first=F. J. R.|chapter=Phylum Dinoflagellata|pages=419–437|title=Handbook of Protoctista|date=1990|editor-first=Lynn|editor-last=Margulis|editor-first2=John O.|editor-last2=Corliss|editor-first3=Michael|editor-last3=Melkonian|editor-first4=David J.|editor-last4=Chapman|isbn=0-86720-052-9|publisher=Jones and Bartlett Publishers|location=Boston|url=https://archive.org/details/handbookofprotoc0000unse|url-access=registration|chapter-url=https://archive.org/details/handbookofprotoc0000unse/page/418}}</ref><ref name="Esteban-2020a">{{cite book|last=Esteban|first=Genoveva F.|last2=Fenchel|first2=Tom M.|date=2020|chapter=What is a protozoon?|pages=1–14|title=Ecology of Protozoa: The Biology of Free-living Phagotrophic Protists|publisher=Springer|location=Cham|doi=10.1007/978-3-030-59979-9_1}} </ref>

* Amoebae are known for their often flexible shape and ability to form extensions of the cytoplasm known as pseudopodia. These extensions come in various forms, such as lobose (blunt, rounded, as in ''Amoeba''), filose (thin, tapering, as in cercozoans), or reticulose (branching networks, as in foraminifers). Some, called axopodia, take the shape of radiating projections supported by microtubules, characteristic of heliozoa and radiolaria.<ref name="Hausmann-1996"/>{{rp|p=23}} Some amoebae can grow to sizes visible to the naked eye, reaching up to 20 cm.<ref>{{cite journal |last1=Gooday |first1=A.J |last2=Aranda da Silva |first2=A. |last3=Pawlowski |first3=J. |title=Xenophyophores (Rhizaria, Foraminifera) from the Nazare Canyon (Portuguese margin, NE Atlantic) |journal=Deep-Sea Research Part II |date=2011 |volume=58 |issue=23–24 |pages=2401–2419 |doi=10.1016/j.dsr2.2011.04.005|bibcode=2011DSRII..58.2401G }}</ref> Amoeboflagellates can produce both pseudopodia and flagella within the same life cycle.<ref name="Pánek-2017"/>

* Ciliates have larger cells with two types of nuclei and rows of small flagella, known as cilia.{{efn|name=cilia}} They are often at the top of the microbial food web. Although ciliates compose a single lineage,<ref>{{cite book|first=Denis|last=H. Lynn|pages=679–730|chapter=Ciliophora|volume=1|doi=10.1007/978-3-319-28149-0_23|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer International Publishing|location=Cham|url=https://ficoherb.fciencias.unam.mx/Recursos/Bibl/Handbookoftheprotists.pdf|isbn=978-3-319-28149-0|lccn=2017945328|date=2017}}</ref> some protists have evolved unusual large ciliate-like cells, such as the opalines.<ref name="Kostka-2017">{{cite book|first=Martin|last=Kostka|pages=543–565|chapter=Opalinata|volume=1|doi=10.1007/978-3-319-28149-0_4|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer International Publishing|location=Cham|url=https://ficoherb.fciencias.unam.mx/Recursos/Bibl/Handbookoftheprotists.pdf|isbn=978-3-319-28149-0|lccn=2017945328|date=2017}}</ref>

* Certain parasitic protists traditionally described as sporozoa<ref>{{cite book|last=Margulis|first=Lynn|author-link=Lynn Margulis|chapter=Introduction|pages=xi–xiii|title=Handbook of Protoctista|date=1990|editor-first=Lynn|editor-last=Margulis|editor-first2=John O.|editor-last2=Corliss|editor-first3=Michael|editor-last3=Melkonian|editor-first4=David J.|editor-last4=Chapman|isbn=0-86720-052-9|publisher=Jones and Bartlett Publishers|location=Boston|url=https://archive.org/details/handbookofprotoc0000unse|url-access=registration}}</ref> are immobile in their adult stage and reproduce through thick-walled spores, such as haplosporidians and apicomplexans. This term also included fungi and animals that have evolved a similar lifestyle—microsporidians and myxozoans, respectively. Other than their lifestyle, they have very little in common, and have evolved in distantly related groups.<ref name="Levine-1980">{{cite journal|last1=Levine|first1=N. D.|last2=Corliss|first2=J. O.|last3=Cox|first3=F. E. G.|last4=Deroux|first4=G.|last5=Grain|first5=J.|last6=Honigberg|first6=B. M.|last7=Leedale|first7=G. F.|last8=Loeblich|first8=A. R.|last9=Lom|first9=Iii. J.|last10=Lynn|first10=D.|last11=Merinfeld|first11=E. G.|last12=Page|first12=F. C.|last13=Poljansky|first13=G.|last14=Sprague|first14=V.|last15=Vavra|first15=J.|last16=Wallace|first16=F. G.|display-authors=6|title=A newly revised classification of the Protozoa|journal=Journal of Protozoology|volume=27|issue=1|date=1980|pages=37–58|doi=10.1111/j.1550-7408.1980.tb04228.x|pmid=6989987|doi-access=free}}</ref><ref name="Sleigh-1989-1"/>{{rp|p=8–9}}

Other single-celled algae exist in forms beyond the motile flagellates. Some are non-motile and encased in hard cell walls (coccoid, like diatoms) or embedded in a mucilage matrix (capsalean, like glaucophytes);<ref name="Price-2017">{{cite book|first1=Dana C.|last1=Price|first2=J��rgen M.|last2=Steiner|first3=Hwan Su|last3=Yoon|first4=Debashish|last4=Bhattacharya|first5=Wolfgang|last5=Löffelhardt|chapter=Glaucophyta|pages=23–87|chapter-url=https://tuaulavirtual.educatic.unam.mx/pluginfile.php/1747217/mod_resource/content/1/Price2016_ReferenceWorkEntry_Glaucophyta.pdf|access-date=9 June 2025|doi=10.1007/978-3-319-28149-0_42|volume=1|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer International Publishing|location=Cham|url=https://ficoherb.fciencias.unam.mx/Recursos/Bibl/Handbookoftheprotists.pdf|isbn=978-3-319-28149-0|lccn=2017945328|date=2017}}</ref> others are amoeboid, like the reticulose chlorarachniophytes.<ref name="Keeling-2017a">{{cite book|first=Patrick J.|last=Keeling|pages=765–781|chapter=Chlorarachniophytes|volume=1|doi=10.1007/978-3-319-28149-0_34|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer International Publishing|location=Cham|url=https://ficoherb.fciencias.unam.mx/Recursos/Bibl/Handbookoftheprotists.pdf|isbn=978-3-319-28149-0|lccn=2017945328|date=2017}}</ref><ref name="Graham-2022-1">{{cite book|first1=Linda E.|last1=Graham|first2=James M.|last2=Graham|first3=Lee W.|last3=Wilcox|first4=Martha E.|last4=Cook|title=Algae|edition=4th|publisher=LJLM Press|date=2022|isbn=978-0-9863935-4-9|chapter=Chapter 1. Introduction to the Algae|pages=1-1–1-21}}</ref>

{{multiple image | perrow = 2 | total_width = 300 | title = Examples of multicellular protists | image1 = Kelp-forest-Monterey.jpg | caption1 = A kelp forest with ''Macrocystis'', the giant kelp | image2 = Badhamia_utricularis_mature.jpg | caption2 = Sporocarps of the myxomycete ''Badhamia'' | image3 = Fonticula alba black background.jpg | caption3 = Sorocarps of the nucleariid ''Fonticula'' | image4 = Chara_sp_reproductive_structure.JPG | caption4 = Reproductive structures of the green alga ''Chara'' }}

Multicellularity has evolved numerous times to various degrees among protists, resulting in organisms built either by cells aggregating together (aggregative) or by cells dividing without separating (clonal).<ref name="Lamża-2023" /> For example:

* Fungus-like protists, traditionally studied by mycologists, are osmotrophic and produce macroscopic fruiting bodies for dispersal (sorocarps if aggregative, sporocarps if clonal). They may have hyphae-like bodies, or they may be slime molds, composed of individual amoebae until they form fruiting bodies.<ref name="Spiegel-2016">{{cite book|last=Spiegel|first=F.W.|title=Encyclopedia of Evolutionary Biology|chapter=Unikonts, Evolution and Diversification of (with Emphasis on Fungal-Like Forms)|publisher=Elsevier|date=2016|isbn=978-0-12-800426-5|doi=10.1016/b978-0-12-800049-6.00248-1|url=https://linkinghub.elsevier.com/retrieve/pii/B9780128000496002481|access-date=12 May 2025|page=325–332}}</ref><ref>{{cite journal|last1=Gleason|first1=Frank H.|last2=Lilje|first2=Osu|last3=Lange|first3=Lene|title=What has happened to the "aquatic phycomycetes" (sensu Sparrow)? Part II: Shared properties of zoosporic true fungi and fungus-like microorganisms|journal=Fungal Biology Reviews|volume=32|issue=2|pages=52–61|date=2018|doi=10.1016/j.fbr.2017.09.003|bibcode=2018FunBR..32...52G }}</ref><ref>{{cite book|chapter=An introduction to fungus-like microorganisms|title=Marine fungi|first1=Sigrid|last1=Neuhauser|first2=Sally L.|last2=Glockling|first3=Eduardo M.|last3=Leaño|first4=Osu|last4=Lilje|first5=Agostina V.|last5=Marano|first6=Frank H.|last6=Gleason|editor-first1=E. B. Gareth|editor-last1=Jones|editor-first2=Ka-Lai|editor-last2=Pang|series=Marine and Freshwater Botany|doi=10.1515/9783110264067.137|publisher=De Gruyter|date=2012|pages=137–152|isbn= 978-3-11-026406-7}}</ref> Some can reach lengths of several meters.<ref name="Hausmann-1996" />{{rp|p=29}}

* Multicellular algae range from simple filaments and coenocytes to the highly complex brown algae, which have fully differentiated tissues (parenchymatous) resembling those of plants, or the red algae, with only partial differentiation (pseudoparenchymatous).<ref name="Graham-2022-1" /><ref name="Guiry 2023">{{cite journal|title=Validation of the phylum name ''Heterokontophyta''|author1=Michael D. Guiry|author2=Øjvind Moestrup|author3=Robert A. Andersen|journal=Notulae Algarum|volume=2023|issue=297|date=11 October 2023|url=https://www.notulaealgarum.com/2023/documents/Notulae%20Algarum%20No.%20297.pdf}}</ref> In green algae alone multicellularity is thought to have evolved over 20 separate times, with some groups like the stoneworts developing specialized reproductive organs.<ref>{{cite journal|last1=Umen|first1=James|last2=Herron|first2=Matthew D.|title=Green Algal Models for Multicellularity|journal=Annual Review of Genetics|volume=55|issue=1|date=23 November 2021|issn=0066-4197|doi=10.1146/annurev-genet-032321-091533|pages=603–632|url=https://www.annualreviews.org/docserver/fulltext/genet/55/1/annurev-genet-032321-091533.pdf|access-date=27 February 2026}}</ref><ref name="Lamża-2023" />

Other multicellular protists include amoebae that fuse into large networks, and colonial heliozoa and ciliates with new features not seen in solitary cells. The ciliate ''Haplozoon'' is interpreted to have animal-like embryonic development and cell type differentiation.<ref name="Lamża-2023" /> Choanoflagellates, the closest living relatives of animals, include alternating cell types that are interpreted as early stages of animal multicellularity.<ref>{{cite journal|last1=Laundon|first1=Davis|last2=Larson|first2=Ben T.|last3=McDonald|first3=Kent|last4=King|first4=Nicole|last5=Burkhardt|first5=Pawel|title=The architecture of cell differentiation in choanoflagellates and sponge choanocytes|journal=PLOS Biology|volume=17|issue=4|date=12 April 2019|issn=1545-7885|pmid=30978201|pmc=6481868|doi=10.1371/journal.pbio.3000226|doi-access=free|article-number=e3000226}}</ref>

== Classification ==

=== Brief history ===

Starting in the 17th and 18th centuries, after the discovery of microscopic life by Antonie van Leeuwenhoek, the classification of single-celled protists was largely based on observations under light microscopy. Protists were incorporated into the traditional dichotomy that defined all life as either plant or animal: non-motile algae were considered part of the plant kingdom, and all other protists joined the animal kingdom.<ref name="Scamardella-1999">{{cite journal | first = Joseph M. | last = Scamardella |title = Not plants or animals: A brief history of the origin of Kingdoms Protozoa, Protista, and Protoctista | year = 1999 | journal = International Microbiology | volume = 2 |issue = 4 | pages = 207–221 |pmid = 10943416 | url = http://revistes.iec.cat/index.php/IM/article/viewFile/9219/9217}}</ref><ref name="Yazaki-2025">{{cite journal|last1=Yazaki|first1=Euki|last2=Shiratori|first2=Takashi|last3=Inagaki|first3=Yuji|title=Protists with Uncertain Phylogenetic Affiliations for Resolving the Deep Tree of Eukaryotes|journal=Microorganisms|volume=13|issue=8|date=18 August 2025|issn=2076-2607|pmid=40871430|pmc=12388492|doi=10.3390/microorganisms13081926|doi-access=free|article-number=1926}}</ref> They were popularly known as "infusion animals" or infusoria, together with bacteria and small invertebrates. Otto Friedrich Müller was the first to introduce microbial protists to the Linnean system of binomial nomenclature.<ref>{{cite book|title=The Flagellates. Unity, diversity and evolution|editor1=Barry S. C. Leadbeater|editor2=J. C. Green|chapter=Chapter 1. The flagellates: historical perspectives|author1=Barry S. C. Leadbeater|author2=Sharon M. M. McReady|publisher=Taylor & Francis|publication-place=London|date=2000|pages=1–26|doi=10.1201/9781482268225|isbn=978-0-429-18213-6 }}</ref><ref name="Ratcliff-2009">{{cite book|author=Marc J. Ratcliff|publisher=Ashgate|date=2009|url=https://books.google.com/books?id=GTZt1c5IYioC|chapter=The Emergence of the Systematics of Infusoria|title=The Quest for the Invisible: Microscopy in the Enlightenment|pages=177–216|isbn=978-1-4094-8026-6 }}</ref>

[[File:Haeckel arbol bn.png|thumb|Haeckel's 1866 tree of life, with the third kingdom Protista.]]

During the 19th century, after several waves of naturalist studies,<ref name="Ratcliff-2009"/> it became clear that these microorganisms were distinct from animals and plants. John Hogg and Ernst Haeckel proposed a separate kingdom of life, named '''Protoctista'''<ref name="Hogg-1860">{{cite journal|author=John Hogg|date=1860|title=On the distinctions of a Plant and an Animal, and on a Fourth Kingdom of Nature|url=https://babel.hathitrust.org/cgi/pt?id=hvd.32044089575245;view=1up;seq=232 |journal=Edinburgh New Philosophical Journal|series=2nd series|volume=12|pages=216–225|quote-page=223|quote=... I here suggest a ''fourth'' or an additional kingdom, under the title of the ''Primigenal'' kingdom, ... This ''Primigenal'' kingdom would comprise all the lower creatures, or the primary organic beings, – 'Protoctista,' – from πρώτος, ''first'', and χτιστά, ''created beings''; ...}}</ref> or '''Protista''',<ref name="Haeckel-1866">{{cite book |last1=Haeckel |first1=Ernst |title=Generelle Morphologie der Organismen |trans-title=The General Morphology of Organisms |date=1866 |publisher=G. Reimer |location=Berlin, (Germany) |volume=1 |pages=215ff |url=https://archive.org/details/generellemorphol01haec/page/214 |language=de}} From p. 215: "VII. Character des Protistenreiches." (VII. Character of the kingdom of Protists.)</ref> respectively, to accommodate the predominantly unicellular eukaryotes, and initially bacteria, which were later excluded.<ref name="Scamardella-1999"/> The classical framework of protist classification was established, as exemplified by the works of Otto Bütschli, where they were grouped according to morphological and locomotive features, such as Mastigophora (flagellates), Rhizopoda (amoebae), Sporozoa (spore-forming parasites), and Infusoria (ciliates).<ref name="Yazaki-2025"/> However, Bütschli retained a division between the ''Protozoa'' (animal-like protists) and ''Protophyta'' (plant-like). This dogma remained dominant throughout the early 20th century.<ref name="Scamardella-1999"/>

From the mid-20th century, eukaryotes were firmly split from bacteria (prokaryotes) due to the presence of the cell nucleus,<ref name="Copeland-1938">{{cite journal | first = Herbert F. | last = Copeland | year = 1938 | title = The Kingdoms of Organisms | journal = Quarterly Review of Biology | volume = 13 | issue = 4 | pages = 383–420 | doi = 10.1086/394568 | jstor=2808554| s2cid = 84634277 }}</ref> and protists (or protoctists) were more popularly accepted as a separate kingdom of eukaryotes.<ref>{{cite book|title=Kingdoms & Domains: An Illustrated Guide to the Phyla of Life on Earth|first1=Lynn|last1=Margulis|author-link1=Lynn Margulis|first2=Michael J.|last2=Chapman|publisher=AP|date=2009|isbn=978-0-12-373621-5|edition=4th}}</ref><ref name="Whittaker-1969">{{cite journal | first = R. H. | last = Whittaker | title = New concepts of kingdoms or organisms: Evolutionary relations are better represented by new classifications than by the traditional two kingdoms | journal = Science | volume = 163 | issue = 3863 | pages = 150–160 | date = January 1969 | pmid = 5762760 | doi = 10.1126/science.163.3863.150 | bibcode = 1969Sci...163..150W | citeseerx = 10.1.1.403.5430 }}</ref><ref name="Hagen-2012">{{cite journal | title = Five Kingdoms, More or Less: Robert Whittaker and the Broad Classification of Organisms | journal = BioScience | volume = 62 | pages = 67–74 | doi = 10.1525/bio.2012.62.1.11 | year = 2012 | last1 = Hagen | first1 = Joel B. | doi-access = free }}</ref> The advent of electron microscopy shifted the methods of classification, as it revealed previously unrecognized cellular characteristics (i.e., ultrastructure, particularly of the flagellar apparatus and the cytoskeleton) that suggested evolutionary affinities between superficially disparate lineages. For example, the tripartite flagellar mastigonemes were used to group heterokont algae, oomycetes and opalines into the Stramenopiles; the discovery of cortical alveoli showed affinities between dinoflagellates and ciliates, which now belong to the Alveolata; and disc-shaped mitochondrial cristae were shared by kinetoplastids and euglenids, now united as Euglenozoa. The algae-protozoa dichotomy became obsolete.<ref name=":2">{{cite book|last=Patterson|first=David J.|author-link=David J. Patterson|date=1994|chapter=Protozoa: Evolution and Systematics|pages=1–14|title=Progress in Protozoology: Proceedings of the IX International Congress of Protozoology, Berlin 25–31 July 1993|publisher=Gustav Fischer|editor-first1=Klaus|editor-last1=Hausmann|editor-first2=Norbert|editor-last2=Hülsmann|url=https://archive.org/details/progressinprotoz0000klau|url-access=registration|chapter-url=https://archive.org/details/progressinprotoz0000klau/page/1}}</ref><ref>{{cite journal|last1=Patterson|first1=David J.|author-link1=David J. Patterson|title=The Diversity of Eukaryotes|journal=The American Naturalist|date=October 1999|volume=154|issue=S4|url=https://www.journals.uchicago.edu/doi/epdf/10.1086/303287|url-access=subscription|doi=10.1086/303287|pages=S96–S124|pmid=10527921}}</ref><ref name="Yazaki-2025"/>

[[File:Eukaryotic tree of life (Burki et al 2020).jpg|thumb|right|upright=1.5|Phylogenomic tree of eukaryotes, as regarded in 2020. Supergroups are in color.]]

Since the 1990s, molecular phylogenetic analyses based primarily on the SSU rRNA gene demonstrated that protists were a paraphyletic assemblage of clades spanning the entire eukaryotic tree of life, from which the other three "kingdoms" (animals, plants, and fungi) had evolved.<ref name="Schlegel-2007">{{cite journal | doi=10.1016/j.ode.2006.11.001 | title=Protists – A textbook example for a paraphyletic taxon☆ | journal=Organisms Diversity & Evolution| volume=7 | issue=2 | pages=166–172 | year=2007 | last1=Schlegel | first1=M.| last2=Hulsmann | first2=N.| bibcode=2007ODivE...7..166S }}</ref><ref name="Yazaki-2025"/> Beginning in the 2000s, single-cell sequencing and phylogenomics technologies progressively improved the resolution of deeper evolutionary relationships. Altogether, these innovations led to successive revisions of protist classification,<ref name="Yazaki-2025"/> such as the ones published by the International Society of Protistologists.<ref name="Adl-2005">{{cite journal|first1=Sina M.|last1=Adl|first2=Alastair G. B.|last2=Simpson|first3=Mark A.|last3=Farmer|first4=Robert A.|last4=Andersen|first5=O. Roger|last5=Anderson|first6=John R.|last6=Barta|first7=Samuel S.|last7=Bowser|first8=Guy|last8=Brugerolle|first9=Robert A.|last9=Fensome|first10=Suzanne|last10=Fredericq|first11=Timothy Y.|last11=James|first12=Sergei|last12=Karpov|first13=Paul|last13=Kugrens|first14=John|last14=Krug|first15=Christopher E.|last15=Lane|first16=Louise A.|last16=Lewis|first17=Jean|last17=Lodge|first18=Denis H.|last18=Lynn|first19=David G.|last19=Mann|first20=Richard M.|last20=Mccourt|first21=Leonel|last21=Mendoza|first22=Øjvind|last22=Moestrup|first23=Sharon E.|last23=Mozley-Standridge|first24=Thomas A.|last24=Nerad|first25=Carol A.|last25=Shearer|first26=Alexey V.|last26=Smirnov|first27=Frederick W.|last27=Spiegel|first28=Max F. J. R.|last28=Taylor|title=The New Higher Level Classification of Eukaryotes with Emphasis on the Taxonomy of Protists|date=19 October 2005|volume=52|issue=2|pages=399–451|journal=The Journal of Eukaryotic Microbiology|doi=10.1111/j.1550-7408.2005.00053.x|doi-access=free|pmid=16248873|display-authors=5}}</ref><ref name="Adl-2012">{{cite journal|first1=Sina M.|last1=Adl|first2=Alastair G. B.|last2=Simpson|first3=Christopher E.|last3=Lane|first4=Julius|last4=Lukeš|first5=David|last5=Bass|first6=Samuel S.|last6=Bowser|first7=Matthew W.|last7=Brown|first8=Fabien|last8=Burki|first9=Micah|last9=Dunthorn|first10=Vladimir|last10=Hampl|first11=Aaron|last11=Heiss|first12=Mona|last12=Hoppenrath|first13=Enrique|last13=Lara|first14=Line|last14=le Gall|first15=Denis H.|last15=Lynn|first16=Hilary|last16=McManus|first17=Edward A. D.|last17=Mitchell|first18=Sharon E.|last18=Mozley-Stanridge|first19=Laura W.|last19=Parfrey|first20=Jan|last20=Pawlowski|first21=Sonja|last21=Rueckert|first22=Laura|last22=Shadwick|first23=Conrad L.|last23=Schoch|first24=Alexey|last24=Smirnov|first25=Frederick W.|last25=Spiegel|title=The Revised Classification of Eukaryotes|journal=The Journal of Eukaryotic Microbiology|date=28 September 2012|volume=59|issue=2|pages=429–514|doi=10.1111/j.1550-7408.2012.00644.x|doi-access=free|pmc=3483872|pmid=23020233|display-authors=5}}</ref><ref name="Adl-2019">{{cite journal | doi=10.1111/JEU.12691 | title=Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes | date=2019 | last1=Adl | first1=Sina M. | last2=Bass | first2=David | last3=Lane | first3=Christopher E. | last4=Lukeš | first4=Julius | last5=Schoch | first5=Conrad L. | last6=Smirnov | first6=Alexey | last7=Agatha | first7=Sabine | last8=Berney | first8=Cedric | last9=Brown | first9=Matthew W. | last10=Burki | first10=Fabien | last11=Cárdenas | first11=Paco | last12=Čepička | first12=Ivan | last13=Chistyakova | first13=Lyudmila | last14=Del Campo | first14=Javier | last15=Dunthorn | first15=Micah | last16=Edvardsen | first16=Bente | last17=Eglit | first17=Yana | last18=Guillou | first18=Laure | last19=Hampl | first19=Vladimír | last20=Heiss | first20=Aaron A. | last21=Hoppenrath | first21=Mona | last22=James | first22=Timothy Y. | last23=Karnkowska | first23=Anna | last24=Karpov | first24=Sergey | last25=Kim | first25=Eunsoo | last26=Kolisko | first26=Martin | last27=Kudryavtsev | first27=Alexander | last28=Lahr | first28=Daniel J.G. | last29=Lara | first29=Enrique | last30=Le Gall | first30=Line | journal=Journal of Eukaryotic Microbiology | volume=66 | issue=1 | pages=4–119 | pmid=30257078 | pmc=6492006 | display-authors=5}}</ref> Eukaryotes could no longer be divided into four monophyletic kingdoms,{{efn|There was, however, one kingdom-based system that persisted into the 21st century, developed by Thomas Cavalier-Smith. He proposed two non-monophyletic kingdoms of protists, the Protozoa and the Chromista. He argued that protists with red algal-derived plastids and their heterotrophic relatives (i.e., Stramenopiles, Alveolata, Haptista and Cryptista) shared a single common photosynthetic ancestor, and composed the Chromalveolata or, later with the addition of Rhizaria, the Chromista,<ref name="Cavalier-Smith-1998"/><ref>{{cite journal|last1=Ruggiero|first1=Michael A.|last2=Gordon|first2=Dennis P.|last3=Orrell|first3=Thomas M.|last4=Bailly|first4=Nicolas|last5=Bourgoin|first5=Thierry|last6=Brusca|first6=Richard C.|last7=Cavalier-Smith|first7=Thomas|last8=Guiry|first8=Michael D.|last9=Kirk|first9=Paul M.|last10=Thuesen|first10=Erik V.|title=A higher level classification of all living organisms|journal=PLOS ONE|date=2015|volume=10|issue=4|article-number=e0119248|doi=10.1371/journal.pone.0119248|pmid=25923521|pmc=4418965|bibcode=2015PLoSO..1019248R|doi-access=free}}</ref> which was polyphyletic.<ref>{{cite journal|last1=Burki|first1=Fabien|last2=Kaplan|first2=Maia|last3=Tikhonenkov|first3=Denis V.|last4=Zlatogursky|first4=Vasily|last5=Minh|first5=Bui Quang|last6=Radaykina|first6=Liudmila V.|last7=Smirnov|first7=Alexey|last8=Mylnikov|first8=Alexander P.|last9=Keeling|first9=Patrick J.|display-authors=5|title=Untangling the early diversification of eukaryotes: a phylogenomic study of the evolutionary origins of Centrohelida, Haptophyta and Cryptista|journal=Proceedings of the Royal Society B: Biological Sciences|volume=283|issue=1823|date=27 January 2016|issn=0962-8452|pmid=26817772|pmc=4795036|doi=10.1098/rspb.2015.2802|doi-access=free|article-number=20152802}}</ref> This scheme endured until 2022, the year of his last publication.<ref name="Cavalier-Smith-2022">{{cite journal|first=Thomas|last=Cavalier-Smith|title=Ciliary transition zone evolution and the root of the eukaryote tree: implications for opisthokont origin and classification of kingdoms Protozoa, Plantae, and Fungi|journal=Protoplasma|date=May 2022|volume=259|issue=3|pages=487–593|doi=10.1007/s00709-021-01665-7|pmid=34940909|pmc=9010356|bibcode=2022Prpls.259..487C }}</ref>}} and instead are arranged in "supergroups", each often encompassing an unexpected variety of morphologies and lifestyles that do not resemble one another. New deep-branching groups are added to the tree at a rate of nearly one per year.<ref name="Lax-2018"/><ref>{{cite journal |last1=Burki |first1=Fabien |last2=Roger |first2=Andrew J. |last3=Brown |first3=Matthew W. |last4=Simpson |first4=Alastair G.B. |title=The New Tree of Eukaryotes |journal=Trends in Ecology & Evolution |volume=35 |issue=1 |date=9 October 2019 |doi=10.1016/j.tree.2019.08.008 |doi-access=free |pages=43–55 |pmid=31606140 |bibcode=2020TEcoE..35...43B |url=https://www.cell.com/article/S0169534719302575/pdf |access-date=2025-06-05}}</ref><ref name="Tikhonenkov-2022a">{{cite journal|last1=Tikhonenkov|first1=Denis V.|last2=Mikhailov|first2=Kirill V.|last3=Gawryluk|first3=Ryan M. R.|last4=Belyaev|first4=Artem O.|last5=Mathur|first5=Varsha|last6=Karpov|first6=Sergey A.|last7=Zagumyonnyi|first7=Dmitry G.|last8=Borodina|first8=Anastasia S.|last9=Prokina|first9=Kristina I.|last10=Mylnikov|first10=Alexander P.|last11=Aleoshin|first11=Vladimir V.|last12=Keeling|first12=Patrick J.|display-authors=5|title=Microbial predators form a new supergroup of eukaryotes|journal=Nature|date=2022|volume=612 |issue=7941 |pages=714–719 |doi=10.1038/s41586-022-05511-5|pmid=36477531 |bibcode=2022Natur.612..714T |s2cid=254436650 }}</ref><ref name="Eglit-2024">{{Cite journal|title=''Meteora sporadica'', a protist with incredible cell architecture, is related to Hemimastigophora|journal=Current Biology|doi=10.1016/j.cub.2023.12.032|volume=34|pages=451–459|date=22 January 2024|first1=Yana|last1=Eglit|first2=Takashi|last2=Shiratori|first3=Jon|last3=Jerlström-Hultqvist|first4=Kelsey|last4=Williamson|first5=Andrew J.|last5=Roger|first6=Ken-Ichiro|last6=Ishida|first7=Alastair G.B.|last7=Simpson|issue=2 |pmid=38262350 |bibcode= 2024CBio...34..451E}}</ref>

=== Modern classification and diversity === {{Further|Protist classification}} Protists are currently divided among a number of clades informally named supergroups. Most of these supergroups fall under either of two large clades of eukaryotes: Amorphea and Diaphoretickes. The animals and fungi belong to the Opisthokonta supergroup in the Amorphea clade, along with several other groups of protists (e.g., Amoebozoa).<ref name="Adl-2019"/> Diaphoretickes contains the diverse supergroups Archaeplastida (including plants), Stramenopiles, Alveolata, Rhizaria (combined as the SAR supergroup), and the less species-rich Cryptista, Haptista, Telonemia, and Disparia.<ref name="Burki-2007">{{cite journal|last1=Burki|first1=Fabien|last2=Shalchian-Tabrizi|first2=Kamran|last3=Minge|first3=Marianne|last4=Skjæveland|first4=Åsmund|last5=Nikolaev|first5=Sergey I.|last6=Jakobsen|first6=Kjetill S.|last7=Pawlowski|first7=Jan|display-authors=5|date=August 2007 |title=Phylogenomics reshuffles the eukaryotic supergroups |journal=PLOS ONE |volume=2 |issue=8 |article-number=e790 |bibcode=2007PLoSO...2..790B |doi=10.1371/journal.pone.0000790 |pmc=1949142 |pmid=17726520 |doi-access=free}}</ref><ref>{{Cite book|chapter= Protist Diversity and Eukaryote Phylogeny|title=Handbook of the Protists|date=2017|isbn=978-3-319-28147-6|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer|pages=1–22|last1=Simpson|first1=Alastair G. B.|last2=Slamovits|first2=Claudio H.|last3=Archibald|first3=John M.|volume=1|doi=10.1007/978-3-319-28149-0_45}}</ref><ref name="Valt-2025"/> Outside of these larger clades, various groups of protists with primitive cell architecture (Discoba, Metamonada, and Malawimonadida) are collectively known as the excavates or "Excavata". The name 'excavate' refers to the shared characteristic of a ventral groove in the cell used for feeding, which is considered an ancestral trait present in the last eukaryotic common ancestor.<ref name=":3">{{cite journal|first1=Alastair G. B.|last1=Simpson|date=2003|title=Cytoskeletal organization, phylogenetic affinities and systematics in the contentious taxon Excavata (Eukaryota)|journal=International Journal of Systematic and Evolutionary Microbiology|volume=53|issue=6|pages=1759–1777|doi=10.1099/ijs.0.02578-0|pmid=14657103 |bibcode=2003IJSEM..53.1759S }}</ref><ref name="Madigan-2019">{{cite book|chapter=Diversity of Microbial ''Eukarya''|pages=593–618|title=Brock Biology of Microorganisms|edition=15th, Global|last1=Madigan|first1=Michael T.|last2=Bender|first2=Kelly S.|last3=Buckley|first3=Daniel H.|last4=Sattley|first4=W. Matthew|last5=Stahl|first5=David A.|date=2019|isbn=978-1-292-23510-3|publisher=Pearson}}</ref>{{rp|597}}<ref name=":4">{{cite journal|last1=Suzuki-Tellier|first1=Sei|last2=Kiørboe|first2=Thomas|last3=Simpson|first3=Alastair G. B.|title=The function of the feeding groove of 'typical excavate' flagellates|journal=Journal of Eukaryotic Microbiology|date=2023|volume=71|article-number=e13016|doi=10.1111/jeu.13016|pmid=38108228|issue=2}}</ref>

The following table lists estimated numbers of described extant species for all known protist supergroups, and provides an overview of their diversity in terms of morphologies, habitats, and nutritional modes. For large groups, the overview is not exhaustive and only mentions the most characteristic members.

{| class="wikitable sortable plainrowheaders" style="width:100%;font-size:90%" table-layout:auto;" ! Clade ! colspan="2" |Supergroup!! Example !! Brief description of morphology, lifestyle and habitat !! Living species |- | rowspan="8" |'''Diaph.''' | rowspan="3" |SAR |Stramenopiles |frameless|90x90px |Ancestrally flagellates distinguished by two 'heterokont' (unequal) flagella, one with tripartite mastigonemes. Present in virtually all habitats. The most species-rich lineage, the ochrophytes, are algae of diverse morphologies, ranging from flagellates (like golden algae) to walled ornamented cells (like diatoms, ''pictured'') to truly multicellular macroalgae with differentiated tissues (brown algae such as kelp). All other lineages are composed of heterotrophs: bacterivorous flagellates (e.g., bicosoecids, bigyromonads), fungus-like osmotrophs (oomycetes, hyphochytrids, and labyrinthulomycetes), heliozoan amoebae (actinophryids), and ciliate-like obligate symbionts of animals (opalinids).<ref>{{cite journal|last1=Jirsová|first1=Dagmar|last2=Wideman|first2=Jeremy G|title=Integrated overview of stramenopile ecology, taxonomy, and heterotrophic origin|journal=The ISME Journal|volume=18|issue=1|date=8 January 2024|issn=1751-7362|pmid=39077993|pmc=11412368|doi=10.1093/ismejo/wrae150|article-number=wrae150}}</ref> |{{nts|100000|prefix=over }}<ref name="Yoon 2009">{{cite book |last1=H.S. Yoon |last2=R.A. Andersen |last3=S.M. Boo |last4=D. Bhattacharya |chapter=Stramenopiles|title=Encyclopedia of Microbiology |edition=Third |date=17 February 2009 |pages=721–731 |doi=10.1016/B978-012373944-5.00253-4 |isbn=978-0-12-373944-5 |chapter-url=https://www.sciencedirect.com/science/article/abs/pii/B9780123739445002534 |access-date=2 March 2024|chapter-url-access=subscription }}</ref> |- |Alveolata |frameless|90x90px |Ancestrally flagellated predators with cortical alveoli. The colponemids represent these ancestral characteristics.<ref name="Tikhonenkov-2020"/> The most diverse group are the ciliates (''pictured''), with large cells covered in rows of cilia, usually at the top of the microbial food chain.<ref>{{cite book|first=Denis|last=H. Lynn|pages=679–730|chapter=Ciliophora|volume=1|doi=10.1007/978-3-319-28149-0_23|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer International Publishing|location=Cham|url=https://ficoherb.fciencias.unam.mx/Recursos/Bibl/Handbookoftheprotists.pdf|isbn=978-3-319-28149-0|lccn=2017945328|date=2017}}</ref> The remaining alveolates belong to the clade Myzozoa and are ancestrally photosynthetic; some have retained their photosynthetic ability (chromerids and many dinoflagellates), while others have evolved into parasites of animals and algae (apicomplexans, perkinsozoans, and some dinoflagellates).<ref name="Janouškovec-2015">{{cite journal|first1=Jan|last1=Janouškovec|first2=Denis V.|last2=Tikhonenkov|first3=Fabien|last3=Burki|first4=Alexis T.|last4=Howe|first5=Martin|last5=Kolísko|first6=Alexander P.|last6=Mylnikov|first7=Patrick J.|last7=Keeling|title=Factors mediating plastid dependency and the origins of parasitism in apicomplexans and their close relatives|journal=PNAS|date=2015|volume=112|issue=33|pages=10200–10207|doi=10.1073/pnas.1423790112|doi-access=free |pmid=25717057|pmc=4547307|bibcode=2015PNAS..11210200J }}</ref><ref name="Adl-2019"/><ref>{{cite journal|last1=Mathur|first1=Varsha|last2=Kolísko|first2=Martin|last3=Hehenberger|first3=Elisabeth|last4=Irwin|first4=Nicholas A.T.|last5=Leander|first5=Brian S.|last6=Kristmundsson|first6=Árni|last7=Freeman|first7=Mark A.|last8=Keeling|first8=Patrick J.|title=Multiple Independent Origins of Apicomplexan-Like Parasites|journal=Current Biology|volume=29|issue=17|date=2019|doi=10.1016/j.cub.2019.07.019|pmid=31422883|doi-access=free|pages=2936–2941|display-authors=5}}</ref><ref>{{cite journal|last1=Holt|first1=Corey C.|last2=Hehenberger|first2=Elisabeth|last3=Tikhonenkov|first3=Denis V.|last4=Jacko-Reynolds|first4=Victoria K. L.|last5=Okamoto|first5=Noriko|last6=Cooney|first6=Elizabeth C.|last7=Irwin|first7=Nicholas A. T.|last8=Keeling|first8=Patrick J.|title=Multiple parallel origins of parasitic Marine Alveolates|journal=Nature Communications|volume=14|issue=1|date=3 November 2023|issn=2041-1723|pmid=37923716|pmc=10624901|doi=10.1038/s41467-023-42807-0|article-number=7049}}</ref> |{{nts|10000|prefix=over }}<ref name="Tikhonenkov-2020">{{cite journal|last1=Tikhonenkov|first1=Denis V.|last2=Strassert|first2=Jürgen F.H.|last3=Janouškovec|first3=Jan|last4=Mylnikov|first4=Alexander P.|last5=Aleoshin|first5=Vladimir V.|last6=Burki|first6=Fabien|last7=Keeling|first7=Patrick J.|title=Predatory colponemids are the sister group to all other alveolates|journal=Molecular Phylogenetics and Evolution|volume=149|date=2020|doi=10.1016/j.ympev.2020.106839|article-number=106839|url=https://linkinghub.elsevier.com/retrieve/pii/S1055790320301111|access-date=19 February 2026}}</ref> |- |Rhizaria |frameless|90x90px |Amoebae with filose or reticulose pseudopodia.<ref name="Cavalier-Smith-2018">{{cite journal|last1=Cavalier-Smith|first1=Thomas|last2=Chao|first2=Ema E.|last3=Lewis|first3=Rhodri|title=Multigene phylogeny and cell evolution of chromist infrakingdom Rhizaria: contrasting cell organisation of sister phyla Cercozoa and Retaria|journal=Protoplasma|date=2018|volume=255|issue=5 |pages=1517–1574|doi=10.1007/s00709-018-1241-1|pmid=29666938|pmc=6133090|bibcode=2018Prpls.255.1517C }}</ref> The most species-rich group is Retaria, home to conspicuous marine amoebae encased in hard skeletons (radiolarians) or multichambered tests (foraminifers, ''pictured''). Secondly is Cercozoa, with an extreme diversity of morphologies: small flagellates, amoeboflagellates, aggregative slime molds,<ref>{{cite journal|last1=Brown|first1=Matthew W.|last2=Kolisko|first2=Martin|last3=Silberman|first3=Jeffrey D.|last4=Rogers |first4=Andrew J.|title=Aggregative multicellularity evolved independently in the eukaryotic supergroup Rhizaria|journal=Current Biology|date=2012 |volume=22|issue=12 |pages=1123–1127|doi=10.1016/j.cub.2012.04.021|pmid=22608512|bibcode=2012CBio...22.1123B }}</ref> testate amoebae, heliozoa, and massive radiolarian-like cells (phaeodarians);<ref>{{cite journal|last1=Lax|first1=Gordon|last2=Cooney|first2=Elizabeth C.|last3=Zlatogursky|first3=Vasily|last4=Mtawali|first4=Mahara|last5=Okamoto|first5=Noriko|last6=Jacko-Reynolds|first6=Victoria K. L.|last7=Bjornson|first7=Saelin|last8=Holt|first8=Corey|last9=Hurdeal|first9=Vedprakash G.|last10=Giannotti|first10=Daniele|last11=Keeling|first11=Patrick J.|display-authors=5|title=Phylogenomic tree of Cercozoa based on single-cell transcriptomes from 100 uncultured cells|journal=BMC Biology|volume=24|issue=1|date=30 January 2026|issn=1741-7007|pmid=41618358|pmc=12930898|doi=10.1186/s12915-026-02536-4|doi-access=free|article-number=55}}</ref> some are capable of photosynthesis (e.g., chlorarachniophytes). Lastly, Endomyxa contains both free-living predatory amoebae (e.g., vampyrellids) and obligate parasites of animals, plants, and algae (e.g., phytomyxeans and ascetosporeans).<ref name="Burki-2014">{{cite journal|last1=Burki|first1=Fabien|last2=Keeling|first2=Patrick J.|title=Rhizaria|journal=Current Biology|volume=24|issue=3|date=2014|doi=10.1016/j.cub.2013.12.025|pages=R103–R107}}</ref><ref name="Adl-2019"/> |{{nts|11000|prefix=over }}<ref name="Adl-2007">{{cite journal|last1=Adl|first1=Sina M.|last2=Leander|first2=Brian S.|last3=Simpson|first3=Alastair G. B.|last4=Archibald|first4=John M.|last5=Anderson|first5=O. Roger.|last6=Bass|first6=David|last7=Bowser|first7=Samuel S.|last8=Brugerolle|first8=Guy|last9=Farmer|first9=Mark A.|last10=Karpov|first10=Sergey|last11=Kolisko|first11=Martin|last12=Lane|first12=Christopher E.|last13=Lodge|first13=Deborah J.|last14=Mann|first14=David G.|last15=Meisterfeld|first15=Ralf|last16=Mendoza|first16=Leonel|last17=Moestrup|first17=Øjvind|last18=Mozley-Standridge|first18=Sharon E.|last19=Smirnov|first19=Alexey V.|last20=Spiegel|first20=Frederick|title=Diversity, Nomenclature, and Taxonomy of Protists|journal=Systematic Biology|volume=56|issue=4|date=1 August 2007|issn=1076-836X|doi=10.1080/10635150701494127 |pages=684–689|pmid=17661235 |url=https://academic.oup.com/sysbio/article-pdf/56/4/684/24203799/56-4-684.pdf|access-date=15 July 2025|display-authors=5}}</ref> |- | colspan="2" |Telonemia |frameless|90x90px |Free-living flagellates with a unique cytoskeleton and a combination of cell structures. Present in all marine and freshwater environments feeding on bacteria.<ref name="Tikhonenkov-2022">{{cite journal | last1=Tikhonenkov | first1=Denis V. | last2=Jamy | first2=Mahwash | last3=Borodina | first3=Anastasia S. | last4=Belyaev | first4=Artem O. | last5=Zagumyonnyi | first5=Dmitry G. | last6=Prokina | first6=Kristina I. | last7=Mylnikov | first7=Alexander P. | last8=Burki | first8=Fabien | last9=Karpov | first9=Sergey A. | title=On the origin of TSAR: morphology, diversity and phylogeny of Telonemia | journal=Open Biology | publisher=The Royal Society | volume=12 | issue=3 | year=2022 | issn=2046-2441 | doi=10.1098/rsob.210325| pmid=35291881 | pmc=8924772 | doi-access=free }}</ref> |{{nts|10}}<ref name="Zlatogursky-2025">{{cite journal|last1=Zlatogursky|first1=Vasily|last2=Boscaro|first2=Vittorio|last3=Lax|first3=Gordon|last4=Wanntorp|first4=Matias|last5=Pohl|first5=Nina|last6=Burki|first6=Fabien|last7=Keeling|first7=Patrick J.|title=Phylogenetic position and mitochondrial genome evolution of "orphan" eukaryotic lineages|journal=iScience|volume=28|issue=8|date=15 August 2025|pmid=40948565|pmc=12432456|doi=10.1016/j.isci.2025.113184|article-number=113184|doi-access=free}}</ref><ref name="Mostazo-Zapata-2025">{{cite journal|last1=Mostazo‐Zapata|first1=Helena|last2=Gàlvez‐Morante|first2=Alex|last3=Berney|first3=Cédric|last4=Maya‐Figuerola|first4=Xènia|last5=Sigona|first5=Cristiana|last6=López‐Escardó|first6=David|last7=Sà|first7=Elisabet L.|last8=Vaqué|first8=Dolors|last9=Richter|first9=Daniel J.|title=Description of a New Telonemia Genus and Species With Novel Observations Providing Insights Into Its Hidden Diversity|journal=Journal of Eukaryotic Microbiology|volume=72|issue=6|date=2025|issn=1066-5234|pmid=41131848|pmc=12550356|doi=10.1111/jeu.70050|doi-access=free|article-number=e70050}}</ref> |- | colspan="2" |Haptista |frameless|90x90px |Two groups of different free-living single-celled protists: centrohelids—predatory heliozoan amoebae, widespread in aquatic and soil environments<ref>{{cite journal|last1=Zagumyonnyi|first1=Dmitry G.|last2=Radaykina|first2=Liudmila V.|last3=Tikhonenkov|first3=Denis V.|title=''Triangulopteris lacunata'' gen. et sp. nov. (Centroplasthelida), a New Centrohelid Heliozoan from Soil|journal=Diversity|volume=13|issue=12|date=11 December 2021|issn=1424-2818|doi=10.3390/d13120658|doi-access=free|article-number=658}}</ref>—and haptophytes—coccoid or flagellated photosynthetic algae, mostly marine (e.g., coccolithophores, ''pictured'').<ref>{{cite book|first1=Wenche|last1=Eikrem|first2=Linda K.|last2=Medlin|first3=Jorijntje|last3=Henderiks|first4=Sebastian|last4=Rokitta|first5=Björn|last5=Rost|first6=Ian|last6=Probert|first7=Jahn|last7=Throndsen|first8=Bente|last8=Edvardsen|chapter=Haptophyta|volume=2|pages=893–953|doi=10.1007/978-3-319-28149-0_38|chapter-url=https://tuaulavirtual.educatic.unam.mx/pluginfile.php/1747249/mod_resource/content/1/Eikrem2017_ReferenceWorkEntry_Haptophyta.pdf|access-date=9 June 2025|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer International Publishing|location=Cham|url=https://ficoherb.fciencias.unam.mx/Recursos/Bibl/Handbookoftheprotists.pdf|isbn=978-3-319-28149-0|lccn=2017945328|date=2017|display-authors=5}}</ref> Both can produce an outer coat of complex mineralized scales.<ref name="Adl-2019"/> |{{nts|600|prefix=over }} |- | colspan="2" |Pancryptista |frameless|94x94px |Free-living flagellates, except one species of heliozoan amoebae, ''Microheliella maris''.<ref>{{Cite journal |last=Yazaki |first=Euki |last2=Yabuki |first2=Akinori |last3=Imaizumi |first3=Ayaka |last4=Kume |first4=Keitaro |last5=Hashimoto |first5=Tetsuo |last6=Inagaki |first6=Yuji |date=2022-04-13 |title=The closest lineage of Archaeplastida is revealed by phylogenomics analyses that include Microheliella maris |url=https://royalsocietypublishing.org/doi/10.1098/rsob.210376 |journal=Open Biology |volume=12 |issue=4 |article-number=210376 |doi=10.1098/rsob.210376 |pmc=9006020 |pmid=35414259 |doi-access=free}}</ref> Almost all of the flagellates are distinguished by specialized ribbon-shaped extrusomes known as ejectisomes. Many are photosynthetic, known as cryptomonads (''pictured''), while the rest are phagotrophs, consumers of bacteria. Present in aquatic environments worldwide.<ref>{{cite book|first1=Kerstin|last1=Hoef-Emden|first2=John M.|last2=Archibald|chapter=Cryptophyta (Cryptomonads)|volume=2|pages=851–891|doi=10.1007/978-3-319-28149-0_35|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer International Publishing|location=Cham|url=https://ficoherb.fciencias.unam.mx/Recursos/Bibl/Handbookoftheprotists.pdf|isbn=978-3-319-28149-0|lccn=2017945328|date=2017}}</ref><ref name="Adl-2019"/> |{{nts|100|prefix=over }} |- | colspan="2" |Archaeplastida* |frameless|90x90px |Algae with chloroplasts derived from primary endosymbiosis with a cyanobacterium. Found in all environments. Almost entirely photosynthetic, with the exception of two small groups of phagotrophic flagellates, rhodelphids<ref name="Prokina-2023">{{cite journal|first1=Kristina I.|last1=Prokina|first2=Denis V.|last2=Tikhonenkov|first3=Purificación|last3=López-García|first4=David|last4=Moreira|date=2023|title=Morphological and molecular characterization of a new member of the phylum Rhodelphidia|journal=Journal of Eukaryotic Microbiology|volume=71 |issue=2 |article-number=e12995|doi=10.1111/jeu.12995|pmid=37548159 |doi-access=free}}</ref> and picozoans.<ref name="Schön-2021">{{cite journal|first1=Max E.|last1=Schön|first2=Vasily V.|last2=Zlatogursky|first3=Rohan P.|last3=Singh|first4=Camille|last4=Poirier|first5=Susanne|last5=Wilken|first6=Varsha|last6=Mathur|first7=Jürgen F. H.|last7=Strassert|first8=Jarone|last8=Pinhassi|first9=Alexandra|last9=Z. Worden|first10=Patrick J.|last10=Keeling|first11=Thijs J. G.|last11=Ettema|first12=Jeremy G.|last12=Wideman|first13=Fabien|last13=Burki|title=Single cell genomics reveals plastid-lacking Picozoa are close relatives of red algae|journal=Nature Communications|doi=10.1038/s41467-021-26918-0|date=2021|volume=12|issue=1 |page=6651|pmid=34789758 |pmc=8599508 |bibcode=2021NatCo..12.6651S }}</ref> The two major groups, red algae and green algae (''pictured''), exhibit diverse morphologies, ranging from single cells—coccoid, palmelloid, sarcinoid, flagellated—to colonies, simple filaments, and macroscopic thalli with varying degrees of complexity (e.g., coralline algae, sea lettuce, stoneworts). Also included are glaucophytes, rare blue-green algae found in surface waters.<ref>{{cite book|first1=Dana C.|last1=Price|first2=Jürgen M.|last2=Steiner|first3=Hwan Su|last3=Yoon|first4=Debashish|last4=Bhattacharya|first5=Wolfgang|last5=Löffelhardt|chapter=Glaucophyta|pages=23–87|chapter-url=https://tuaulavirtual.educatic.unam.mx/pluginfile.php/1747217/mod_resource/content/1/Price2016_ReferenceWorkEntry_Glaucophyta.pdf|access-date=9 June 2025|doi=10.1007/978-3-319-28149-0_42|volume=1|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer International Publishing|location=Cham|url=https://ficoherb.fciencias.unam.mx/Recursos/Bibl/Handbookoftheprotists.pdf|isbn=978-3-319-28149-0|lccn=2017945328|date=2017}}</ref> |{{nts|20500|prefix=over }}<ref name="Bowles-2023">{{cite journal|last1=Bowles|first1=Alexander M.C.|last2=Williamson|first2=Christopher J.|last3=Williams|first3=Tom A.|last4=Lenton|first4=Timothy M.|last5=Donoghue|first5=Philip C.J.|title=The origin and early evolution of plants|journal=Trends in Plant Science|volume=28|issue=3|date=2023|doi=10.1016/j.tplants.2022.09.009|pages=312–329|doi-access=free}}</ref>* |- | colspan="2" |Disparia |frameless|90x90px |Three lineages of free-living predatory flagellates with unique cytoskeletons. These are: Hemimastigophora, with two rows of flagella, present in soils and aquatic sediments;<ref name="Lax-2018">{{Cite journal |last1=Lax |first1=Gordon |last2=Eglit |first2=Yana |last3=Eme |first3=Laura |last4=Bertrand |first4=Erin M. |last5=Roger |first5=Andrew J. |last6=Simpson |first6=Alastair G. B. |date=2018-11-14 |df=dmy-all |title=Hemimastigophora is a novel supra-kingdom-level lineage of eukaryotes |journal=Nature |volume=564 |issue=7736 |pages=410–414 |language=En |doi=10.1038/s41586-018-0708-8 |pmid=30429611 |issn=0028-0836 |bibcode=2018Natur.564..410L|s2cid=205570993 }}</ref> Provora, fast-swimming predators of other protists through a strong feeding apparatus resembling jaws, found in low abundance in marine environments globally;<ref>{{cite journal|last1=Tikhonenkov|first1=Denis V.|last2=Mikhailov|first2=Kirill V.|last3=Gawryluk|first3=Ryan M. R.|last4=Belyaev|first4=Artem O.|last5=Mathur|first5=Varsha|last6=Karpov|first6=Sergey A.|last7=Zagumyonnyi|first7=Dmitry G.|last8=Borodina|first8=Anastasia S.|last9=Prokina|first9=Kristina I.|last10=Mylnikov|first10=Alexander P.|last11=Aleoshin|first11=Vladimir V.|last12=Keeling|first12=Patrick J.|display-authors=5|title=Microbial predators form a new supergroup of eukaryotes|journal=Nature|volume=612|issue=7941|date=22 December 2022|doi=10.1038/s41586-022-05511-5|pages=714–719|pmid=36477531}}</ref><ref name="Belyaev-2024">{{cite journal|title=The nature of 'jaws': a new predatory representative of Provora and the ultrastructure of nibbling protists|first1=Artem O.|last1=Belyaev|first2=Sergey A.|last2=Karpov|first3=Patrick J.|last3=Keeling|first4=Denis V.|last4=Tikhonenkov|journal=Open Biology|date=18 December 2024|volume=14|issue=12|doi=10.1098/rsob.240158|doi-access=free|article-number=240158|pmid=39689855 |pmc=11651884 }}</ref> and Caelestes (''pictured''), rare inhabitants of the marine benthos whose cells protrude arms or stalks used for movement or prey capture.<ref name="Valt-2025">{{Cite journal |last1=Valt |first1=Marek |last2=Pánek |first2=Tomáš |last3=Mirzoyan |first3=Seda |last4=Tice |first4=Alexander K. |last5=Jones |first5=Robert E. |last6=Dohnálek |first6=Vít |last7=Doležal |first7=Pavel |last8=Mikšátko |first8=Jiří |last9=Rotterová |first9=Johana |last10=Hrubá |first10=Pavla |last11=Brown |first11=Matthew W. |last12=Čepička |first12=Ivan |display-authors=5 |date=19 November 2025 |title=Rare microbial relict sheds light on an ancient eukaryotic supergroup |journal=Nature |language=en |doi=10.1038/s41586-025-09750-0 |issn=0028-0836 |url = https://www.researchsquare.com/article/rs-5245440/v1.pdf |access-date=25 November 2025}}</ref> |{{nts|20}} |- | rowspan="4" |'''Amorph.''' | colspan="2" |Amoebozoa |frameless|90x90px |Amoebae of diverse morphologies, with lobose or filose pseudopodia, and sometimes with flagella. Most are free-living phagotrophs found across terrestrial and aquatic environments, such as the archetypal genus ''Amoeba'' itself,<ref name="Smirnov-2005">{{cite journal|first1=Alexey|last1=Smirnov|first2=Elena|last2=Nassonova|first3=Cédric|last3=Berney|first4=José|last4=Fahrni|first5=Ignacio|last5=Bolivar|first6=Jan|last6=Pawlowski|title=Molecular Phylogeny and Classification of the Lobose Amoebae|journal=Protist|date=2005|volume=156|issue=2|pages=129–142|doi=10.1016/j.protis.2005.06.002|pmid=16171181}}</ref><ref>{{cite journal|first1=Seungho|last1=Kang|first2=Alexander K |last2=Tice|first3=Frederick W |last3=Spiegel|first4=Jeffrey D |last4=Silberman|first5=Tomáš |last5=Pánek|first6=Ivan |last6=Čepička|first7=Martin |last7=Kostka|first8=Anush |last8=Kosakyan|first9=Daniel M C |last9=Alcântara|first10=Andrew J |last10=Roger|first11=Lora L |last11=Shadwick|first12=Alexey |last12=Smirnov|first13=Alexander |last13=Kudryavtsev|first14=Daniel J G |last14=Lahr|first15=Matthew W |last15=Brown|display-authors=5|title=Between a Pod and a Hard Test: The Deep Evolution of Amoebae|journal=Molecular Biology and Evolution|volume=34|issue=9|date=September 2017|pages=2258–2270|pmid=28505375|pmc=5850466|doi=10.1093/molbev/msx162}}</ref><ref name="Tekle-2022">{{cite journal|first1=Yonas I.|last1=Tekle|first2=Fang|last2=Wang|first3=Fiona C.|last3=Wood|first4=O. Roger|last4=Anderson|first5=Alexey|last5=Smirnov|title=New insights on the evolutionary relationships between the major lineages of Amoebozoa|journal=Scientific Reports|date=1 July 2022|volume=12|page=11173|doi=10.1038/s41598-022-15372-7|issue=1|pmc=9249873|pmid=35778543|bibcode=2022NatSR..1211173T }}</ref> or the testate amoebae Arcellinida, one of the most conspicuous groups of protists.<ref name="González-Miguéns-2022">{{cite journal|first1=Rubén |last1=González-Miguéns|first2=Milcho |last2=Todorov|first3=Quentin |last3=Blandenier|first4=Clément |last4=Duckert|first5=Alfredo L. |last5=Porfirio-Sousa|first6=Giulia M. |last6=Ribeiro|first7=Diana |last7=Ramos|first8=Daniel J.G.|last8=Lahr|first9=David|last9=Buckley|first10=Enrique|last10=Lara|title=Deconstructing ''Difflugia'': The tangled evolution of lobose testate amoebae shells (Amoebozoa: Arcellinida) illustrates the importance of convergent evolution in protist phylogeny|journal=Molecular Phylogenetics and Evolution|volume=175|date=2022|article-number=107557|doi=10.1016/j.ympev.2022.107557|pmid=35777650|doi-access=free|bibcode=2022MolPE.17507557G |hdl=10261/281619|hdl-access=free}}</ref> Numerous groups have independently evolved fungus-like fruiting bodies,<ref name="Brown-2010a">{{cite journal|first1=Matthew W.|last1=Brown|first2=Jeffrey D.|last2=Silberman|first3=Frederick W.|last3=Spiegel|title="Slime Molds" among the Tubulinea (Amoebozoa): Molecular Systematics and Taxonomy of ''Copromyxa''|journal=Protist|volume=162|issue=2|date=26 November 2010|pages=277–287|doi=10.1016/j.protis.2010.09.003|pmid=21112814}}</ref> such as myxomycetes (''pictured'').<ref name="Lamza-2023">{{cite journal|last=Lamża|first=Łukasz|title=Diversity of 'simple' multicellular eukaryotes: 45 independent cases and six types of multicellularity|journal=Biological Reviews|volume=98|issue=6|date=2023|issn=1464-7931|doi=10.1111/brv.13001|pages=2188–2209}}</ref> Some of the free-living amoebae are important vectors of pathogenic bacteria or are pathogenic themselves (e.g., ''Acanthamoeba'').<ref>{{cite journal|last1=Martinez|first1=Augusto Julio|last2=Visvesvara|first2=Govinda S.|title=Free‐living, Amphizoic and Opportunistic Amebas|journal=Brain Pathology|volume=7|issue=1|date=1997|issn=1015-6305|pmid=9034567|pmc=8098488|doi=10.1111/j.1750-3639.1997.tb01076.x|pages=583–598}}</ref> Others are anaerobic intestinal symbionts (e.g., ''Entamoeba'').<ref name="Adl-2019"/> |{{nts|2400|prefix=over }}<ref name="Pawlowski-2012">{{cite journal|last1=Pawlowski|first1=Jan|last2=Audic|first2=Stéphane|last3=Adl|first3=Sina|last4=Bass|first4=David|last5=Belbahri|first5=Lassaâd|last6=Berney|first6=Cédric|last7=Bowser|first7=Samuel S.|last8=Cepicka|first8=Ivan|last9=Decelle|first9=Johan|last10=Dunthorn|first10=Micah|last11=Fiore-Donno|first11=Anna Maria|last12=Gile|first12=Gillian H.|last13=Holzmann|first13=Maria|last14=Jahn|first14=Regine|last15=Jirků|first15=Miloslav|last16=Keeling|first16=Patrick J.|last17=Kostka|first17=Martin|last18=Kudryavtsev|first18=Alexander|last19=Lara|first19=Enrique|last20=Lukeš|first20=Julius|last21=Mann|first21=David G.|last22=Mitchell|first22=Edward A. D.|last23=Nitsche|first23=Frank|last24=Romeralo|first24=Maria|last25=Saunders|first25=Gary W.|last26=Simpson|first26=Alastair G. B.|last27=Smirnov|first27=Alexey V.|last28=Spouge|first28=John L.|last29=Stern|first29=Rowena F.|last30=Stoeck|first30=Thorsten|last31=Zimmermann|first31=Jonas|last32=Schindel|first32=David|last33=de Vargas|first33=Colomban|display-authors=5|title=CBOL Protist Working Group: Barcoding Eukaryotic Richness beyond the Animal, Plant, and Fungal Kingdoms|journal=PLoS Biology|volume=10|issue=11|date=6 November 2012|issn=1545-7885|pmid=23139639|pmc=3491025|s2cid=6330045|doi=10.1371/journal.pbio.1001419|doi-access=free|article-number=e1001419}}</ref> |- | colspan="2" |Breviatea |frameless|90x90px |Anaerobic free-living amoeboflagellates with fine pseudopodia and modified mitochondria. Present only in low-oxygen marine and brackish sediments, their growth depends on mutualistic interactions with prokaryotes.<ref>{{cite journal|last1=Aguilera-Campos|first1=Karla Iveth|last2=Boisard|first2=Julie|last3=Törnblom|first3=Viktor|last4=Jerlström-Hultqvist|first4=Jon|last5=Behncké-Serra|first5=Ada|last6=Cotillas|first6=Elena Aramendia|last7=Stairs|first7=Courtney Weir|title=Anaerobic breviate protist survival in microcosms depends on microbiome metabolic function|journal=The ISME Journal|volume=19|issue=1|date=2 January 2025|issn=1751-7362|pmid=40795332|pmc=12453579|doi=10.1093/ismejo/wraf171|article-number=wraf171}}</ref> |{{nts|4}}<ref name="Heiss-2017">{{cite book|last1=Heiss|first1=Aaron A.|first2=Matthew W.|last2=Brown|first3=Alastair G. B.|last3=Simpson|chapter=Apusomonadida|pages=1619–1645|volume=2|doi=10.1007/978-3-319-28149-0_12|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer International Publishing|location=Cham|url=https://ficoherb.fciencias.unam.mx/Recursos/Bibl/Handbookoftheprotists.pdf|isbn=978-3-319-28149-0|lccn=2017945328|date=2017}}</ref> |- | colspan="2" |Apusomonadida |frameless|90x90px |Free-living flagellates distinguished by a proboscis, a sleeve-like structure that envelops one of their two flagella.<ref name="Heiss-2017"/> Found gliding on wet soil and aquatic sediments worldwide.<ref name="Torruella-2022"/> |{{nts|28}}<ref name="Torruella-2022">{{cite journal|last1=Torruella|first1=Guifré|last2=Galindo|first2=Luis Javier|last3=Moreira|first3=David|last4=Ciobanu|first4=Maria|last5=Heiss|first5=Aaron A.|last6=Yubuki|first6=Naoji|last7=Kim|first7=Eunsoo|last8=López‐García|first8=Purificación|display-authors=5|title=Expanding the molecular and morphological diversity of Apusomonadida, a deep‐branching group of gliding bacterivorous protists|journal=Journal of Eukaryotic Microbiology|volume=70|issue=2|date=2023|doi=10.1111/jeu.12956|article-number=e12956|pmid=36453005|hdl=2117/404026|s2cid=253460648|url=https://hal.science/hal-04384861|hdl-access=free}}</ref> |- | colspan="2" |Opisthokonta** |frameless|91x91px |Flagellates distinguished by a single posterior flagellum, many with complex life cycles and varying degrees of multicellularity.<ref name="Lamza-2023"/> Some are entirely amoeboid, with fine pseudopodia (e.g., filastereans and nucleariids, including slime molds),<ref>{{cite journal|last1=Torruella|first1=Guifré|last2=de Mendoza|first2=Alex|last3=Grau-Bové|first3=Xavier|last4=Antó|first4=Meritxell|last5=Chaplin|first5=Mark A.|last6=del Campo|first6=Javier|last7=Eme|first7=Laura|last8=Pérez-Cordón|first8=Gregorio|last9=Whipps|first9=Christopher M.|last10=Nichols|first10=Krista M.|last11=Paley|first11=Richard|last12=Roger|first12=Andrew J.|last13=Sitjà-Bobadilla|first13=Ariadna|last14=Donachie|first14=Stuart|last15=Ruiz-Trillo|first15=Iñaki|display-authors=5|title=Phylogenomics Reveals Convergent Evolution of Lifestyles in Close Relatives of Animals and Fungi|journal=Current Biology|volume=25|issue=18|date=2015|doi=10.1016/j.cub.2015.07.053|pages=2404–2410|pmid=26365255|doi-access=free}}</ref><ref name="Gabaldón-2022">{{cite journal|first1=Toni|last1=Gabaldón|first2=Eckhard|last2=Völcker|first3=Guifré|last3=Torruella|title=On the Biology, Diversity and Evolution of Nucleariid Amoebae (Amorphea, Obazoa, Opisthokonta)|journal=Protist|volume=173|date=2022|article-number=125895|doi=10.1016/j.protis.2022.125895|pmid=35841659|issue=4|hdl=2117/369912|hdl-access=free}}</ref> while others become amoeboid temporarily (e.g., choanoflagellates, ''pictured'').<ref>{{cite journal|last1=Brunet|first1=Thibaut|last2=Albert|first2=Marvin|last3=Roman|first3=William|last4=Coyle|first4=Maxwell C|last5=Spitzer|first5=Danielle C|last6=King|first6=Nicole|title=A flagellate-to-amoeboid switch in the closest living relatives of animals|journal=eLife|volume=10|date=15 January 2021|issn=2050-084X|doi=10.7554/eLife.61037|doi-access=free|article-number=e61037}}</ref> Most species are free-living filter-feeders or predators,<ref>{{cite book|last1=Richter|first1=Daniel J.|last2=Nitsche|first2=Frank|pages=1479–1496|chapter=Choanoflagellatea|doi=10.1007/978-3-319-28149-0_5|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer|isbn=978-3-319-28147-6|date=2017|volume=2}}</ref><ref name="Gabaldón-2022"/><ref>{{cite journal|first1=Elisabeth|last1=Hehenberger|first2=Denis V.|last2=Tikhonenkov|first3=Martin|last3=Kolisko|first4=Javier|last4=del Campo|first5=Anton S.|last5=Esaulov|first6=Alexander P.|last6=Mylnikov|first7=Patrick J.|last7=Keeling|title=Novel Predators Reshape Holozoan Phylogeny and Reveal the Presence of a Two-Component Signaling System in the Ancestor of Animals|journal=Current Biology|volume=27|issue=13|pages=2043–2050|date=10 July 2017|doi=10.1016/j.cub.2017.06.006|pmid=28648822|bibcode=2017CBio...27E2043H }}</ref><ref>{{cite journal | doi=10.1016/J.CUB.2020.08.061 | title=New Lineage of Microbial Predators Adds Complexity to Reconstructing the Evolutionary Origin of Animals | date=16 November 2020 | last1=Tikhonenkov | first1=Denis V. | last2=Mikhailov | first2=Kirill V. | last3=Hehenberger | first3=Elisabeth | last4=Karpov | first4=Sergei A. | last5=Prokina | first5=Kristina I. | last6=Esaulov | first6=Anton S. | last7=Belyakova | first7=Olga I. | last8=Mazei | first8=Yuri A. | last9=Mylnikov | first9=Alexander P. | last10=Aleoshin | first10=Vladimir V. | last11=Keeling | first11=Patrick J. | journal=Current Biology | volume=30 | issue=22 | pages=4500–4509.e5 | pmid=32976804 | bibcode=2020CBio...30E4500T | doi-access=free | display-authors = 5}}</ref> but some lineages (e.g., ichthyosporids) evolved into osmotrophic parasites of animals.<ref>{{cite journal|first1=Sally L.|last1=Glockling|first2=Wyth L.|last2=Marshall|first3=Frank H.|last3=Gleason|title=Phylogenetic interpretations and ecological potentials of the Mesomycetozoea (Ichthyosporea)|journal=Fungal Ecology|volume=6|issue=4|date=25 April 2013|pages=237–247|doi=10.1016/j.funeco.2013.03.005}}</ref><ref>{{cite journal|first1=Ander|last1=Urrutia|first2=Konstantina|last2=Mitsi|first3=Rachel|last3=Foster|first4=Stuart|last4=Ross|first5=Martin|last5=Carr|first6=Georgia M.|last6=Ward|first7=Ronny|last7=van Aerle|first8=Ionan|last8=Marigomez|first9=Michelle M.|last9=Leger|first10=Iñaqui|last10=Ruiz-Trillo|first11=Stephen W.|last11=Feist|first12=David|last12=Bass|display-authors=5|title=''Txikispora philomaios'' n. sp., n. g., a micro-eukaryotic pathogen of amphipods, reveals parasitism and hidden diversity in Class Filasterea|journal=Journal of Eukaryotic Microbiology|date=2022|volume=69|article-number=e12875|doi=10.1111/jeu.12875|issue=2|pmid=34726818|url=https://pure.hud.ac.uk/en/publications/1dd4e1c5-f638-48cf-ab99-95c9c90636a1 }}</ref> |{{nts|300|prefix=approx. }}<ref name="Adl-2007"/>** |- | rowspan="3" |'''Excavat.''' | colspan="2" |Discoba |frameless|90x90px |Flagellates with very different lifestyles, present in aquatic and terrestrial environments, ranging from aerobes to anaerobes. The most diverse group, Euglenozoa, includes free-living osmotrophs, phagotrophs, phototrophs (euglenophytes, ''pictured''), and pathogens (kinetoplastids).<ref name="Madigan-2019"/><ref name="Kostygov-2021">{{cite journal|last1=Kostygov|first1=Alexei Y.|first2=Anna|last2=Karnkowska|first3=Jan|last3=Votýpka|first4=Daria|last4=Tashyreva|first5=Kacper|last5=Maciszewski|first6=Vyacheslav|last6=Yurchenko|first7=Julius|last7=Lukeš|date=2021|title=Euglenozoa: taxonomy, diversity and ecology, symbioses and viruses|volume=11|article-number=200407|journal= Open Biology|issue=3 |doi=10.1098/rsob.200407 |doi-access=free|pmid=33715388|pmc=8061765}}</ref> The less diverse Heterolobosea are primarily amoeboflagellates, and include some slime molds (acrasids) and well-known opportunistic parasites (e.g., ''Naegleria fowleri'').<ref name="Pánek-2017">{{cite book|first1=Tomáš|last1=Pánek|first2=Alastair G. B.|last2=Simpson|first3=Matthew W.|last3=Brown|first4=Betsey Dexter|last4=Dyer|chapter=Heterolobosea|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer|isbn=978-3-319-28147-6|date=2017|volume=2|pages=1005–1046|doi=10.1007/978-3-319-28149-0_10}}</ref> The smallest group, Jakobida, consume bacteria by suspension feeding.<ref name="Simpson-2017b">{{cite book|chapter=Jakobida|doi=10.1007/978-3-319-28149-0_6|last1=Simpson|first1=Alastair G. B.|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer|isbn=978-3-319-28147-6|date=2017|pages=973–1004|volume=2}}</ref> |{{nts|2200|prefix=over }}<ref name="Guiry-2024">{{cite journal |first1=Michael D. |last1=Guiry |author-link=Michael D. Guiry |title=How many species of algae are there? A reprise. Four kingdoms, 14 phyla, 63 classes and still growing |journal=Journal of Phycology |date=2024 |volume=60 |issue=2 |pages=214–228 |doi=10.1111/jpy.13431|pmid=38245909|bibcode=2024JPcgy..60..214G |doi-access=free }}</ref><ref>{{cite journal|last1=Pánek|first1=Tomáš|last2=Tice|first2=Alexander K.|last3=Corre|first3=Pia|last4=Hrubá|first4=Pavla|last5=Žihala|first5=David|last6=Kamikawa|first6=Ryoma|last7=Yazaki|first7=Euki|last8=Shiratori|first8=Takashi|last9=Kume|first9=Keitaro|last10=Hashimoto|first10=Tetsuo|last11=Ishida|first11=Ken-ichiro|last12=Hradilová|first12=Miluše|last13=Silberman|first13=Jeffrey D.|last14=Roger|first14=Andrew|last15=Inagaki|first15=Yuji|last16=Eliáš|first16=Marek|last17=Brown|first17=Matthew W.|last18=Čepička|first18=Ivan|title=An expanded phylogenomic analysis of Heterolobosea reveals the deep relationships, non-canonical genetic codes, and cryptic flagellate stages in the group|journal=Molecular Phylogenetics and Evolution|volume=204|date=16 January 2025|doi=10.1016/j.ympev.2025.108289|article-number=108289|pmid=39826589|url=https://linkinghub.elsevier.com/retrieve/pii/S1055-7903(25)00006-5|url-access=subscription|display-authors=5}}</ref> |- | colspan="2" |Metamonada |frameless|90x90px |Anaerobic or microaerophilic flagellates, amoebae, or amoeboflagellates,<ref name="Boscaro-2024"/> with reduced or completely lost<ref>{{cite journal|last1=Karnkowska|first1=Anna|last2=Vacek|first2=Vojtěch|last3=Zubáčová|first3=Zuzana|last4=Treitli|first4=Sebastian C.|last5=Petrželková|first5=Romana|last6=Eme|first6=Laura|last7=Novák|first7=Lukáš|last8=Žárský|first8=Vojtěch|last9=Barlow|first9=Lael D.|last10=Herman|first10=Emily K.|last11=Soukal|first11=Petr|last12=Hroudová|first12=Miluše|last13=Doležal|first13=Pavel|last14=Stairs|first14=Courtney W.|last15=Roger|first15=Andrew J.|last16=Eliáš|first16=Marek|last17=Dacks|first17=Joel B.|last18=Vlček|first18=Čestmír|last19=Hampl|first19=Vladimír|display-authors=5|title=A Eukaryote without a Mitochondrial Organelle|journal=Current Biology|volume=26|issue=10|date=2016|doi=10.1016/j.cub.2016.03.053|pages=1274–1284}}</ref> mitochondria. A few are free-living, found in aquatic hypoxic sediments, but most species are obligate parasites (e.g., ''Giardia'', ''pictured'') or commensals in animal intestines (e.g., parabasalids). Many have a high number of flagella.<ref>{{cite journal|last1=Yubuki|first1=Naoji|last2=Huang|first2=Sam S.C.|last3=Leander|first3=Brian S.|title=Comparative Ultrastructure of Fornicate Excavates, Including a Novel Free-living Relative of Diplomonads: ''Aduncisulcus paluster'' gen. et sp. nov.|journal=Protist|volume=167|issue=6|date=2016|doi=10.1016/j.protis.2016.10.001|pages=584–596}}</ref><ref name="Boscaro-2024">{{cite journal|last1=Boscaro|first1=Vittorio|last2=James|first2=Erick R.|last3=Fiorito|first3=Rebecca|last4=del Campo|first4=Javier|last5=Scheffrahn|first5=Rudolf H.|last6=Keeling|first6=Patrick J.|title=Updated classification of the phylum Parabasalia|journal=Journal of Eukaryotic Microbiology|volume=71|issue=4|date=2024|issn=1066-5234|doi=10.1111/jeu.13035|doi-access=free|article-number=e13035}}</ref><ref name="Adl-2019"/> |{{nts|800|prefix=approx. }}<ref name="Adl-2007"/> |- | colspan="2" |Malawimonadida |frameless|90x90px |Free-living bacterivorous flagellates that feed by suspension feeding, present in marine or fresh waters.<ref name="Heiss-2021">{{cite journal|last1=Heiss|first1=Aaron A.|last2=Warring|first2=Sally D.|last3=Lukacs|first3=Kaleigh|last4=Favate|first4=John|last5=Yang|first5=Ashley|last6=Gyaltshen|first6=Yangtsho|last7=Filardi|first7=Christopher|last8=Simpson|first8=Alastair G.B.|last9=Kim|first9=Eunsoo|display-authors=5|title=Description of ''Imasa heleensis'' , gen. nov., sp. nov. (Imasidae, fam. nov.), a Deep‐Branching Marine Malawimonad and Possible Key Taxon in Understanding Early Eukaryotic Evolution|journal=Journal of Eukaryotic Microbiology|volume=68|issue=2|date=2021|doi=10.1111/jeu.12837|article-number=e12837}}</ref> |{{nts|3}}<ref name="Heiss-2021"/> |- | rowspan="2" |Other | colspan="2" |Ancyromonadida |{{center|frameless|90x90px}} |Tiny free-living aquatic flagellates composed of flattened cells with an inflexible pellicle and a lateral rostrum with extrusomes. Found in most aquatic habitats.<ref name="Yubuki-2023"/> |{{nts|20|prefix=over }}<ref name="Heiss-2017"/><ref name="Yubuki-2023">{{cite journal |last1=Yubuki |first1=Naoji |last2=Torruella |first2=Guifré |last3=Galindo |first3=Luis Javier |last4=Heiss |first4=Aaron A. |last5=Ciobanu |first5=Maria Cristina |last6=Shiratori |first6=Takashi |last7=Ishida |first7=Ken-ichiro |last8=Blaz |first8=Jazmin |last9=Kim |first9=Eunsoo |last10=Moreira |first10=David |last11=López-García |first11=Purificación |last12=Eme |first12=Laura |title=Molecular and morphological characterization of four new ancyromonad genera and proposal for an updated taxonomy of the Ancyromonadida |journal=Journal of Eukaryotic Microbiology |date=22 August 2023|volume=70 |issue=6|article-number=e12997 |doi=10.1111/jeu.12997 |issn=1550-7408|hdl=2117/404022 |hdl-access=free|doi-access=free|pmid=37606230|display-authors=5}}</ref> |- | colspan="2" |CRuMs |frameless|90x90px |Free-living flagellates and filose amoebae with a pellicle underneath the cell membrane. Almost all flagellated members can produce filose pseudopodia. Found in aquatic environments.<ref name="Yazaki-2025"/> |{{nts|14}}<ref name="Yazaki-2025"/> |- | colspan="6" |*Excluding plants. **Excluding animals and fungi. |}

There are also many protists of uncertain position because their DNA has not been sequenced, and consequently their phylogenetic affinities are unknown.<ref name="Adl-2019"/>

=== Predicted diversity === thumb|upright=1.2|Difference between catalogued species (A) and genetic diversity (B) of eukaryotes. In the legend, "Archaeplastida" and "Opisthokonta" exclude Streptophyta, Metazoa and Fungi.<ref name="Pawlowski-2012" />

The total species diversity of protists is severely underestimated by traditional methods that differentiate species based on morphological characteristics. The number of catalogued protist species is very low (ranging from 26,000<ref>{{cite journal|last1=Mora|first1=Camilo|last2=Tittensor|first2=Derek P.|last3=Adl|first3=Sina|last4=Simpson|first4=Alastair G. B.|last5=Worm|first5=Boris|date=2011|title=How Many Species Are There on Earth and in the Ocean?|journal=PLOS Biology|volume=9|issue=8|article-number=e1001127|doi=10.1371/journal.pbio.1001127|pmid=21886479 |pmc=3160336 |doi-access=free }}</ref> to over 76,000){{efn|A 2007 report on protist diversity included a table listing the described number of species for protist and fungal groups. The total sum of the listed species, excluding fungi, is 76,144.<ref name="Adl-2007"/>}} in comparison to the diversity of land plants, animals and fungi, which are historically and biologically well-known and studied. The predicted number of species also varies greatly, ranging from 140,000 to 1,600,000, and in several groups the number of predicted species is arbitrarily doubled. Most of these predictions are highly subjective. Molecular techniques such as environmental DNA barcoding have revealed a vast diversity of undescribed protists that accounts for the majority of eukaryotic sequences or operational taxonomic units (OTUs), dwarfing those from land plants, animals and fungi.<ref name="Pawlowski-2012"/> As such, it is considered that protists dominate eukaryotic diversity.<ref name="Burki-2021">{{cite journal|first1=Fabien|last1=Burki|first2=Miguel M.|last2=Sandin|first3=Mahwash|last3=Jamy|title=Diversity and ecology of protists revealed by metabarcoding|journal=Current Biology|volume=31|issue=19|date=2021|pages=R1267–R1280|doi=10.1016/j.cub.2021.07.066|pmid=34637739 |s2cid=238588753 |doi-access=free|bibcode=2021CBio...31R1267B }}</ref>

==Biology==

In general, protists have typical eukaryotic cells that follow the same principles of biology described for those cells within the "higher" eukaryotes (animals, fungi and land plants).<ref>{{cite journal | first = Helmut | last = Plattner | year = 2018 | title = Evolutionary cell biology of proteins from protists to humans and plants | url = http://nbn-resolving.de/urn:nbn:de:bsz:352-2-1kezn7g0ruv3m7| journal = J. Eukaryot. Microbiol. | volume = 65 | issue = 2| pages = 255–289 | doi = 10.1111/jeu.12449 | pmid = 28719054 | s2cid = 206055044 }}</ref> However, many have evolved a variety of unique physiological adaptations that do not appear in the remaining eukaryotes,<ref name="Levandowsky-2012">{{cite book|first1=Michael|last1=Levandowsky|chapter=Physiological Adaptations of Protists|title=Cell Physiology Sourcebook: Essentials of Membrane Biophysics|editor-first1=Nicholas|editor-last1=Sperelakis|edition=Fourth|publication-place=Amsterdam; Boston|publisher=Elsevier/AP|date=2012|pages=874–890|isbn=978-0-12-387738-3|doi=10.1016/B978-012656976-6/50153-0}}</ref> and in fact protists encompass almost all of the broad spectrum of biological characteristics expected in eukaryotes.<ref name="Burki-2021"/>

=== Nutrition ===

Protists display a wide variety of food preferences and feeding mechanisms.<ref name="Adl-2019"/><ref name="Wiser-2024">{{cite journal|last1=Wiser|first1=Mark F.|title=Feeding Mechanisms of Pathogenic Protozoa with a Focus on Endocytosis and the Digestive Vacuole|journal=Parasitologia|volume=4|issue=3|pages=222–237|doi=10.3390/parasitologia4030019|doi-access=free|date=1 July 2024}}</ref> According to the nutrient source, they can be divided into autotrophs (or phototrophs,<ref name="Lwoff-1946">{{cite book | first = A. | last = Lwoff | first2 = C. B. | last2 = van Niel | first3 = F. J. | last3 = Ryan | first4 = E. L. | last4 = Tatum | chapter = Appendix: Nomenclature of nutritional types of microorganisms | series = Cold Spring Harbor Symposia on Quantitative Biology | volume = XI | title = Heredity and Variation in Microorganisms | date = 1946 | pages = 302–303 | url = https://symposium.cshlp.org/content/11 | chapter-url = http://symposium.cshlp.org/content/11/local/back-matter.pdf }}</ref> producers, traditionally algae), which photosynthesize their own organic molecules, and heterotrophs (consumers, traditionally protozoa), which obtain organic molecules from the environment, either by passive feeding of small particles (i.e., osmotrophs) or by engulfing whole cells or parts of cells of other organisms (phagotrophs).<ref name="Hickman-2017">{{cite book|chapter=Unicellular Eukaryotes: Protozoan Groups|pages=216–245|title=Integrated Principles of Zoology|edition=7th|first1=Cleveland P. Jr.|last1=Hickman|first2=Susan L.|last2=Keen|first3=David J.|last3=Eisenhour|first4=Allan|last4=Larson|first5=Helen|last5=l'Anson|publisher=McGraw Hill|lccn=2016026850|isbn=978-1-259-56231-0|date=2017|location=New York}}</ref>

==== Phagotrophy ====

[[File:A sol mit Paramecium.jpg|thumb|The heliozoan ''Actinophrys sol'' phagocytosing a ''Paramecium'' ciliate]]

Phagotrophic protists feed by phagocytosis, a process unique to eukaryotes<ref name="Leander-2020">{{cite journal |last=Leander |first=Brian S. |date=2020 |title=Predatory protists |journal=Current Biology |volume=30 |issue=10 |pages=R510–R516 |doi=10.1016/j.cub.2020.03.052}}</ref> where food particles or cells are digested into a vacuole, the phagosome.<ref name="Wiser-2024" /> This is the general mode of nutrition for protists, and has resulted in a diverse array of strategies for hunting and digestion.<ref name="Leander-2020" /> Usually, digestion occurs at a specialized mouth-like region of the cell, the cytostome, which may be followed by the cytopharynx,<ref name="Ruppert-2004">{{cite book|chapter=Protozoa|pages=22–57|title=Invertebrate Zoology: A Functional Evolutionary Approach|edition=7th|first1=Edward E.|last1=Ruppert|first2=Richard S.|last2=Fox|first3=Robert D.|last3=Barnes|publisher=Thomson Brooks/Cole|date=2004|isbn=0-03-025982-7}}</ref> a tract supported by microtubules.<ref name="Wiser-2024" /> In amoebae, phagocytosis takes place anywhere on the cell surface.<ref name="Esteban-2020b">{{cite book|chapter=Feeding|pages=33–54|doi=10.1007/978-3-030-59979-9_4|first1=Genoveva F.|last1=Esteban|first2=Tom M.|last2=Fenchel|title=Ecology of Protozoa: The Biology of Free-living Phagotrophic Protists|edition=2nd|isbn=978-3-030-59979-9|publisher=Springer Nature Switzerland AG|location=Cham|date=2020}}</ref> [[File:Vampyrella_lateritia_emptying_algal_cell_2H.png|thumb|''Vampyrella lateritia'' extracting algal cell content with a pseudopodium (arrow)]] According to the method of digestion, protists can be divided into filter, raptorial, or diffusion feeders. Filter feeders accumulate small suspended particles into the cytostome by filtering them through pseudopodia or rigid tentacles, like choanoflagellates, or by generating water currents around the cytostome, like ciliates.<ref name="Adl-2019" /> Raptorial feeders capture whole cells, either grazing on surfaces like bacterial lawns or actively preying on larger cells of other organisms.<ref name="Esteban-2020b" /><ref name="Adl-2019" /> Diffusion feeders, like suctorian ciliates and heliozoans, passively engulf prey that happen to collide with their tentacles or pseudopodia and are immobilized.<ref name="Esteban-2020b" /> Certain protists exhibit a variation of predation known as myzocytosis, where they perforate the prey cell and suck out its contents or ingest them from the inside, leaving behind an empty shell; this is the case for vampyrellids, viridiraptorids, and many alveolates.<ref name="Leander-2020" /><ref name="Adl-2019" />

Different predatory protists have sophisticated structures for capturing prey, such as the ventral groove of excavates, the hood-like extension or 'pallium' of some dinoflagellates, or the expandable oral pocket of ciliates. Many euglenids have a system of rods and vanes that grab and pull in prey cells, similarly to a Chinese finger trap.<ref name="Leander-2020" />

Preys of phagotrophic protists range from prokaryotes (i.e., bacterivores) to other eukaryotes, including single-celled protists, algae,<ref name="Adl-2019" /> fungi,<ref name="Geisen-2016">{{cite journal|first1=Stefan|last1=Geisen|first2=Robert|last2=Koller|first3=Maike|last3=Hünninghaus|first4=Kenneth|last4=Dumack|first5=Tim|last5=Urich|first6=Michael|last6=Bonkowski|title=The soil food web revisited: Diverse and widespread mycophagous soil protists|journal=Soil Biology and Biochemistry|volume=94|date=2016|pages=10–18|doi=10.1016/j.soilbio.2015.11.010|bibcode=2016SBiBi..94...10G }}</ref> nematodes,<ref name="Geisen-2015">{{cite journal|last1=Geisen|first1=Stefan|last2=Rosengarten|first2=Jamila|last3=Koller|first3=Robert|last4=Mulder|first4=Christian|last5=Urich|first5=Tim|last6=Bonkowski|first6=Michael|title=Pack-hunting protists attacking nematodes|journal=Environmental Microbiology|volume=17|issue=11|pages=4538–4546|doi=10.1111/1462-2920.12949|date=16 June 2015|pmid=26079718}}</ref> or tissues of larger animals.<ref name="Esteban-2020b" /> Prey specificity varies, with some groups specialized in eating only one type of organisms,<ref name="Geisen-2016" /> or only a particular strain.<ref name="Karnkowska-2023" /> Traditionally, protists were considered primarily bacterivorous due to biases in cultivation techniques, but most are omnivores.<ref name="Geisen-2016" />

==== Osmotrophy ==== Osmotrophic protists absorb soluble<ref name="Hickman-2017" /> or very small (under 0.5 μm) molecules by diffusion, membrane channels and carriers, or pinocytosis, where nutrients are engulfed into small vacuoles or endosomes.<ref name="Adl-2019" /><ref name="Wiser-2024" /> Some osmotrophs, known as saprotrophs or lysotrophs, perform external digestion by releasing digestive enzymes into the environment and decomposing organic matter<ref name="Adl-2019" /> into simpler molecules that can be absorbed, allowing finer control over substances that enter the cell and minimizing the risk of harmful substances or infection.<ref name="Richards-2013">{{cite journal|last1=Richards|first1=Thomas A.|last2=Talbot|first2=Nicholas J.|title=Horizontal gene transfer in osmotrophs: playing with public goods|journal=Nature Reviews Microbiology|volume=11|issue=10|pages=720–727|date=10 September 2013|doi=10.1038/nrmicro3108|pmid=24018383|hdl=10871/15898|hdl-access=free}}</ref> Probably all eukaryotes are capable of osmotrophy, but some have no alternative of acquiring nutrients. Obligate osmotrophs include the aphagean euglenids, some green algae, the human parasite ''Blastocystis'', some metamonads,<ref name="Adl-2019" /> the parasitic trypanosomatids,<ref>{{cite journal|first1=Jane|last1=Harmer|first2=Vyacheslav|last2=Yurchenko|first3=Anna|last3=Nenarokova|first4=Julius|last4=Lukeš|first5=Michael L.|last5=Ginger|title=Farming, slaving and enslavement: histories of endosymbioses during kinetoplastid evolution|journal=Parasitology|date=13 June 2018|volume=145|issue=10|pages=1311–1323|doi=10.1017/S0031182018000781|pmid=29895336|url=https://pure.hud.ac.uk/ws/files/13639420/Harmer_et_al_2018a_Parasitology_in_press.pdf }}</ref> and the fungus-like oomycetes and hyphochytrids.<ref name="Richards-2013" />

==== Mixotrophy ====

{{multiple image | align = right | total_width = 350 | image1 = Rapaza viridis 2012 Yamaguchi et al fig1e.webp | caption1 = | image2 = Rapaza viridis 2012 Yamaguchi et al fig1f.webp | caption2 = | image3 = Rapaza viridis 2012 Yamaguchi et al fig1g.webp | image4 = Rapaza viridis 2012 Yamaguchi et al fig1h.webp | footer = ''Rapaza viridis'' is a species of obligate specialist mixotrophs: it survives through the predation of ''Tetraselmis'' algae and acquisition of their chloroplasts. It rejects any other prey cells. Even when well fed, it cannot survive without a light source, as it needs to photosynthesize with those chloroplasts.<ref name="Karnkowska-2023">{{cite journal|last1=Karnkowska|first1=Anna|last2=Yubuki|first2=Naoji|last3=Maruyama|first3=Moe|last4=Yamaguchi|first4=Aika|last5=Kashiyama|first5=Yuichiro|last6=Suzaki|first6=Toshinobu|last7=Keeling|first7=Patrick J.|last8=Hampl|first8=Vladimír|last9=Leander|first9=Brian S.|display-authors=5|title=Euglenozoan kleptoplasty illuminates the early evolution of photoendosymbiosis|journal=Proceedings of the National Academy of Sciences of the United States of America|date=21 March 2023|volume=120|issue=12|article-number=e2220100120|doi=10.1073/pnas.2220100120|doi-access=free |pmid=36927158|pmc=10041101|bibcode=2023PNAS..12020100K }}</ref> }}

Most photosynthetic protists are mixotrophs,<ref name="Sanders-2011">{{cite journal|last=Sanders|first=Robert W.|date=2011|title=Alternative Nutritional Strategies in Protists: Symposium Introduction and a Review of Freshwater Protists that Combine Photosynthesis and Heterotrophy|journal=Journal of Eukaryotic Microbiology|volume=58|pages=181–184|issue=3|doi=10.1111/j.1550-7408.2011.00543.x|pmid=21477096 }}</ref> as they combine photosynthesis with phagocytosis.{{efn|The terms "mixotroph" and "mixoplankton" almost exclusively refer to protists that perform photosynthesis and phagocytosis (photo-phagotrophs). Osmotrophy is always present, but not taken into account. As such, "pure" phototrophs (incapable of phagocytosis) and "pure" phagotrophs (incapable of photosynthesis) are technically mixotrophic due to their innate ability for osmotrophy, but are not usually reported in this sense.<ref>{{cite journal|first1=Kevin J.|last1=Flynn|first2=Aditee|last2=Mitra|first3=Konstantinos|last3=Anestis|first4=Anna A.|last4=Anschütz|first5=Albert|last5=Calbet|first6=Guilherme Duarte|last6=Ferreira|first7=Nathalie|last7=Gypens|first8=Per J.|last8=Hansen|first9=Uwe|last9=John|first10=Jon Lapeyra|last10=Martin|first11=Joost S.|last11=Mansour|first12=Maira|last12=Maselli|first13=Nikola|last13=Medić|first14=Andreas|last14=Norlin|first15=Fabrice|last15=Not|first16=Paraskevi|last16=Pitta|first17=Filomena|last17=Romano|first18=Enric|last18=Saiz|first19=Lisa K.|last19=Schneider|first20=Willem|last20=Stolte|first21=Claudia|last21=Traboni|title=Mixotrophic protists and a new paradigm for marine ecology: where does plankton research go now?|journal=Journal of Plankton Research|volume=41|issue=4|date=15 July 2019|pages=375–391|doi=10.1093/plankt/fbz026|doi-access=free|hdl=10261/192145|hdl-access=free}}</ref>}} While some mixotrophs already have chloroplasts (i.e., algae), others acquire chloroplasts by stealing them from their prey, a process known as kleptoplasty. Kleptoplastic protists may be generalists, able to steal chloroplasts from a variety of prey, like some ciliates, or they may be specialists, only capable of obtaining chloroplasts from very specific prey. Specialists may keep the entire prey inside of their cells, as do many foraminifers and radiolarians, or they may only engulf the plastids and discard the rest.<ref name="Mitra-2016">{{cite journal|first1=Aditee|last1=Mitra|first2=Kevin J.|last2=Flynn|first3=Urban|last3=Tillmann|first4=John A.|last4=Raven|first5=David|last5=Caron|first6=Diane K.|last6=Stoecker|first7=Fabrice|last7=Not|first8=Per J.|last8=Hansen|first9=Gustaaf|last9=Hallegraeff|first10=Robert|last10=Sanders|first11=Susanne|last11=Wilken|first12=George|last12=McManus|first13=Mathew|last13=Johnson|first14=Paraskevi|last14=Pitta|first15=Selina|last15=Våge|first16=Terje|last16=Berge|first17=Albert|last17=Calbet|first18=Frede|last18=Thingstad|first19=Hae Jin|last19=Jeong|first20=JoAnn|last20=Burkholder|first21=Patricia M.|last21=Glibert|first22=Edna|last22=Granéli|first23=Veronica|last23=Lundgren|display-authors=5|title=Defining Planktonic Protist Functional Groups on Mechanisms for Energy and Nutrient Acquisition: Incorporation of Diverse Mixotrophic Strategies|journal=Protist|volume=167|issue=2|date=2016|pages=106–120|doi=10.1016/j.protis.2016.01.003|pmid=26927496 |doi-access=free|hdl=10261/131722|hdl-access=free}}</ref><ref name="Faure-2019">{{cite journal|last1=Faure|first1=Emile|last2=Not|first2=Fabrice|last3=Benoiston|first3=Anne-Sophie|last4=Labadie|first4=Karine|last5=Bittner|first5=Lucie|last6=Ayata|first6=Sakina-Dorothée|date=April 2019|title=Mixotrophic protists display contrasted biogeographies in the global ocean|journal=The ISME Journal|volume=13|issue=4|pages=1072–1083|pmc=6461780|pmid=30643201|doi=10.1038/s41396-018-0340-5|bibcode=2019ISMEJ..13.1072F }}</ref>

Among exclusively heterotrophic protists, variation of nutritional modes is also observed. The diplonemids, which inhabit deep waters where photosynthesis is absent, can flexibly switch between osmotrophy and bacterivory depending on the environmental conditions.<ref name="Prokopchuk-2022">{{cite journal|first1=Galina|last1=Prokopchuk|first2=Tomáš|last2=Korytář|first3=Valéria|last3=Juricová|first4=Jovana|last4=Majstorović|first5=Aleš|last5=Horák|first6=Karel|last6=Šimek|first7=Julius|last7=Lukeš|title=Trophic flexibility of marine diplonemids - switching from osmotrophy to bacterivory|journal=The ISME Journal|volume=16|issue=5|date=18 January 2022|pages=1409–1419|doi=10.1038/s41396-022-01192-0|pmid=35042972|pmc=9039065|bibcode=2022ISMEJ..16.1409P }}</ref>

=== Homeostasis ===

[[File:Paramecium contractile vacuoles.jpg|thumb|upright=0.8|Contractile vacuoles in ''Paramecium aurelia'']]

Many freshwater protists need to osmoregulate (i.e., remove excess water volume to adjust the ion concentrations) because non-saline water enters in excess from the environment.<ref name="Patterson-1980">{{cite journal|last1=Patterson|first1=D. J.|date=February 1980|title=Contractile vacuoles and associated structures: their organization and function|volume=55|issue=1|pages=1–46|doi=10.1111/j.1469-185x.1980.tb00686.x|journal=Biological Reviews |bibcode=1980BioRv..55....1P }}</ref><ref name="Ruppert-2004"/> Osmoregulation is done through ion transporters of the cell membrane and through contractile vacuoles, specialized organelles unique to protists that periodically excrete fluid high in potassium and sodium through a cycle of contractions.<ref name="Levandowsky-2012"/> These vacuoles are surrounded by the spongiome, a system of vesicles or tubes that slowly collect fluid from the cytoplasm into the vacuoles, which then contract and discharge the fluid through a pore. The mechanism, location, and structure of this system vary across protists. For example, ciliates contract the vacuoles by actin and microtubule filaments; dinoflagellates contract it through a sheath formed by a flagellar rootlet, known as the pusule. Marine, parasitic, or thick-walled protists lack these vacuoles.<ref name="Patterson-1980" />

Certain protists have acidic organelles known as acidocalcisomes, which store high concentrations of phosphorus, calcium, and enzymes related to their metabolism. Among their proposed functions are osmoregulation and maintenance of pH and calcium homeostasis.<ref>{{cite journal |last1=Moreno |first1=Silvia N. J. |last2=Docampo |first2=Roberto |date=2009 |title=The Role of Acidocalcisomes in Parasitic Protists 1 |journal=Journal of Eukaryotic Microbiology |volume=56 |issue=3 |pages=208–213 |doi=10.1111/j.1550-7408.2009.00404.x |issn=1066-5234 |pmc=2802266 |pmid=19527347}}</ref>

=== Mitochondria and respiration ===

The last eukaryotic common ancestor was aerobic, bearing mitochondria that synthesize ATP through oxidative respiration, which requires oxygen. Most protists are aerobes, but many lineages of free-living and parasitic protists have independently adapted to inhabit anaerobic or microaerophilic (low-oxygen) habitats by modifying their mitochondria into organelles collectively known as mitochondrion-related organelles (MROs). These exist in a continuum from lower to higher degrees of reduction. For example, hydrogenosomes have lost the electron transport chain used in respiration, as well as other features of classical mitochondria (their DNA, the Krebs cycle, etc.), but can still generate ATP anaerobically through the fermentation of pyruvate, releasing hydrogen gas as a byproduct. Mitosomes have lost both the respiratory chain and the production of ATP. One group of protists, the genus ''Monocercomonoides'', has lost its mitochondria entirely.<ref name="Levandowsky-2012" /><ref>{{cite journal |last1=Gawryluk |first1=Ryan M.R. |last2=Kamikawa |first2=Ryoma |last3=Stairs |first3=Courtney W. |last4=Silberman |first4=Jeffrey D. |last5=Brown |first5=Matthew W. |last6=Roger |first6=Andrew J. |date=2016 |title=The Earliest Stages of Mitochondrial Adaptation to Low Oxygen Revealed in a Novel Rhizarian |journal=Current Biology |volume=26 |issue=20 |pages=2729–2738 |doi=10.1016/j.cub.2016.08.025}}</ref><ref>{{cite journal |last1=Gawryluk |first1=Ryan M.R. |last2=Stairs |first2=Courtney W. |date=2021 |title=Diversity of electron transport chains in anaerobic protists |journal=Biochimica et Biophysica Acta (BBA) - Bioenergetics |volume=1862 |issue=1 |doi=10.1016/j.bbabio.2020.148334 |article-number=148334}}</ref> In a similar manner, the oxidative peroxisome evolved into a fermentative glycosome in trypanosomatids.<ref name="Levandowsky-2012" /> thumb|upright=1.5|Diversity of mitochondria (in red) across protists Besides metabolic reduction, the shape, composition, and number of mitochondria varies greatly across protists. Apicomplexans and kinetoplastids have a single large mitochondrion that divides synchronously with the cell, while some amoebae can present hundreds of mitochondria.<ref>{{cite journal |last1=Voleman |first1=Luboš |last2=Doležal |first2=Pavel |date=21 November 2019 |title=Mitochondrial dynamics in parasitic protists |journal=PLOS Pathogens |volume=15 |issue=11 |doi=10.1371/journal.ppat.1008008 |issn=1553-7374 |doi-access=free |article-number=e1008008}}</ref> Mitochondrial genomes (mitogenomes), typically composed of one circular chromosome, can appear as numerous linear chromosomes in many unrelated protists, such as ''Amoebidium'', with hundreds of chromosomes.<ref>{{cite journal |last1=Burger |first1=Gertraud |last2=Gray |first2=Michael W |last3=Franz Lang |first3=B |date=2003 |title=Mitochondrial genomes: anything goes |journal=Trends in Genetics |volume=19 |issue=12 |pages=709–716 |doi=10.1016/j.tig.2003.10.012}}</ref> The large mitogenome of kinetoplastids is condensed into a kinetoplast, which is physically tied to the flagellar apparatus. The smallest known mitogenome belongs to the symbiotic alga ''Chromera velia''.<ref>{{cite journal |last1=Zíková |first1=Alena |last2=Hampl |first2=Vladimír |last3=Paris |first3=Zdeněk |last4=Týč |first4=Jiří |last5=Lukeš |first5=Julius |date=2016 |title=Aerobic mitochondria of parasitic protists: Diverse genomes and complex functions |journal=Molecular and Biochemical Parasitology |volume=209 |issue=1-2 |pages=46–57 |doi=10.1016/j.molbiopara.2016.02.007}}</ref>

Mitochondrial cristae, foldings of the inner membrane, have been used to classify protists since the advent of electron microscopy.<ref name=":2" /> Flat cristae are the ancestral trait, tubular cristae are present in the SAR supergroup and Amoebozoa, and discoid cristae distinguish the Discoba.<ref>{{cite journal |last1=Pánek |first1=Tomáš |last2=Eliáš |first2=Marek |last3=Vancová |first3=Marie |last4=Lukeš |first4=Julius |last5=Hashimi |first5=Hassan |date=2020 |title=Returning to the Fold for Lessons in Mitochondrial Crista Diversity and Evolution |journal=Current Biology |volume=30 |issue=10 |pages=R575–R588 |doi=10.1016/j.cub.2020.02.053}}</ref>

=== Cytoskeleton === The cytoskeleton of protists generally consists of an array of microtubules and other fibers that radiate from a complex flagellar apparatus. This structure—sometimes known as the mastigont<ref>{{cite book|chapter=The Mastigont System in Trichomonads|first=Marlene|last=Benchimol|editor-first1=W.|editor-last1=de Souza|title=Structures and Organelles in Pathogenic Protists|series=Microbiology Monographs|volume=17|date=2010|publisher=Springer-Verlag|location=Berlin|doi=10.1007/978-3-642-12863-9_1|pages=1–26}}</ref>—was present in the ancestor of all eukaryotes, and is fundamental to the structure, movement and division of cells. It is one of the only cellular features that can be compared across all protists, as it is relatively conserved.<ref name=":5">{{cite journal |last1=Yubuki |first1=Naoji |last2=Čepička |first2=Ivan |last3=Leander |first3=Brian S. |date=2016 |title=Evolution of the microtubular cytoskeleton (flagellar apparatus) in parasitic protists |journal=Molecular and Biochemical Parasitology |volume=209 |issue=1-2 |pages=26–34 |doi=10.1016/j.molbiopara.2016.02.002}}</ref><ref name=":6" />

The interior of the flagellum, the axoneme, consists of a common structure of nine pairs of microtubules surrounding two central microtubules, known as the 9+2 structure. At its base is a basal body or kinetosome, a complex proteic structure that forms the centrioles and behaves as the microtubule organizing center. Microtubules emerge from each basal body in the form of one or two 'roots'.<ref>{{cite book |last=Moestrup |first=Øjvind |url=https://archive.org/details/flagellatesunity0000unse |title=The flagellates: Unity, diversity and evolution |date=2000 |publisher=Taylor & Francis |editor-last1=Leadbeater |editor-first1=Barry S. C. |series=The Systematics Association Special Volume Series |volume=59 |location=London |pages=69–94 |chapter=The flagellate cytoskeleton: Introduction of a general terminology for microtubular flagellar roots in protists |editor-last2=Green |editor-first2=J. C. |chapter-url=https://archive.org/details/flagellatesunity0000unse/page/69/mode/1up |url-access=registration}}</ref> The basic plan of the flagellar apparatus consists of two basal bodies (B1 and B2), one for each flagellum, followed by four primary microtubular 'roots' (named R1 through R4) and a 'singlet root' (SR) formed by a single microtubule and originating from B1. Attached to the R1 is a multilayered structure, also known as C fiber.<ref name=":6">{{cite journal |last1=Yubuki |first1=Naoji |last2=Leander |first2=Brian S. |date=2013 |title=Evolution of microtubule organizing centers across the tree of eukaryotes |journal=The Plant Journal |volume=75 |issue=2 |pages=230–244 |doi=10.1111/tpj.12145 |issn=0960-7412}}</ref>

Each protist group has modifications or secondary losses of this standard organization.<ref name=":5" /> In groups where the standard structure is mostly untouched (e.g., excavates, stramenopiles, and apusomonads), the R1, R2 and SR roots provide reinforcement for the ventral feeding groove, and the R3 supports the dorsal side of the cell.<ref name=":6" /> In opisthokonts, one flagellum and all the microtubular roots were lost, but both basal bodies remain.<ref name=":6" /><ref>{{cite journal|last=Karpov|first=Sergey A.|title=Flagellar apparatus structure of choanoflagellates|journal=Cilia|volume=5|issue=1|date=2016|issn=2046-2530|pmid=27148446|pmc=4855756|doi=10.1186/s13630-016-0033-5|doi-access=free|article-number=11}}</ref> In archaeplastids, the SR and R2 supporting the feeding groove were lost, likely due to their shift to autotrophic nutrition.<ref name=":6" /> In certain protists the flagellar apparatus is physically linked to the nucleus, forming what is known as the karyomastigont.<ref>{{cite journal | last1=Chapman | first1=Michael | last2=Alliegro | first2=Mark C. | title=The Karyomastigont as an Evolutionary Seme | journal=The Quarterly Review of Biology | volume=87 | issue=4 | date=2012 | issn=0033-5770 | doi=10.1086/668165 | pages=315–324}}</ref> This connection is often done through different kinds of filamentous structures, variously called rhizoplasts or internal flagellar roots.<ref>{{cite journal |last1=Moestrup |first1=Øjvind |date=1982 |title=Phycological Reviews 7. Flagellar structure in algae: a review, with new observations particularly on the Chrysophyceae, Phaeophyceae (Fucophyceae), Euglenophyceae and ''Reckertia'' |journal=Phycologia |volume=21 |issue=4 |pages=427–528 |doi=10.2216/i0031-8884-21-4-427.1}}</ref>

=== Sensory perception ===

[[File:Bmc evol bio hoppenrath proterythropsis ocelloid fig1a.png|thumb|A dinoflagellate with an ocelloid (double arrowhead)|alt=An image of a single cell featuring a large nucleus and an ocelloid, which is composed of a roundish "lens" and a darkly pigmented disc-shaped retinal body.|178x178px]]

Many flagellates and probably all motile algae exhibit a positive phototaxis (i.e. they swim or glide toward a source of light). For this purpose, they exhibit photoreceptors of varying degrees of complexity, from simple receptors with light antennae (as in the eyespot apparatus of many algae), to receptors with opaque screens, to complex ocelloids with intracellular lenses (as in the dinoflagellate family Warnowiaceae).<ref name="Levandowsky-2012" /><ref name="Hoppenrath-2009">{{cite journal|last1=Hoppenrath|first1=M|last2=Bachvaroff|first2=TR|last3=Handy|first3=SM|last4=Delwiche|first4=CF|last5=Leander|first5=BS|title=Molecular phylogeny of ocelloid-bearing dinoflagellates (Warnowiaceae) as inferred from SSU and LSU rDNA sequences.|journal=BMC Evolutionary Biology|date=25 May 2009|volume=9|issue=1|page=116|pmid=19467154|doi=10.1186/1471-2148-9-116|pmc=2694157|doi-access=free|bibcode=2009BMCEE...9..116H}}</ref> Some ciliates orient themselves in relation to the Earth's gravitational field while moving (geotaxis), and others swim in relation to the concentration of dissolved oxygen in the water.<ref name="Levandowsky-2012" />

=== Symbionts === [[File:Bihospites_bacati_SEM_frontiersin_2018.webp|thumb|Scanning electron micrographs of a symbiontid showing two types of epibiotic bacteria: rod-shaped proteobacteria (white arrow) and spherical verrucomicrobia (black arrow) that discharge threads of DNA (white arrowheads).]] Protist cells have an accentuated tendency to harbor mutualistic symbionts, which have produced new physiological opportunities. Some associations are permanent, others more transient.<ref name="Levandowsky-2012"/> Many protists maintain cyanobacteria or other algae as endosymbionts, to benefit from their photosynthesis—especially in radiolarians, foraminifers, and other planktonic marine protists<ref>{{cite book|last=Decelle|first=Johan|last2=Colin|first2=Sébastien|last3=Foster|first3=Rachel A.|date=2015|chapter=Photosymbiosis in Marine Planktonic Protists|title=Marine Protists: Diversity and Dynamics|editor-first1=Susumu|editor-last1=Ohtsuka|editor-first2=Toshinobu|editor-last2=Suzaki|editor-first3=Takeo|editor-last3=Horiguchi|editor-first4=Noritoshi|editor-last4=Suzuki|editor-first5=Fabrice|editor-last5=Not|pages=465–500|publisher=Springer|location=Tokyo|doi=10.1007/978-4-431-55130-0_19}}</ref>—or their nitrogen fixation.<ref>{{cite journal | last1=Foster | first1=Rachel A. | last2=Zehr | first2=Jonathan P. | title=Diversity, Genomics, and Distribution of Phytoplankton-Cyanobacterium Single-Cell Symbiotic Associations | journal=Annual Review of Microbiology | volume=73 | issue=1 | date=8 September 2019 | issn=0066-4227 | doi=10.1146/annurev-micro-090817-062650 | pages=435–456}}</ref> Others maintain only the chloroplasts of algae they ingest, and dispose of the remaining cellular structures (i.e., kleptoplasty).<ref name="Karnkowska-2023"/> Two species of ciliates have been observed hosting purple bacteria, which perform photosynthesis without oxygen.<ref>{{cite journal | last1=Fenchel | first1=Tom | last2=Bernard | first2=Catherine | title=Endosymbiotic purple non-sulphur bacteria in an anaerobic ciliated protozoon | journal=FEMS Microbiology Letters | volume=110 | issue=1 | date=1993 | doi=10.1111/j.1574-6968.1993.tb06289.x | pages=21–25}}</ref><ref>{{cite journal | last1=Muñoz-Gómez | first1=Sergio A. | last2=Kreutz | first2=Martin | last3=Hess | first3=Sebastian | title=A microbial eukaryote with a unique combination of purple bacteria and green algae as endosymbionts | journal=Science Advances | volume=7 | issue=24 | date=11 June 2021 | issn=2375-2548 | pmid=34117067 | pmc=8195481 | doi=10.1126/sciadv.abg4102 | url=https://www.science.org/doi/10.1126/sciadv.abg4102 | access-date=26 March 2026 | article-number=eabg4102}}</ref>

Several groups of protists host non-photosynthetic prokaryotes, often maintaining an anaerobic lifestyle through the metabolism of their symbionts. Xenosomes are bacterial endosymbionts with a methanogenic role, found in anaerobic ciliates.<ref name="Levandowsky-2012"/> Symbiontid euglenozoans and select ciliates have sulfur-oxidizing bacteria living as epibionts on their surfaces.<ref name="Yubuki-2018"/><ref>{{cite journal | last1=Seah | first1=Brandon K. B. | last2=Schwaha | first2=Thomas | last3=Volland | first3=Jean-Marie | last4=Huettel | first4=Bruno | last5=Dubilier | first5=Nicole | last6=Gruber-Vodicka | first6=Harald R. | title=Specificity in diversity: single origin of a widespread ciliate-bacteria symbiosis | journal=Proceedings of the Royal Society B: Biological Sciences | volume=284 | issue=1858 | date=12 July 2017 | issn=0962-8452 | pmid=28701560 | pmc=5524500 | doi=10.1098/rspb.2017.0764 | article-number=20170764}}</ref><ref>{{cite journal | last1=Bright | first1=Monika | last2=Espada-Hinojosa | first2=Salvador | last3=Lagkouvardos | first3=Ilias | last4=Volland | first4=Jean-Marie | title=The giant ciliate ''Zoothamnium niveum'' and its thiotrophic epibiont ''Candidatus'' Thiobios zoothamnicoli: a model system to study interspecies cooperation | journal=Frontiers in Microbiology | volume=5 | date=7 April 2014 | issn=1664-302X | pmid=24778630 | pmc=3985026 | doi=10.3389/fmicb.2014.00145 | doi-access=free | article-number=145}}</ref> Similarly, breviates have hydrogen-oxidizing epibiotic bacteria.<ref>{{cite journal | last1=Hamann | first1=Emmo | last2=Gruber-Vodicka | first2=Harald | last3=Kleiner | first3=Manuel | last4=Tegetmeyer | first4=Halina E. | last5=Riedel | first5=Dietmar | last6=Littmann | first6=Sten | last7=Chen | first7=Jianwei | last8=Milucka | first8=Jana | last9=Viehweger | first9=Bernhard | last10=Becker | first10=Kevin W. | last11=Dong | first11=Xiaoli | last12=Stairs | first12=Courtney W. | last13=Hinrichs | first13=Kai-Uwe | last14=Brown | first14=Matthew W. | last15=Roger | first15=Andrew J. | last16=Strous | first16=Marc | display-authors=5 | title=Environmental Breviatea harbour mutualistic Arcobacter epibionts | journal=Nature | volume=534 | issue=7606 | date=9 June 2016 | issn=0028-0836 | pmid=27279223 | pmc=4900452 | doi=10.1038/nature18297 | pages=254–258}}</ref> Metamonads, particularly parabasalids and oxymonads found in the hindgut of termites, typically host methanogenic archaea as epi- or endobionts.<ref>{{cite journal | last1=Husnik | first1=Filip | last2=Tashyreva | first2=Daria | last3=Boscaro | first3=Vittorio | last4=George | first4=Emma E. | last5=Lukeš | first5=Julius | last6=Keeling | first6=Patrick J. | title=Bacterial and archaeal symbioses with protists | journal=Current Biology | volume=31 | issue=13 | date=2021 | doi=10.1016/j.cub.2021.05.049 | pages=R862–R877 | url=https://linkinghub.elsevier.com/retrieve/pii/S0960982221007478 | access-date=26 March 2026}}</ref>

Some rare associations involve prokaryotes that defend the protist host against potential predators, namely in symbiontids and in the ciliate ''Euplotidium'', where the epibionts are verrucomicrobia that eject genetic material as a defense mechanism.<ref>{{cite journal | last1=Petroni | first1=Giulio | last2=Spring | first2=Stefan | last3=Schleifer | first3=Karl-Heinz | last4=Verni | first4=Franco | last5=Rosati | first5=Giovanna | title=Defensive extrusive ectosymbionts of ''Euplotidium'' (Ciliophora) that contain microtubule-like structures are bacteria related to Verrucomicrobia | journal=Proceedings of the National Academy of Sciences | volume=97 | issue=4 | date=15 February 2000 | issn=0027-8424 | pmid=10660683 | pmc=26518 | doi=10.1073/pnas.030438197 | pages=1813–1817}}</ref><ref name="Yubuki-2018">{{cite journal | last1=Yubuki | first1=Naoji | last2=Leander | first2=Brian S. | title=Diversity and Evolutionary History of the Symbiontida (Euglenozoa) | journal=Frontiers in Ecology and Evolution | volume=6 | date=18 July 2018 | issn=2296-701X | doi=10.3389/fevo.2018.00100 | doi-access=free | article-number=100}}</ref> There are also some species of oxymonads whose epibionts function as chemosensors, providing their host with information on the surrounding chemical gradient.<ref>{{cite journal | last1=Dyer | first1=Betsey Dexter | last2=Khalsa | first2=Ongkar | title=Surface bacteria of Streblomastix strix are sensory symbionts | journal=Biosystems | volume=31 | issue=2-3 | date=1993 | doi=10.1016/0303-2647(93)90046-F | pages=169–180}}</ref>

Besides algae, occurrence of mutualistic eukaryotic symbionts is rare among protists. In the genus ''Neoparamoeba'', some species have endosymbionts that resemble ''Perkinsela amoebae'', a species of trypanosomatids. Although no benefits are yet known from this association, their evolution matches almost perfectly, suggesting that the symbionts are inherited.<ref>{{cite journal | last1=Young | first1=Neil D. | last2=Dyková | first2=Iva | last3=Crosbie | first3=Philip B.B. | last4=Wolf | first4=Matthias | last5=Morrison | first5=Richard N. | last6=Bridle | first6=Andrew R. | last7=Nowak | first7=Barbara F. | display-authors=5 | title=Support for the coevolution of ''Neoparamoeba'' and their endosymbionts, ''Perkinsela amoebae''-like organisms | journal=European Journal of Protistology | volume=50 | issue=5 | date=2014 | doi=10.1016/j.ejop.2014.07.004 | pages=509–523}}</ref>

== Life cycle and reproduction == [[File:Protist life cycle consensus.svg|thumb|upright=1.5|Consensus life cycle of free-living protists, showing the ploidy (''n'') at each stage.{{Efn|name=ploidy}} Purple arrows represent a full haplo-diploid cycle, while the green and blue arrows represent the variations present in the haploid (meiosis immediately after syngamy) and the diploid (syngamy immediately after meiosis) cycles, respectively. Vegetative reproduction is shown in pink.]] Protists exhibit a large variability of life cycles and strategies involving multiple stages of different morphologies which have allowed them to thrive in most environments. Nevertheless, most research concerning protist life cycles corresponds to model organisms and important parasites; knowledge on the life cycles of the free-living majority remains fragmentary.<ref name="Rizos-2024">{{cite journal|first1=Iris|last1=Rizos|first2=Miguel J.|last2=Frada|first3=Lucie|last3=Bittner|first4=Fabrice|last4=Not|title=Life cycle strategies in free- living unicellular eukaryotes: Diversity, evolution, and current molecular tools to unravel the private life of microorganisms|date=31 July 2024|volume=71|article-number=e13052|doi=10.1111/jeu.13052|journal=Journal of Eukaryotic Microbiology|issue=6 |doi-access=free|pmid=39085163 |pmc=11603280}}</ref>

=== Asexual reproduction ===

Protists typically reproduce asexually under favorable environmental conditions,<ref name="Bernstein-2012"/> allowing for rapid exponential population growth with minimal genetic variation. This occurs through mitosis and has historically been considered the main reproductive mode in protists.<ref name="Rizos-2024"/> Unicellular protists often multiply via binary fission, like bacteria;<ref name="Rizos-2024" /> they can also divide through budding, similarly to yeasts, or through multiple fissions, a process known as schizogony.<ref name="Milgroom-2023" /> In multicellular protists, this process is often known as vegetative reproduction, only performed by the 'vegetative stage' or individual. It can take the form of fragmentation of body parts, or specialized propagules composed of numerous cells (e.g., in red algae).<ref name="Cecere-2011">{{cite journal|last1=Cecere|first1=Ester|first2=Antonella|last2=Petrocelli|first3=Marc|last3=Verlaque|date=2011|title=Vegetative reproduction by multicellular propagules in Rhodophyta: an overview|journal=Marine Ecology|volume=32|issue=4 |pages=419–437|doi=10.1111/j.1439-0485.2011.00448.x|bibcode=2011MarEc..32..419C }}</ref>

=== Sexual reproduction ===

Sexual reproduction is a fundamental characteristic of eukaryotes.<ref name="Hörandl-2020">{{cite book|chapter=Chapter 7. Genome Evolution of Asexual Organisms and the Paradox of Sex in Eukaryotes|first1=Elvira|last1=Hörandl|first2=Jens|last2=Bast|first3=Alexander|last3=Brandt|first4=Stefan|last4=Scheu|first5=Christoph|last5=Bleidorn|first6=Mathilde|last6=Cordellier|first7=Minou|last7=Nowrousian|first8=Dominik|last8=Begerow|first9=Anja|last9=Sturm|first10=Koen|last10=Verhoeven|first11=Jens|last11=Boenigk|first12=Thomas|last12=Friedl|first13=Micah|last13=Dunthorn|title=Evolutionary Biology—A Transdisciplinary Approach|date=2020|publisher=Springer Nature Switzerland AG|location=Cham|doi=10.1007/978-3-030-57246-4_7|isbn=978-3-030-57246-4|chapter-url=https://serval.unil.ch/notice/serval:BIB_E19BFD9D1BE0 |editor-first1=Pierre|editor-last1=Pontarotti}}</ref><ref name="Gibson-2021"/> It involves meiosis (a specialized nuclear division enabling genetic recombination) and syngamy (the fusion of nuclei from two parents),<ref name="Rizos-2024"/> two processes thought to have been present in the last eukaryotic common ancestor,<ref name="Malik-2007">{{cite journal|last1=Malik|first1=Shehre-Banoo|last2=Pightling|first2=Arthur W.|last3=Stefaniak|first3=Lauren M.|last4=Schurko|first4=Andrew M.|last5=Logsdon|first5=John M. | title = An expanded inventory of conserved meiotic genes provides evidence for sex in Trichomonas vaginalis | journal = PLOS ONE | volume = 3 | issue = 8 | article-number = e2879 | date = August 2007 | pmid = 18663385 | pmc = 2488364 | doi = 10.1371/journal.pone.0002879 | editor1-last = Hahn | bibcode = 2008PLoSO...3.2879M | editor1-first = Matthew W | doi-access = free }}</ref> which likely had the ability to reproduce sexually on a facultative (non-obligate) basis.<ref>{{cite journal|last1=Dacks|first1=Joel|last2=Roger|first2=Andrew J.| s2cid = 9441768 | title = The first sexual lineage and the relevance of facultative sex | journal = Journal of Molecular Evolution | volume = 48 | issue = 6 | pages = 779–783 | date = June 1999 | pmid = 10229582 | doi = 10.1007/PL00013156 | bibcode = 1999JMolE..48..779D }}</ref> Even protists that no longer reproduce sexually still retain a core set of meiosis-related genes, reflecting their descent from sexual ancestors.<ref>{{cite journal|last1=Ramesh|first1=Marilee A.|last2=Malik|first2=Shehre-Banoo|last3=Logsdon|first3=John M. | s2cid = 17013247 | title = A phylogenomic inventory of meiotic genes; evidence for sex in Giardia and an early eukaryotic origin of meiosis | journal = Current Biology | volume = 15 | issue = 2 | pages = 185–191 | date = January 2005 | pmid = 15668177 | doi = 10.1016/j.cub.2005.01.003 | doi-access = free | bibcode = 2005CBio...15..185R }}</ref><ref>{{cite journal|last1=Cooper|first1=Margarethe A.|last2=Adam|first2=Rodney D.|last3=Worobey|first3=Michael|last4=Sterling|first4=Charles R. | s2cid = 15991722 | title = Population genetics provides evidence for recombination in Giardia | journal = Current Biology | volume = 17 | issue = 22 | pages = 1984–1988 | date = November 2007 | pmid = 17980591 | doi = 10.1016/j.cub.2007.10.020 | doi-access = free | bibcode = 2007CBio...17.1984C }}</ref> For example, although amoebae are traditionally considered asexual organisms, most asexual amoebae likely arose recently and independently from sexually reproducing amoeboid ancestors.<ref name="Lahr-2011">{{cite journal|last1=Lahr|first1=Daniel J. G.|last2=Parfrey|first2=Laura Wegener|last3=Mitchell|first3=Edward A. D.|last4=Katz|first4=Laura A.|last5=Lara|first5=Enrique | title = The chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms | journal = Proceedings of the Royal Society B: Biological Sciences| volume = 278 | issue = 1715 | pages = 2081–2090 | date = July 2011 | pmid = 21429931 | pmc = 3107637 | doi = 10.1098/rspb.2011.0289 | bibcode = 2011PBioS.278.2081L }}</ref> Even in the early 20th century, some researchers interpreted phenomena related to chromidia (chromatin granules free in the cytoplasm) in amoebae as sexual reproduction.<ref name="Dobell-1909">{{cite journal |last1=Dobell|first1=C. |year=1909 |title=Chromidia and the binuclearity hypotheses: A review and a criticism |journal=Quarterly Journal of Microscopical Science |volume=53 |pages=279–326 |url=http://jcs.biologists.org/content/s2-53/210/279.full.pdf}}</ref> Three distinguishable sexual cycles are observed in protists depending on the ploidy{{Efn|name=ploidy|Every sexual cycle involves syngamy and meiosis, which increase or decrease the ploidy (i.e., number of chromosome sets, represented by the letter ''n''), respectively. Syngamy is the fusion of two haploid (1''n'') reproductive cells, known as gametes, into a diploid (2''n'') cell called zygote, which then undergoes meiosis to generate haploid cells.<ref name="Rizos-2024"/>}} of the individual or vegetative stage:<ref name="Rizos-2024"/>

* Haploid cycle (as in most fungi): the individual is haploid and differentiates through mitosis into haploid gametes, which fuse into a zygote that immediately undergoes meiosis to generate new haploid individuals.<ref name="Rizos-2024"/> This is the case for some green algae (such as volvocine algae), most dinoflagellates, dictyostelids, some metamonads, and apicomplexans.<ref name="Ruppert-2004"/>{{rp|p=26}}<ref name="Mulisch-1996">{{cite book|first=Maria|last=Mulisch|chapter=Nuclei and Sexual Reproduction|pages=241–255|title=Protistology|edition=3rd|publisher=E. Schweizerbart'sche Verlagsbuchhandlung|location=Berlin|editor-first1=Klaus|editor-last1=Hausmann|editor-first2=Norbert|editor-last2=Hülsmann|editor-first3=Renate|editor-last3=Radek|date=1996|isbn=3-510-65208-8|url=https://archive.org/details/protistology0000haus|url-access=registration}}</ref> Zygnematophytes, a group of green algae, fuse vegetative cells directly by conjugation instead of producing gametes.<ref>{{cite book|first1=John D.|last1=Hall|first2=Richard M.|last2=McCourt|chapter=Zygnematophyta|pages=135–330|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|volume=1|doi=10.1007/978-3-319-28149-0_33|publisher=Springer International Publishing|location=Cham|url=https://ficoherb.fciencias.unam.mx/Recursos/Bibl/Handbookoftheprotists.pdf|isbn=978-3-319-28149-0|lccn=2017945328|date=2017}}</ref>

[[File: Coleps-Konjugation.jpg|upright=0.6|thumb|Two ciliates join during conjugation to exchange their haploid nuclei via a cytoplasmic bridge.]]

* Diploid cycle (as in animals): the individual is diploid and undergoes meiosis to generate haploid gametes, which fuse into a zygote that develops as a new individual.<ref name="Rizos-2024"/> This is the case for some metamonads, heliozoans, many green algae, diatoms, and labyrinthulids.<ref name="Ruppert-2004"/>{{rp|p=26}}<ref name="Mulisch-1996"/> Ciliates are also diploid, but instead of producing gametes they divide their micronucleus into two haploid nuclei, exchange one of them by conjugation with another ciliate, and fuse the two nuclei into a new diploid nucleus.<ref name="Lynn-2017">{{cite book|first1=Denis H.|last1=Lynn|pages=679–730|chapter=Ciliophora|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer|isbn=978-3-319-28147-6|date=2017|volume=1|doi=10.1007/978-3-319-28149-0_23}}</ref> * Haplo-diploid cycle (as in land plants): there are two alternating generations of individuals. One, the diploid ''agamont'' (or sporophyte), undergoes meiosis to generate haploid cells (called spores) that develop into the other generation, the haploid ''gamont'' (or gametophyte). The gamont then generates gametes by mitosis, which fuse to form the diploid zygote that develops into the agamont.<ref name="Rizos-2024"/> This is the case for many foraminifera and many algae.<ref name="Ruppert-2004"/>{{rp|p=26}} Depending on the relative growth and lifespan of one generation compared to the other, life cycles may be haploid-dominant, diploid-dominant, or with equally dominant generations. Brown algae exhibit the full range of these modes.<ref name="Heesch-2021">{{cite journal|first1=Svenja|last1=Heesch|first2=Martha|last2=Serrano-Serrano|first3=Josué|last3=Barrera-Redondo|first4=Rémy|last4=Luthringer|first5=Akira F.|last5=Peters|first6=Christophe|last6=Destombe|first7=J. Mark|last7=Cock|first8=Myriam|last8=Valero|first9=Denis|last9=Roze|first10=Nicolas|last10=Salamin|first11=Susana M.|last11=Coelho|title=Evolution of life cycles and reproductive traits: Insights from the brown algae|journal=Journal of Evolutionary Biology|volume=34|issue=7|date=1 July 2021|pages=992–1009|doi=10.1111/jeb.13880|pmid=34096650|bibcode=2021JEBio..34..992H |doi-access=free}}</ref> Many red algae have a three-generational cycle with a carposporophyte, whose spores germinate into a tetrasporophyte, whose spores develop into the gametophyte.<ref>{{cite book|first1=Hwan Su|last1=Yoon|first2=Wendy|last2=Nelson|first3=Sandra C.|last3=Lindstrom|first4=Sung Min|last4=Boo|first5=Curt|last5=Pueschel|first6=Huan|last6=Qiu|first7=Debashish|last7=Bhattacharya|chapter=Rhodophyta|pages=89–135|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|volume=1|doi=10.1007/978-3-319-28149-0_33|publisher=Springer International Publishing|location=Cham|url=https://ficoherb.fciencias.unam.mx/Recursos/Bibl/Handbookoftheprotists.pdf|isbn=978-3-319-28149-0|lccn=2017945328|date=2017}}</ref>

==== Factors inducing sexual cycles ==== Free-living protists tend to reproduce sexually under stressful conditions, such as starvation or heat shock. Oxidative stress, which leads to DNA damage, also appears to be an important factor in the induction of sex in protists.<ref name="Bernstein-2012">{{cite book|first=Harris|last=Bernstein|first2=Carol|last2=Bernstein|first3=Richard E.|last3=Michod|date=2012|chapter=Chapter 1. DNA repair as the primary adaptive function of sex in bacteria and eukaryotes|pages=1–49|title=DNA Repair: New Research|veditors=Kimura S, Shimizu S|publisher=Nova Science Publishers|publication-place=Hauppauge, N.Y.|isbn=978-1-62100-756-2|url=https://www.researchgate.net/publication/285932986}}</ref>

Several protists synchronize their life cycles (namely the formation or release of gametes) according to environmental factors such as nutrient or light levels, resulting in synchronization with the day-night cycle, the lunar cycle, or the seasons. The malaria agent ''Plasmodium falciparum'' synchronizes its life cycle with the host's levels of melatonin.<ref>{{cite journal |last1=Timmons |first1=Caitlin |last2=Le |first2=Kristine |last3=Rappaport |first3=H. B. |last4=Sterner |first4=Elinor G. |last5=Maurer-Alcalá |first5=Xyrus X. |last6=Goldstein |first6=Susan T. |last7=Katz |first7=Laura A. |display-authors=5 |date=13 March 2024 |title=Foraminifera as a model of eukaryotic genome dynamism |journal=mBio |volume=15 |issue=3 |doi=10.1128/mbio.03379-23 |issn=2150-7511 |pmc=10936158 |pmid=38329358 |doi-access=free |article-number=e03379-23}}</ref>

=== Cycles in pathogenic protists === Pathogenic protists tend to have extremely complex life cycles that involve multiple forms of the organism, some of which reproduce sexually and others asexually.<ref>{{cite journal|last1=Talman|first1=Arthur M|last2=Domarle|first2=Olivier|last3=McKenzie|first3=F|last4=Ariey|first4=Frédéric|last5=Robert|first5=Vincent| title = Gametocytogenesis: the puberty of ''Plasmodium falciparum'' | journal = Malaria Journal | volume = 3 | page = 24 | date = July 2004 | pmid = 15253774 | pmc = 497046 | doi = 10.1186/1475-2875-3-24 | doi-access = free }}</ref> The stages that feed and multiply inside the host are generally known as trophozoites ({{etymology|gre|trophos|nutrition||zoia|animals}}), but the names of each stage vary depending on the protist group<ref name="Milgroom-2023">{{cite book|first1=Michael G.|last1=Milgroom|date=26 November 2023|chapter=Protozoa|title=Biology of Infectious Disease|publisher=Springer|location=Cham|doi=10.1007/978-3-031-38941-2_6|pages=71–87|isbn=978-3-031-38940-5 }}</ref> (e.g., ''sporozoites'' and ''merozoites'' in apicomplexans;<ref name="Votýpka-2017">{{cite book|first1=Jan|last1=Votýpka|first2=David|last2=Modrý|first3=Miroslav|last3=Oborník|first4=Jan|last4=Šlapeta|first5=Julius|last5=Lukeš|pages=567–624|chapter=Apicomplexa|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer|isbn=978-3-319-28147-6|date=2017|volume=1|doi=10.1007/978-3-319-28149-0_20}}</ref><ref name="Gibson-2021">{{cite journal|last1=Gibson|first1=Wendy|title=The sexual side of parasitic protists|journal=Molecular and Biochemical Parasitology|date=16 April 2021|volume=243|article-number=111371|doi=10.1016/j.molbiopara.2021.111371|pmid=33872659}}</ref> ''primary'' and ''secondary zoospores'' in phytomyxeans).<ref>{{cite book|last1=Bulman|first1=Simon|last2=Neuhauser|first2=Sigrid|pages=783–803|chapter=Phytomyxea|doi=10.1007/978-3-319-28149-0_24|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer|isbn=978-3-319-28147-6|date=2017|volume=1}}</ref>

Some pathogenic protists undergo asexual reproduction in a wide variety of organisms – which act as secondary or intermediate hosts – but can undergo sexual reproduction only in the primary or definitive host (e.g., ''Toxoplasma gondii'' in felids such as domestic cats).<ref>{{cite book|first1=Jitender P.|last1=Dubey|chapter=The History and Life Cycle of ''Toxoplasma gondii''|doi=10.1016/B978-0-12-396481-6.00001-5|title=Toxoplasma gondii: The Model Apicomplexan - Perspectives and Methods|isbn=978-0-12-396481-6|date=2014|publisher=Academic Press|editor-first1=Louis M.|editor-last1=Weiss|editor-first2=Kami|editor-last2=Kim|edition=2nd|pages=1–17}}</ref> Others, such as ''Leishmania'', are capable of performing syngamy in the secondary vector.<ref>{{cite journal|last1=Akopyants|first1=Natalia S.|last2=Kimblin|first2=Nicola|last3=Secundino|first3=Nagila|last4=Patrick|first4=Rachel|last5=Peters|first5=Nathan|last6=Lawyer|first6=Phillip|last7=Dobson|first7=Deborah E.|last8=Beverley|first8=Stephen M.|last9=Sacks|first9=David L.| title = Demonstration of genetic exchange during cyclical development of ''Leishmania'' in the sand fly vector | journal = Science | volume = 324 | issue = 5924 | pages = 265–268 | date = April 2009 | pmid = 19359589 | pmc = 2729066 | doi = 10.1126/science.1169464 | bibcode = 2009Sci...324..265A | display-authors = 1 }}</ref> In apicomplexans, sexual reproduction is obligatory for parasite transmission.<ref name="da Silva-2022">{{cite journal|last1=da Silva|first1=Verônica Santana|last2=Machado|first2=Carlos Renato|date=2022|title=Sex in protists: A new perspective on the reproduction mechanisms of trypanosomatids|journal=Genetics and Molecular Biology|volume=45|issue=3|pages=e20220065|doi=10.1590/1678-4685-GMB-2022-0065|pmid=36218381 |pmc=9552303}}</ref>

Despite undergoing sexual reproduction, it is unclear how frequently there is genetic exchange between different strains of pathogenic protists, as most populations may be clonal lines that rarely exchange genes with other members of their species.<ref>{{cite journal|last1=Tibayrenc|first1=M.|last2=Kjellberg|first2=F.|last3=Arnaud|first3=J.|last4=Oury|first4=B.|last5=Brenière|first5=S. F.|last6=Dardé|first6=M. L.|last7=Ayala|first7=F. J.| title = Are eukaryotic microorganisms clonal or sexual? A population genetics vantage | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 88 | issue = 12 | pages = 5129–33 | date = June 1991 | pmid = 1675793 | pmc = 51825 | doi = 10.1073/pnas.88.12.5129 | bibcode = 1991PNAS...88.5129T | display-authors = 1 | doi-access = free }}</ref>

== Habitats ==

Protist diversity, as detected through environmental DNA surveys, is vast in every sampled environment, but it is mostly undescribed.<ref>{{cite journal|last1=Epstein|first1=Slava|last2=López-García|first2=Purificación|title="Missing" protists: a molecular perspective|journal=Biodiversity and Conservation|volume=17|issue=2|pages=261–276|doi=10.1007/s10531-007-9250-y|date=2007|s2cid=3960288 }}</ref> The richest protist communities appear in soils, followed by oceanic and lastly freshwater habitats, mostly as part of the plankton.<ref name="Singer-2021">{{cite journal|first1=David|last1=Singer|first2=Christophe V.W.|last2=Seppey|first3=Guillaume|last3=Lentendu|first4=Micah|last4=Dunthorn|first5=David|last5=Bass|first6=Lassâad|last6=Belbahri|first7=Quentin|last7=Blandenier|first8=Didier|last8=Debroas|first9=G. Arjen|last9=de Groot|first10=Colomban|last10=de Vargas|first11=Isabelle|last11=Domaizon|first12=Clément |last12=Duckert|first13=Irina|last13=Izaguirre|first14=Isabelle|last14=Koenig|first15=Gabriela|last15=Mataloni|first16=M. Romina|last16=Schiaffino|first17=Edward A.D.|last17=Mitchell|first18=Stefan|last18=Geisen|first19=Enrique|last19=Lara|title=Protist taxonomic and functional diversity in soil, freshwater and marine ecosystems|journal=Environment International|volume=146|date=January 2021|number=106262|article-number=106262 |doi=10.1016/j.envint.2020.106262|pmid=33221595 |doi-access=free|bibcode=2021EnInt.14606262S |hdl=10261/265020|hdl-access=free|display-authors=5}}</ref>

Soil-dwelling protist communities are ecologically the richest, possibly due to the complex and highly dynamic distribution of water in the sediment, which creates extremely heterogenous environmental conditions. The constantly changing environment promotes the activity of only one part of the community at a time, while the rest remains inactive; this phenomenon promotes high microbial diversity in prokaryotes as well as protists.<ref name="Singer-2021" /> Cercozoan amoeboflagellates like the glissomonads and cercomonads are among the most abundant soil protists, their morphological variability well suited for foraging between soil particles. Testate amoebae are also acclimated to the soil environment, as their shells protect against desiccation. Most soil algae are stramenopiles and green algae. Fungus-like protists and slime molds (e.g., oomycetes, myxomycetes, acrasids) are present abundantly as saprotrophs.<ref name="Geisen-2018">{{cite journal|first1=Stefan |last1=Geisen |first2=Edward A. D. |last2=Mitchell |first3=Sina |last3=Adl |first4=Michael |last4=Bonkowski |first5=Micah |last5=Dunthorn |first6=Flemming |last6=Ekelund |first7=Leonardo D. |last7=Fernández |first8=Alexandre |last8=Jousset |first9=Valentyna |last9=Krashevska |first10=David |last10=Singer |first11=Frederick W. |last11=Spiegel |first12=Julia |last12=Walochnik |first13=Enrique |last13=Lara|title=Soil protists: a fertile frontier in soil biology research|journal=FEMS Microbiology Reviews|volume=42|issue=3|date=May 2018|pages=293–323|doi=10.1093/femsre/fuy006|pmid=29447350 |doi-access=free }}</ref> The major parasites in land are the animal-associated apicomplexans<ref>{{cite journal|first1=Frédéric|last1=Mahé|first2=Colomban|last2=de Vargas|first3=David|last3=Bass|first4=Lucas|last4=Czech|first5=Alexandros|last5=Stamatakis|first6=Enrique|last6=Lara|first7=David|last7=Singer|first8=Jordan|last8=Mayor|first9=John|last9=Bunge|first10=Sarah|last10=Sernaker|first11=Tobias|last11=Siemensmeyer|first12=Isabelle|last12=Trautmann|first13=Sarah|last13=Romac|first14=Cédric|last14=Berney|first15=Alexey|last15=Kozlov|first16=Edward A. D.|last16=Mitchell|first17=Christophe V. W.|last17=Seppey|first18=Elianne|last18=Egge|first19=Guillaume|last19=Lentendu|first20=Rainer|last20=Wirth|first21=Gabriel|last21=Trueba|first22=Micah|last22=Dunthorn|display-authors=5|title=Parasites dominate hyperdiverse soil protist communities in Neotropical rainforests|journal=Nature Ecology and Evolution|volume=1|page=0091|date=2017|issue=4 |doi=10.1038/s41559-017-0091|pmid=28812652|url=https://hal.science/hal-03971032 }}</ref> and the plant-associated oomycetes and plasmodiophorids.<ref name="Geisen-2018" />

Marine protists are present in almost the entire range of oceanic conditions, mostly dominating the photic zone. Their abundance depends mostly on the availability of inorganic nutrients, rather than temperature or sunlight, and may vary seasonally.<ref>{{cite journal|last1=Leles|first1=S. G.|last2=Mitra|first2=A.|last3=Flynn|first3=K. J.|last4=Stoecker|first4=D. K.|last5=Hansen|first5=P. J.|last6=Calbet|first6=A.|last7=McManus|first7=G. B.|last8=Sanders|first8=R. W.|last9=Caron|first9=D. A.|last10=Not|first10=F.|last11=Hallegraeff|first11=G. M.|last12=Pitta|first12=P.|last13=Raven|first13=J. A.|last14=Johnson|first14=M. D.|last15=Glibert|first15=P. M.|last16=Våge|first16=S.|display-authors=5|title=Oceanic protists with different forms of acquired phototrophy display contrasting biogeographies and abundance|journal= Proceedings of the Royal Society B: Biological Sciences|date=August 2017|volume=284|issue=1860|article-number=20170664|doi=10.1098/rspb.2017.0664|pmid=28768886|pmc=5563798 |bibcode=2017PBioS.28470664L }}</ref> They are most abundant in coastal waters that receive nutrient-rich run-off from land, and areas where nutrient-rich deep ocean water reaches the surface, namely the upwelling zones in the Arctic Ocean and along continental margins.<ref name="Thoré-2023">{{cite journal|first1=Eli S.J.|last1=Thoré|first2=Koenraad|last2=Muylaert|first3=Michael G.|last3=Bertram|first4=Thomas|last4=Brodin|title=Microalgae|journal=Current Biology|volume=33|issue=3|pages=R91–R95|date=6 February 2023|doi-access=free|doi=10.1016/j.cub.2022.12.032|pmid=36750029|bibcode=2023CBio...33R..91T }}</ref> Radiolarians are widespread as the most dominant marine consumers.<ref name="Faure-2019" /><ref name="Mitra-2016" /><ref name="Singer-2021" /> Macroalgae (namely red algae, green algae and brown algae), unlike plankton, generally require a fixation point, which limits their marine distribution to coastal waters, and particularly to rocky substrates.<ref name=":0" /> Some communities of seaweeds exist adrift on the ocean surface, serving as a refuge and means of dispersal for associated organisms.<ref name=":1" /> Parasitoids such as Syndiniales are abundant pathogens in oceans.<ref name="Singer-2021" />

Freshwater protist communities are characterized by a higher beta diversity (highly heterogeneous between samples) than soil and marine plankton. The high diversity can be a result of the hydrological dynamic of recruiting organisms from different habitats through extreme floods.<ref name="Metz-2022">{{cite journal|last1=Metz|first1=Sebastian|last2=Huber|first2=Paula|last3=Accattatis|first3=Victoria|last4=Lopes dos Santos|first4=Adriana|last5=Bigeard|first5=Estelle|last6=Unrein|first6=Fernando|last7=Chambouvet|first7=Aurélie|last8=Not|first8=Fabrice|last9=Lara|first9=Enrique|last10=Devercelli|first10=Melina|display-authors=5|date=2022|title=Freshwater protists: unveiling the unexplored in a large floodplain system|journal=Environmental Microbiology|volume=24|issue=4 |pages=1731–1745|doi=10.1111/1462-2920.15838|pmid=34783136 |bibcode=2022EnvMi..24.1731M |s2cid=244133100 }}</ref> In freshwater phytoplankton, golden algae, cryptophytes and dinoflagellates are the most abundant groups.<ref name="Singer-2021" />

=== Extreme habitats === {{multiple image | image1 = Examples-of-protists-highlighting-the-morphological-diversity-of-extremophiles-a W640.jpg | width = 400 | title = Examples of extremophilic protists | footer = a) ''Frontonia'', a ciliate from soda lakes in Kenya.{{br}}b) ''Chlamydomonas pitschmanii'', a green alga from hot spring soils.{{br}}c) ''Tetramitus thermacidophilus'', an amoeboflagellate from an acidic geothermal lake in California.{{br}}d) ''Galdieria sulphuraria'', a thermoacidophilic red alga.{{br}}e) ''Halocafeteria seosinensis'', a flagellate from a saltern in Korea. }}

Protists can survive a broad range of extreme conditions, including extreme temperatures (thermophiles or psychrophiles), salinity (halophiles), and pH (alkaliphiles or acidophiles). Most of the extremophilic eukaryotes are algae, specifically chlorophytes, followed by fungi. Other extremophile-abundant groups are heterolobose amoebae, red algae, stramenopiles, and ciliates.<ref name="Rappaport-2023" />

Eukaryotic algae are well-known to withstand high temperatures; for example, the red alga ''Cyanidioschyzon merolae'' persists up to 60°C, similarly to the most extreme thermophilic fungi. Lesser-known thermophilic amoebae and amoeboflagellates (e.g., ''Echinamoeba thermarum'') are repeatedly found in hot environments, including artificially heated systems. While less successful than algae or amoebae, ciliates have also been found in hydrothermal vents up to 52°C. This is still lower than prokaryotes, some of which grow above 80°C.<ref name="Rappaport-2023">{{cite journal |last1=Rappaport |first1=Hannah B. |last2=Oliverio |first2=Angela M. |title=Extreme environments offer an unprecedented opportunity to understand microbial eukaryotic ecology, evolution, and genome biology |journal=Nature Communications |volume=14 |issue=1 |date=16 August 2023 |issn=2041-1723 |pmid=37587119 |pmc=10432404 |doi=10.1038/s41467-023-40657-4 |doi-access=free |article-number=4959 |bibcode=2023NatCo..14.4959R }}</ref> In extremely cold habitats, like snow and the Arctic Ocean, diatoms and green algae are the dominant phototrophs.<ref name="Rappaport-20232">{{cite journal |last1=Rappaport|first1=Hannah B.|last2=Oliverio|first2=Angela M.|title=Extreme environments offer an unprecedented opportunity to understand microbial eukaryotic ecology, evolution, and genome biology|journal=Nature Communications|volume=14|issue=1|date=16 August 2023|issn=2041-1723|pmid=37587119|pmc=10432404|doi=10.1038/s41467-023-40657-4|doi-access=free|article-number=4959|bibcode=2023NatCo..14.4959R}}</ref>

In terms of pH and salinity, protists can withstand similar extremes relative to prokaryotes and fungi, and also persist in polyextreme environments (polyextremophiles). The record for acidophily is also ''C. merolae'', with an observed minimum growth of pH 0. Besides red algae, some species of green algae and amoeboflagellates are found in high-temperature, low-pH geothermal springs. Alkaliphilic protists, primarily represented by ciliates, resist up to pH 10.48, higher than the most alkalophilic bacterium.<ref name="Rappaport-2023" />

Protists are remarkably successful in extreme salinity due to their salt-out strategy, which consists of accumulating organic solutes in the cell instead of ions to counterbalance the hypertonic environment. Examples include the alga ''Dunaliella salina'' and the flagellate ''Halocafeteria seosinensis'', which is able to tolerate up to 36.3% salinity, higher than the maximum reported in bacteria (35%) and fungi (30%).<ref name="Rappaport-2023" />

=== Biomass comparison ===

Protists are abundant in nearly all habitats. They contribute an estimated 4 gigatons (Gt) to Earth's biomass—double that of animals (2 Gt), but less than 1% of the total. Combined, protists, animals, archaea (7 Gt), and fungi (12 Gt) make up less than 10% of global biomass, with plants (450 Gt) and bacteria (70 Gt) dominating.<ref name="Bar-On-2018">{{Cite journal |last1=Bar-On |first1=Yinon M. |last2=Phillips |first2=Rob |last3=Milo |first3=Ron |date=2018-05-17 |df=dmy-all |title=The biomass distribution on Earth |journal=Proceedings of the National Academy of Sciences |volume=115 |issue=25 |language=en |pages=6506–6511 |doi=10.1073/pnas.1711842115 |issn=0027-8424 |pmid=29784790 |pmc=6016768|bibcode=2018PNAS..115.6506B |doi-access=free }}</ref>

== Ecological roles == Protists are indispensable to modern ecosystems worldwide. They also have been the only eukaryotic component of all ecosystems for much of Earth's history, which allowed them to evolve a vast functional diversity that explains their critical ecological significance. They are essential as primary producers, as intermediates in multiple trophic levels, as key regulating parasites or parasitoids, and as partners in diverse symbioses.<ref name="Burki-2021" />

=== Primary producers === Phototrophic protists are the main contributors to the biomass and primary production in nearly all aquatic environments, as part of the phytoplankton.{{Efn|Given the importance of aquatic algae, the equally abundant soil algae may provide a larger contribution to the global carbon cycle than previously thought, but the magnitude of their carbon fixation has yet to be quantified.<ref name="Singer-2021"/>}} Altogether, they are responsible for almost half of the global primary production.<ref name="Thoré-2023" /> They are the main providers of much of the energy and organic matter used by other trophic levels, including essential nutrients such as fatty acids.<ref name="Caron-2009">{{cite journal|first1=David A.|last1=Caron|first2=Alexandra Z.|last2=Worden|first3=Peter D.|last3=Countway|first4=Elif|last4=Demir|first5=Karla B.|last5=Heidelberg|title=Protists are microbes too: a perspective|journal=The ISME Journal|volume=3|issue=1|date=January 2009|pages=4–12|doi=10.1038/ismej.2008.101|pmid=19005497|doi-access=free|bibcode=2009ISMEJ...3....4C }}</ref> Macroalgae support numerous herbivorous animals, especially benthic ones, as both food and refuge from predators.<ref name=":0">{{cite journal|first1=Leonel|last1=Pereira|title=Macroalgae|journal=Encyclopedia|date=7 February 2021|issue=1|volume=1|pages=177–188|doi=10.3390/encyclopedia1010017|doi-access=free}}</ref><ref name=":1">{{cite book|first1=Eva|last1=Rothäusler|first2=Lars|last2=Gutow|first3=Martin|last3=Thiel|chapter=Floating Seaweeds and Their Communities|title=Seaweed Biology: Novel Insights into Ecophysiology, Ecology and Utilization|date=1 January 2012|pages=359–380|doi=10.1007/978-3-642-28451-9_17|publisher=Springer|series=Ecological Studies|volume=219|editor-first1=Christian|editor-last1=Wiencke|editor-first2=Kai|editor-last2=Bischof|isbn=978-3-642-28451-9}}</ref>

=== Consumers === thumb|upright=1.3|Diagram of the soil food web, taking into account the diverse roles of protists as not just bacterivores, but also mycophages and omnivores.<ref name="Geisen-2016" /> Arrows show the flow of nutrients.

Phagotrophic protists are the most diverse functional group in all ecosystems.<ref name="Singer-2021" /> In the trophic webs of soils, protists are the main consumers of both bacteria and fungi, the two main pathways of nutrient flow towards higher trophic levels. As bacterial grazers, they have a significant role in the foodweb: they excrete nitrogen in the form of NH{{sub|3}}, making it available to plants and other microbes.<ref>{{cite journal|last1=Harder|first1=Christoffer Bugge|last2=Rønn|first2=Regin|last3=Brejnrod|first3=Asker|last4=Bass|first4=David|last5=Al-Soud|first5=Waleed Abu|last6=Ekelund|first6=Flemming|title=Local diversity of heathland Cercozoa explored by in-depth sequencing|journal=The ISME Journal|volume=10|pages=2488–2497|date=8 March 2016|issue=10 |doi=10.1038/ismej.2016.31|pmid=26953604 |pmc=5030685 |doi-access=free|bibcode=2016ISMEJ..10.2488H }}</ref>

As part of the plant-associated microbiomes (rhizosphere near the roots, phyllosphere on the leaves), predatory protists such as cercomonads regulate the populations of bacteria and fungi, indirectly improving plant health and growth. They can also have a more direct impact by releasing proteins with antimicrobial activity.<ref>{{cite journal |last1=Santoyo |first1=Gustavo |last2=Orozco-Mosqueda |first2=Ma del Carmen |last3=Babalola |first3=Olubukola Oluranti |date=2025 |title=How protists contribute to plant growth and health: Exploring new interactions with the plant microbiome |journal=The Microbe |volume=7 |doi=10.1016/j.microb.2025.100361 |doi-access=free |article-number=100361}}</ref>

=== Decomposers === Necrophagy (the degradation of dead biomass) among microbes is mainly attributed to bacteria and fungi, but protists have a still poorly recognized role as decomposers with specialized lytic enzymes.<ref name="Moye-2024" /> In marine and estuarine environments, the well-studied thraustochytrids (a type of labyrinthulomycetes) are relevant saprotrophs that decompose various substrates, including dead plant and animal tissue. Various ciliates and testate amoebae scavenge on dead animals. Some nucleariid amoebae specifically consume the contents of dead or damaged cells, but not healthy cells. However, all these examples are of facultative necrophages that also feed on live prey. In contrast, the cercozoan family Viridiraptoridae, present in shallow bog waters, are obligate necrophages of dead algae, potentially fulfilling an important role in cleaning up the environment and releasing nutrients for other microbes.<ref name="Moye-2024">{{cite journal|first1=Jannika|last1=Moye|first2=Sebastian|last2=Hess|title=Broad- range necrophytophagy in the flagellate ''Orciraptor agilis'' (Viridiraptoridae, Cercozoa) and the underappreciated role of scavenging among protists|journal=Journal of Eukaryotic Microbiology|article-number=e13065|doi=10.1111/jeu.13065|doi-access=free|date=3 November 2024|volume=72 |issue=2 |pmid=39489698 |pmc=11822879}}</ref>

=== Parasites and pathogens === Parasitic protists compose around 15–20% of all environmental DNA samples in marine and soil ecosystems, but only around 5% in freshwater systems, where chytrid fungi likely fill that ecological niche.<ref name="Singer-2021"/> They are significant parasites of animals, land plants, fungi,<ref>{{cite book|last=Boddy|first=Lynne|date=2016|chapter=Interactions Between Fungi and Other Microbes|editor-first=Sarah C.|editor-last=Watkinson|editor-first2=Lynne|editor-last2=Boddy|editor-first3=Nicholas P.|editor-last3=Money|title=The Fungi|edition=Third|pages=337–360|publisher=Academic Press|doi=10.1016/B978-0-12-382034-1.00010-4}}</ref> and even of other protists.<ref>{{cite book|chapter=Diseases of Protozoa|first=G.|last=Lauckner|date=1980|title=Diseases of Marine Animals. Volume I. General Aspects. Protozoa to Gastropoda|editor-first=O.|editor-last=Kinne|url=https://www.int-res.com/archive/doma_books/DOMA_Vol_I_(general_aspects,_protozoa_to%20gastropoda).pdf|archive-url=https://web.archive.org/web/20160304042638/https://www.int-res.com/archive/doma_books/DOMA_Vol_I_(general_aspects%2C_protozoa_to%20gastropoda).pdf|archive-date=4 March 2016|pages=75–|publisher=John Wiley & Sons|location=Chichester}}</ref> In plants, oomycetes are the most economically important pathogens (e.g., potato blight),<ref>{{cite journal|last1=Kamoun|first1=Sophien|last2=Furzer|first2=Oliver|last3=Jones|first3=Jonathan D. G.|last4=Judelson|first4=Howard S.|last5=Ali|first5=Gul Shad|last6=Dalio|first6=Ronaldo J. D.|last7=Roy|first7=Sanjoy Guha|last8=Schena|first8=Leonardo|last9=Zambounis|first9=Antonios|last10=Panabières|first10=Franck|last11=Cahill|first11=David|last12=Ruocco|first12=Michelina|last13=Figueiredo|first13=Andreia|last14=Chen|first14=Xiao‐Ren|last15=Hulvey|first15=Jon|last16=Stam|first16=Remco|last17=Lamour|first17=Kurt|last18=Gijzen|first18=Mark|last19=Tyler|first19=Brett M.|last20=Grünwald|first20=Niklaus J.|last21=Mukhtar|first21=M. Shahid|last22=Tomé|first22=Daniel F. A.|last23=Tör|first23=Mahmut|last24=Van Den Ackerveken|first24=Guido|last25=McDowell|first25=John|last26=Daayf|first26=Fouad|last27=Fry|first27=William E.|last28=Lindqvist‐Kreuze|first28=Hannele|last29=Meijer|first29=Harold J. G.|last30=Petre|first30=Benjamin|last31=Ristaino|first31=Jean|last32=Yoshida|first32=Kentaro|last33=Birch|first33=Paul R. J.|last34=Govers|first34=Francine|display-authors=5|title=The Top 10 oomycete pathogens in molecular plant pathology|journal=Molecular Plant Pathology|volume=16|issue=4|date=2015|issn=1464-6722|pmid=25178392|pmc=6638381|doi=10.1111/mpp.12190|pages=413–434}}</ref> but other lesser studied lineages are known to infect plants, such as phytomyxids (e.g., clubroot), labyrinthulids, and trypanosomatids of the genus ''Phytomonas''.<ref>{{cite journal|last1=Schwelm|first1=Arne|last2=Badstöber|first2=Julia|last3=Bulman|first3=Simon|last4=Desoignies|first4=Nicolas|last5=Etemadi|first5=Mohammad|last6=Falloon|first6=Richard E.|last7=Gachon|first7=Claire M. M.|last8=Legreve|first8=Anne|last9=Lukeš|first9=Julius|last10=Merz|first10=Ueli|last11=Nenarokova|first11=Anna|last12=Strittmatter|first12=Martina|last13=Sullivan|first13=Brooke K.|last14=Neuhauser|first14=Sigrid|display-authors=5|title=Not in your usual Top 10: protists that infect plants and algae|journal=Molecular Plant Pathology|volume=19|issue=4|date=2018|issn=1464-6722|pmid=29024322|pmc=5772912|doi=10.1111/mpp.12580|pages=1029–1044}}</ref>

Parasitic protists are among the most well-known human pathogens, causing diseases such as malaria, toxoplasmosis, amoebic meningoencephalitis, sleeping sickness, leishmaniasis, and several diarrheal illnesses like amoebiasis, cryptosporidiosis, and giardiasis.<ref>{{cite journal |last1=Kolářová |first1=Iva |last2=Florent |first2=Isabelle |last3=Valigurová |first3=Andrea |date=3 August 2022 |title=Parasitic Protists: Diversity of Adaptations to a Parasitic Lifestyle |journal=Microorganisms |volume=10 |issue=8 |page=1560 |doi=10.3390/microorganisms10081560 |issn=2076-2607 |pmc=9414628 |pmid=36013978 |doi-access=free}}</ref> Several amoebae are amphizoic, normally free-living but capable of infection.<ref>{{cite journal |last1=Martinez |first1=Augusto Julio |last2=Visvesvara |first2=Govinda S. |date=1997 |title=Free‐living, Amphizoic and Opportunistic Amebas |journal=Brain Pathology |volume=7 |issue=1 |pages=583–598 |doi=10.1111/j.1750-3639.1997.tb01076.x |issn=1015-6305 |pmc=8098488 |pmid=9034567}}</ref>

While parasitic protists are largely studied as protozoa,<ref>{{cite book|last=Baker|first=J. R.|chapter=Systematics of parasitic protozoa|title=Parasitic Protozoa. Volume I. Taxonomy, Kinetoplastids, and Flagellates of Fish|editor-first=Julius P.|editor-last=Kreier|edition=1st|location=New York|publisher=Academic Press|date=1977|url=https://archive.org/details/parasiticprotozo0001unse|url-access=registration|pages=35–56|isbn=0-12-426001-2}}</ref> some are algae, such as the green alga ''Cephaleuros virescens'' which infects plant leaves.<ref>{{cite journal|last1=Brooks|first1=Fred|last2=Rindi|first2=Fabio|last3=Suto|first3=Yasuo|last4=Ohtani|first4=Shuji|last5=Green|first5=Mark|title=The Trentepohliales (Ulvophyceae, Chlorophyta): An Unusual Algal Order and its Novel Plant Pathogen— ''Cephaleuros''|journal=Plant Disease|volume=99|issue=6|date=2015|issn=0191-2917|doi=10.1094/PDIS-01-15-0029-FE|pages=740–753|doi-access=free}}</ref> Hundreds of red algae species parasitize on other red algae, usually closely related species.<ref>{{cite journal|last1=Goff|first1=Lynda J.|last2=Ashen|first2=Jon|last3=Moon|first3=Debra|title=The evolution of parasites from their hosts: a case study in the parasitic red algae|journal=Evolution|volume=51|issue=4|date=1997|issn=0014-3820|doi=10.1111/j.1558-5646.1997.tb03954.x|pages=1068–1078}}</ref>

Certain non-parasitic protists can still be toxic to aquatic animals during periods of excessive growth, either by the release of potent toxins, the depletion of oxygen in the water, or mechanical damage to gills from piercing structures (like the skeletons of silicoflagellates).<ref>{{cite book|last=Haigh|first=Nicola|first2=Tamara|last2=Brown|first3=Devan|last3=Johnson|chapter=Ichthyotoxic skeleton-forming silicoflagellates in British Columbia, Canada; results from the Harmful Algae Monitoring Program, 1999–2017|title=Harmful Algae 2018 – From Ecosystems to Socio-ecosystems. Proceedings of the 18th International Conference on Harmful Algae|editor-first=Philipp|editor-last=Hess|isbn=978-87-990827-7-3|publisher=International Society for the Study of Harmful Algae|location=Nantes, France|pages=167–170|date=2019|url=https://archimer.ifremer.fr/doc/00651/76285/77274.pdf|chapter-url=https://archimer.ifremer.fr/doc/00651/76285/77274.pdf#page=169}}</ref> These phenomena are known as harmful algal blooms, sometimes causing water discoloration (red tides). The most common agents are diatoms and dinoflagellates. When toxins are involved, they can reach human consumption, leading to fish or shellfish poisoning like ciguatera.<ref>{{cite book|last=Hallegraeff|first=Gustaaf M.|chapter=Harmful algal blooms: a global overview|title=Manual on harmful marine microalgae|editor-first=G.M.|editor-last=Hallegraeff|editor-first2=D.M.|editor-last2=Anderson|editor-first3=A.D.|editor-last3=Cembella|date=2003|pages=25–50|isbn=92-3-103948-2|publisher=UNESCO Publishing|location=Landais, France|edition=Second|url=https://www.researchgate.net/profile/Donald-Anderson-3/publication/290125074_IOC_Manuals_and_Guides_No33/links/56994aff08aea14769433603/IOC-Manuals-and-Guides-No33.pdf|chapter-url=https://www.researchgate.net/profile/Donald-Anderson-3/publication/290125074_IOC_Manuals_and_Guides_No33/links/56994aff08aea14769433603/IOC-Manuals-and-Guides-No33.pdf#page=22}}</ref>

=== Mutualists and commensals === Many protists live as endosymbionts in a non-parasitic association, providing their hosts with nutritional advantages. Microalgae, namely zoochlorellae (green algae) and zooxanthellae (dinoflagellates, haptophytes and ochrophytes), are widespread endobionts of other protists, especially foraminifers, radiolarians and ciliates,<ref>{{cite book|last=Esteban|first=Genoveva F.|last2=Fenchel|first2=Tom M.|date=2020|chapter=Symbiosis|title=Ecology of Protozoa: The Biology of Free-living Phagotrophic Protists|publisher=Springer|location=Cham|doi=10.1007/978-3-030-59979-9_8|pages=87–105|isbn=978-3-030-59979-9|edition=2nd}}</ref> but also of animals. The association of zooxanthellae with corals is extensively studied and valued for its importance in reef ecosystems.<ref>{{cite journal | last=Stanley | first=George D. | title=Photosymbiosis and the Evolution of Modern Coral Reefs | journal=Science | volume=312 | issue=5775 | date=12 May 2006 | issn=0036-8075 | doi=10.1126/science.1123701 | pages=857–858}}</ref><ref>{{cite journal | last=Decelle | first=Johan | title=New perspectives on the functioning and evolution of photosymbiosis in plankton: Mutualism or parasitism? | journal=Communicative & Integrative Biology | volume=6 | issue=4 | date=30 July 2013 | issn=1942-0889 | pmid=23986805 | pmc=3742057 | doi=10.4161/cib.24560 | article-number=e24560}}</ref> Climate warming leads to the loss of zooxanthellae, which manifests as coral bleaching and death of the coral hosts.<ref>{{cite journal | last=LaJeunesse | first=Todd C. | title=Zooxanthellae | journal=Current Biology | volume=30 | issue=19 | date=2020 | doi=10.1016/j.cub.2020.03.058 | pages=R1110–R1113}}</ref><ref>{{cite journal | last1=Davy | first1=Simon K. | last2=Allemand | first2=Denis | last3=Weis | first3=Virginia M. | title=Cell Biology of Cnidarian-Dinoflagellate Symbiosis | journal=Microbiology and Molecular Biology Reviews | volume=76 | issue=2 | date=2012 | issn=1092-2172 | doi=10.1128/MMBR.05014-11 | pages=229–261}}</ref> Some molluscs like the giant clams also harbor zooxanthellae.<ref>{{cite journal | last=Hernawan | first=Udhi Eko | title=REVIEW: Symbiosis between the Giant Clams (Bivalvia: Cardiidae) and Zooxanthellae (Dinophyceae) | journal=Biodiversitas Journal of Biological Diversity | volume=9 | issue=1 | date=6 December 2008 | issn=2085-4722 | doi=10.13057/biodiv/d090112 | url=https://smujo.id/biodiv/article/view/414 | access-date=27 March 2026 | pages=53–58 | doi-access=free}}</ref> Sloths are the only mammals with green algae as epibionts on their fur; the hypothesized benefits range from camouflage to sunscreen-like protection or nutritional supplements.<ref>{{cite journal | last1=Kaup | first1=Maya | last2=Trull | first2=Sam | last3=Hom | first3=Erik F. Y. | title=On the move: sloths and their epibionts as model mobile ecosystems | journal=Biological Reviews | volume=96 | issue=6 | date=2021 | issn=1464-7931 | pmid=34309191 | pmc=9290738 | doi=10.1111/brv.12773 | pages=2638–2660}}</ref>

Heterotrophic protists are prevalent members of the gut microbiome of animals, although research has focused almost exclusively on gut bacteria.<ref>{{cite journal | last1=Holt | first1=Corey C. | last2=Boscaro | first2=Vittorio | last3=Van Steenkiste | first3=Niels W. L. | last4=Herranz | first4=Maria | last5=Mathur | first5=Varsha | last6=Irwin | first6=Nicholas A. T. | last7=Buckholtz | first7=Gracy | last8=Leander | first8=Brian S. | last9=Keeling | first9=Patrick J. | display-authors=5 | title=Microscopic marine invertebrates are reservoirs for cryptic and diverse protists and fungi | journal=Microbiome | volume=10 | issue=1 | date=30 September 2022 | issn=2049-2618 | pmid=36180959 | pmc=9523941 | doi=10.1186/s40168-022-01363-3 | doi-access=free | article-number=161}}</ref> The giant metamonad flagellates found in the hindgut of termites and cockroaches allow them to digest wood. This is an obligate mutualism, as termites will starve if cleaned of these protists.<ref name="n885">{{cite journal | last=Gile | first=Gillian H. | title=Protist symbionts of termites: diversity, distribution, and coevolution | journal=Biological Reviews | volume=99 | issue=2 | date=2024 | issn=1464-7931 | doi=10.1111/brv.13038 | pages=622–652}}</ref> However most gut protists are commensals, such as the ciliates abundantly present in the rumen of ruminants, or the ciliate-like opalinids that inhabit amphibian and reptile guts.<ref name="Lind-2025">{{cite journal | last=Lind | first=Abigail | title=Evolution and ecology of commensal gut protists: recent advances | journal=Current Opinion in Genetics & Development | volume=94 | date=2025 | pmid=40683031 | pmc=12369565 | doi=10.1016/j.gde.2025.102382 | article-number=102382 | doi-access=free}}</ref>

=== Biogeochemical cycles ===

Marine protists have a fundamental impact on biogeochemical cycles, particularly the carbon cycle.<ref>{{cite journal|last1=Keeling|first1=Patrick J.|last2=del Campo|first2=Javier|title=Marine Protists Are Not Just Big Bacteria|journal=Current Biology|date=June 2017|volume=27|issue=11|pages=R541–R549|doi=10.1016/j.cub.2017.03.075|pmid=28586691|doi-access=free|bibcode=2017CBio...27.R541K }}</ref> As phytoplankton, they fix as much carbon as all terrestrial plants combined.<ref name="Singer-2021"/> Shells of biomineralizing marine protists accumulate as sediment that forms various geological features, such as chalk deposits, made of coccoliths,<ref>{{cite book|first1=Linda E.|last1=Graham|first2=James M.|last2=Graham|first3=Lee W.|last3=Wilcox|first4=Martha E.|last4=Cook|title=Algae|edition=4th|publisher=LJLM Press|date=2022|isbn=978-0-9863935-4-9|chapter=Chapter 12. The Role of Algae in Biogeochemistry}}</ref> or sand stars, the remains of star-shaped foraminifers.<ref name="Burki-2014"/> Soil protists, particularly testate amoebae, contribute to the silica cycle as much as forest trees through the biomineralization of their shells.<ref name="Geisen-2018"/>

==Evolution and fossil record== Prior to the existence of animals, land plants and fungi, all eukaryotes were protists. As such, questions regarding their origin and evolution are questions on protists. Because of the scarce fossil record prior to <abbr title="circa">ca.</abbr> 750 Mya, dating of early events in eukaryotic evolution relies primarily on molecular clock reconstructions.<ref name="Simpson-2016">{{cite book|last1=Simpson|first1=A.G.B.|last2=Eglit|first2=Y.|title=Encyclopedia of Evolutionary Biology|chapter=Protist Diversification|publisher=Elsevier|date=2016|isbn=978-0-12-800426-5|doi=10.1016/b978-0-12-800049-6.00247-x}}</ref><ref name="Lamza-2025">{{cite journal |last=Lamża |first=Łukasz |date=9 November 2025 |title=Deep‐branching eukaryotes and early events in protist evolution |url=https://www.researchgate.net/profile/Lukasz-Lamza/publication/397445301_Deep-branching_eukaryotes_and_early_events_in_protist_evolution/links/691b6f3cde8143098271e2ac/Deep-branching-eukaryotes-and-early-events-in-protist-evolution.pdf |journal=Biological Reviews |volume=101 |issue=2 |pages=735-750 |doi=10.1111/brv.70101}}</ref> Eukaryotic cells first evolved from archaea<ref>{{cite journal|last=Archibald|first=John M.|title=Genomic clues to the origin of eukaryotic cells|journal=Nature|volume=650|issue=8100|date=5 February 2026|issn=0028-0836|doi=10.1038/d41586-025-04094-1|pages=42–44}}</ref><ref>{{cite journal|last1=Tobiasson|first1=Victor|last2=Luo|first2=Jacob|last3=Wolf|first3=Yuri I.|last4=Koonin|first4=Eugene V.|title=Dominant contribution of Asgard archaea to eukaryogenesis|journal=Nature|volume=650|issue=8100|date=5 February 2026|issn=0028-0836|pmid=41535464|pmc=12872458|doi=10.1038/s41586-025-09960-6|pages=141–149|doi-access=free}}</ref> some time during the late Archean eon (ca. 3 billion years ago),<ref name="Betts-2018">{{cite journal |last1=Betts |first1=Holly C. |last2=Puttick |first2=Mark N. |last3=Clark |first3=James W. |last4=Williams |first4=Tom A. |last5=Donoghue |first5=Philip C. J. |last6=Pisani |first6=Davide |date=20 August 2018 |title=Integrated genomic and fossil evidence illuminates life's early evolution and eukaryote origin |journal=Nature Ecology & Evolution |volume=2 |issue=10 |pages=1556–1562 |doi=10.1038/s41559-018-0644-x |issn=2397-334X |pmc=6152910 |pmid=30127539}}</ref> forming a lineage that eventually gave rise to the last eukaryotic common ancestor (LECA) with the traits associated with crown-group (modern) eukaryotes, namely mitochondria and a complex endomembrane system.<ref name="Butterfield-2015">{{cite journal |last=Butterfield |first=Nicholas J. |date=2015 |title=Early evolution of the Eukaryota |journal=Palaeontology |volume=58 |issue=1 |pages=5–17 |doi=10.1111/pala.12139 |issn=0031-0239}}</ref> Estimations on the date of LECA range from 2.4 to 1.6 billion years ago, during the Paleoproterozoic,<ref name="Strassert-2021"/> leaving a considerably long gap of stem-group (extinct) eukaryotes with intermediate traits.<ref name="Butterfield-2015"/><ref name="Betts-2018"/><ref name="Brocks-2023">{{cite journal|first1=Jochen J.|last1=Brocks|first2=Benjamin J.|last2=Nettersheim|first3=Pierre|last3=Adam|first4=Philippe|last4=Schaeffer|first5=Amber J. M.|last5=Jarrett|first6=Nur|last6=Güneli|first7=Tharika|last7=Liyanage|first8=Lennart M.|last8=van Maldegem|first9=Christian|last9=Hallmann|first10=Janet M.|last10=Hope |display-authors=5 |title=Lost world of complex life and the late rise of the eukaryotic crown |journal=Nature|volume=618|pages=767–773|date=2023|issue=7966 |doi=10.1038/s41586-023-06170-w|pmid=37286610 |bibcode=2023Natur.618..767B |s2cid=259111647 |url=https://hal.science/hal-04273175/file/Brocks%20et%20al.%20Text%202023%20%2B%20Supplementary.pdf }}</ref>

=== First fossils and stem eukaryotes === Fossils before 1 billion years ago are limited and cannot be confidently assigned to modern eukaryotic groups. As such, they are interpreted as potential stem-group eukaryotes, based on their large cell sizes and complex shapes that would still require diagnostic eukaryotic features such as a cytoskeleton and an endomembrane system.<ref name="Butterfield-2015"/><ref name="Brocks-2023"/><ref name="Porter-2023">{{cite journal|last1=Porter|first1=Susannah M.|last2=Riedman|first2=Leigh Anne|title=Frameworks for Interpreting the Early Fossil Record of Eukaryotes|journal=Annual Review of Microbiology|volume=77|issue=1|date=15 September 2023|issn=0066-4227|doi=10.1146/annurev-micro-032421-113254|pages=173–191}}</ref> The earliest potential stem eukaryotes reach back to the late Paleoproterozoic (2–1.6 billion years ago).<ref name="Simpson-2016"/> Examples of these are ''Leiosphaeridia'', ''Tappania'', ''Shuiyousphaeridium'', and ''Grypania.''<ref name="Brocks-2023"/><ref name="Xiao-2018">{{cite journal |last1=Xiao |first1=Shuhai |last2=Tang |first2=Qing |date=28 September 2018 |title=After the boring billion and before the freezing millions: evolutionary patterns and innovations in the Tonian Period |journal=Emerging Topics in Life Sciences |volume=2 |issue=2 |pages=161–171 |doi=10.1042/ETLS20170165 |issn=2397-8554}}</ref> There are also two fossils of putative red algae, ''Ramathallus'' and ''Rafatazmia''.<ref name="Strassert-2021"/>

Sterols, common to modern eukaryotic membranes (e.g., cholesterol), appear comparatively late in the fossil record in comparison to the molecular dating of LECA, suggesting that crown-group eukaryotes flourished relatively late. Instead, stem eukaryotes may have produced simpler protosterols that require less oxygen during biosynthesis. The advanced sterols of modern eukaryotes, although metabolically expensive, likely provided numerous advantages through increased membrane flexibility, such as resilience to osmotic shock during dessication-rehydration cycles, extreme temperatures, oxidative damage, and UV light exposure, allowing them to colonize diverse and harsh environments (e.g., mudflats, rivers, agitated shorelines and land). In contrast, stem eukaryotes remained in low-oxygen marine waters, although at higher abundances.<ref name="Brocks-2023"/>

=== Radiation of crown eukaryotes === In absence of a reliable fossil record, hypotheses regarding the early evolution of crown eukaryotes are based on the phylogenomics of modern protists coupled with morphological and ecological comparisons.<ref name="Lamza-2025"/> LECA is widely hypothesized as having a typical excavate morphology, with two flagella and a ventral groove used for feeding on bacteria by phagocytosis.<ref name=":3" /><ref name=":4" /> In addition, LECA was likely a facultative anaerobe which could live in both aerobic and, more commonly during the Proterozoic, anaerobic environments.<ref>{{cite journal |last1=Müller |first1=Miklós |last2=Mentel |first2=Marek |last3=van Hellemond |first3=Jaap J. |last4=Henze |first4=Katrin |last5=Woehle |first5=Christian |last6=Gould |first6=Sven B. |last7=Yu |first7=Re-Young |last8=van der Giezen |first8=Mark |last9=Tielens |first9=Aloysius G. M. |last10=Martin |first10=William F. |display-authors=5 |date=2012 |title=Biochemistry and Evolution of Anaerobic Energy Metabolism in Eukaryotes |journal=Microbiology and Molecular Biology Reviews |volume=76 |issue=2 |pages=444–495 |doi=10.1128/MMBR.05024-11 |issn=1092-2172}}</ref><ref name="Al-Jewari-2023">{{cite journal |last1=Al Jewari |first1=Caesar |last2=Baldauf |first2=Sandra L. |date=28 April 2023 |title=An excavate root for the eukaryote tree of life |journal=Science Advances |volume=9 |issue=17 |article-number=eade4973 |doi=10.1126/sciadv.ade4973 |issn=2375-2548 |pmc=10146883 |pmid=37115919}}</ref>

Following LECA, a series of ecological and evolutionary innovations took place in the span of 300 million years, between the Paleoproterozoic and Mesoproterozoic, resulting in a rapid radiation that originated all major eukaryotic supergroups.<ref name="Eme-2014">{{cite journal|last1=Eme|first1=L.|last2=Sharpe|first2=S. C.|last3=Brown|first3=M. W.|last4=Roger|first4=A. J.|title=On the Age of Eukaryotes: Evaluating Evidence from Fossils and Molecular Clocks|journal=Cold Spring Harbor Perspectives in Biology|volume=6|issue=8|date=1 August 2014|issn=1943-0264|pmid=25085908|pmc=4107988|doi=10.1101/cshperspect.a016139|pages=a016139–a016139}}</ref><ref name="Porter-2023"/><ref name="Lamza-2025"/> These events can be outlined in four broad stages.{{Efn|The exact temporal relationship between these proposed stages is currently impossible to determine. They merely represent the sequence of ecological and evolutionary innovations that took place between LECA and modern eukaryotes. The second and third stages might have overlapped.<ref name="Lamza-2025"/>}} Firstly, the direct descendants of LECA, excavate-like flagellates feeding on bacteria by altering water currents, diversify and give rise to the most basal groups such as the anaerobic metamonads and the aerobic jakobids. Secondly, surface-associated protists emerge with the development of gliding motility and the cell plasticity to form thin pseudopodia, resulting in lineages such as ancyromonads, apusomonads and CRuMs (i.e., the podiates), which pick bacteria directly off of surfaces. The third stage involves the lineage Diaphoretickes, in which agile swimming motility was prioritized and several adaptations led to an active predatory lifestyle (e.g., provorans, telonemids) hunting both bacteria and, for the first time, other eukaryotes. In response, some prey might have developed stronger pellicles, as seen in rigifilids and apusomonads. Lastly, the fourth stage saw the appearance of both "super-predators" capable of hunting other eukaryovores (as in the SAR lineages), and more specialized bacterivores with amoeboid movement (as in amoebozoans and opisthokonts), resulting in a "mature" biosphere.<ref name="Lamza-2025"/> Early amoebozoans of the Mesoproterozoic adapted to grazing on microbial mats, the dominant food source at the time, resulting in multiple losses of flagella and large amoeboid bodies.<ref>{{cite journal |last=Tekle |first=Yonas I. |last2=Wang |first2=Fang |last3=Wood |first3=Fiona C. |last4=Anderson |first4=O. Roger |last5=Smirnov |first5=Alexey |date=1 July 2022 |title=New insights on the evolutionary relationships between the major lineages of Amoebozoa |journal=Scientific Reports |volume=12 |article-number=11173 |bibcode=2022NatSR..1211173T |doi=10.1038/s41598-022-15372-7 |pmc=9249873 |pmid=35778543 |s2cid=247231712 |doi-access=free |number=11173}}</ref>

Below is a consensus phylogenetic tree of eukaryotes, including all major supergroups that originated from this time period.<ref name="Lamza-2025"/><ref name="Valt-2025"/> The position of the root is still debated, hence the polytomy.{{efn|Some analyses place the root of the eukaryote tree within or next to Metamonada,<ref name="Al-Jewari-2023"/><ref name="Lamza-2025"/> while others split the tree between a branch leading to Diaphoretickes and Discoba ('Diphoda') and a branch leading to Podiata ('Opimoda').<ref>{{cite journal|last1=Williamson|first1=Kelsey|last2=Eme|first2=Laura|last3=Baños|first3=Hector|last4=McCarthy|first4=Charley G. P.|last5=Susko|first5=Edward|last6=Kamikawa|first6=Ryoma|last7=Orr|first7=Russell J. S.|last8=Muñoz-Gómez|first8=Sergio A.|last9=Minh|first9=Bui Quang|last10=Simpson|first10=Alastair G. B.|last11=Roger|first11=Andrew J.|display-authors=5|title=A robustly rooted tree of eukaryotes reveals their excavate ancestry|journal=Nature|volume=640|issue=8060|date=24 April 2025|issn=0028-0836|doi=10.1038/s41586-025-08709-5|pages=974–981}}</ref>}} Excavate groups are marked *. {{clade|label1=LECA|1={{clade|state=dashed |1={{clade|label1=Diaphoretickes |1={{clade |1={{clade |1=SAR 60px |2=Telonemia{{efn|Telonemia may be more closely related to SAR, forming the hypothesized TSAR clade,<ref name="Tikhonenkov-2022b">{{cite journal | last1=Tikhonenkov | first1=Denis V. | last2=Jamy | first2=Mahwash | last3=Borodina | first3=Anastasia S. | last4=Belyaev | first4=Artem O. | last5=Zagumyonnyi | first5=Dmitry G. | last6=Prokina | first6=Kristina I. | last7=Mylnikov | first7=Alexander P. | last8=Burki | first8=Fabien | last9=Karpov | first9=Sergey A. | title=On the origin of TSAR: morphology, diversity and phylogeny of Telonemia | journal= Open Biology| publisher=The Royal Society | volume=12 | issue=3 | year=2022 | article-number=210325 | issn=2046-2441 | doi=10.1098/rsob.210325| pmid=35291881 | pmc=8924772 | doi-access=free }}</ref> or it may branch with Haptista instead.<ref>{{cite journal |last1=Yazaki |first1=Euki |last2=Yabuki |first2=Akinori |last3=Imaizumi |first3=Ayaka |last4=Kume |first4=Keitaro |last5=Hashimoto |first5=Tetsuo |last6=Inagaki |first6=Yuji |date=2022 |title=The closest lineage of Archaeplastida is revealed by phylogenomics analyses that include Microheliella maris |journal=Open Biol. |volume=12 |issue=4 |article-number=210376 |doi=10.1098/rsob.210376 |doi-access=free|pmid=35414259 |pmc=9006020}}</ref><ref>{{cite journal |last1=Torruella |first1=Guifré |last2=Galindo |first2=Luis Javier |last3=Moreira |first3=David |last4=López-García |first4=Purificación |title=Phylogenomics of neglected flagellated protists supports a revised eukaryotic tree of life |journal=Current Biology |date=2025 |volume=35 |issue=1 |biorxiv=10.1101/2024.05.15.594285 |pages=198–207.e4 |doi=10.1016/j.cub.2024.10.075 |pmid=39642877 |bibcode=2025CBio...35..198T |hdl=10481/102343 |hdl-access=free }}</ref>}} 60px|state2=dashed |3=Haptista 50px }} |state2=dashed|2=Disparia 70px |3={{clade  |1=Pancryptista 50px |2=Archaeplastida 50px }} }}}} |2=Discoba* 70px |3=Metamonada* 70px |4=Malawimonada* 70px |5={{clade|state=dashed |1=Ancyromonadida 40px |label2=Podiata|state2=solid|2={{clade |1=CRuMs 60px |label2=Amorphea|2={{clade |1=Amoebozoa 60px |label2=Obazoa|2={{clade |1=Breviatea 60px |2={{clade |1=Apusomonadida 60px |2=Opisthokonta 70px }} }} }} }} }} }}}}

=== Neoproterozoic expansion ===

The presence of modern eukaryotes in the fossil record remained modest{{Efn|There is a considerable fossil gap between the first stem eukaryotes of the late Paleoproterozoic and early Mesoproterozoic (~1.6 billion years ago) and the first crown eukaryotes of the Neoproterozoic (~1 billion years ago). This gap is known as the 'boring billion'.<ref name="Xiao-2018"/>}} until the Tonian period (1000–720 million years ago), when biomarker molecules and microfossils became abundant.<ref name="Butterfield-2015" /> Fossils at around 1000 million years ago (Mya) include the oldest specimens that are assigned with high confidence to crown eukaryotes.<ref name="Simpson-2016" /><ref name="Xiao-2018" /> ''Bangiomorpha'' and ''Proterocladus'' are the oldest multicellular red and green algae, respectively.<ref name="Javaux-2007">{{cite book|last1=Javaux|first1=Emmanuelle J.|date=2007|chapter=The Early Eukaryotic Fossil Record|title=Eukaryotic Membranes and Cytoskeleton|series=Advances in Experimental Medicine and Biology|volume=607|publisher=Springer|location=New York|doi=10.1007/978-0-387-74021-8_1|pages=1–19|pmid=17977455 |isbn=978-0-387-74020-1 |editor-first1=Gáspár|editor-last1=Jékely}}</ref> The oldest fossils of opisthokonts are ''Ourasphaira giraldae'', interpreted as the earliest fungus,<ref name="Brocks-2023" /> and ''Bicellum brasieri'', the earliest holozoan, showing traits associated with animal-like multicellularity such as different cell types.<ref name="Strother-2021">{{Cite journal |last1=Strother |first1=Paul K. |last2=Brasier |first2=Martin D. |last3=Wacey |first3=David |last4=Timpe |first4=Leslie |last5=Saunders |first5=Martin |last6=Wellman |first6=Charles H. |date=April 2021 |title=A possible billion-year-old holozoan with differentiated multicellularity |journal=Current Biology |volume=31 |issue=12 |pages=2658–2665.e2 |doi=10.1016/j.cub.2021.03.051|pmid=33852871 |doi-access=free |bibcode=2021CBio...31E2658S }}</ref> Heterotrophic protists appear abundantly throughout the Tonian, as exemplified by the vase-shaped microfossils (780–720 Mya), interpreted as marine testate amoebae<ref name="Chai-2021" /> and as such composing the oldest representatives of filose (Cercozoa) and lobose (Amoebozoa) amoebae.<ref>{{cite journal|last1=Porter|first1=Susannah M.|last2=Meisterfeld|first2=Ralf|last3=Knoll|first3=Andrew H.|title=Vase-shaped microfossils from the Neoproterozoic Chuar Group, Grand Canyon: a classification guided by modern testate amoebae|journal=Journal of Paleontology|date=May 2003|volume=77|issue=3|pages=409–429|doi=10.1666/0022-3360(2003)077<0409:VMFTNC>2.0.CO;2|bibcode=2003JPal...77..409P | url = http://www.geol.ucsb.edu/faculty/porter/Papers/Porteretal2003_VSMs.pdf|archive-url=https://web.archive.org/web/20220123222203/http://porter.faculty.geol.ucsb.edu/Papers/Porteretal2003_VSMs.pdf|archive-date=23 January 2022}}</ref><ref name="Parfrey-2011">{{cite journal|last1=Parfrey|first1=Laura Wegener|last2=Lahr|first2=Daniel J. G.|last3=Knoll|first3=Andrew H.|last4=Katz|first4=Laura A. | title = Estimating the timing of early eukaryotic diversification with multigene molecular clocks | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 108 | issue = 33 | pages = 13624–9 | date = August 2011 | pmid = 21810989 | pmc = 3158185 | doi = 10.1073/pnas.1110633108 | bibcode = 2011PNAS..10813624P | doi-access = free }}</ref> Other microfossils include some poorly preserved tubular shells from 716–635 Mya rocks.<ref name="Xiao-2021">{{cite journal|first1=Xiao|last1=Min|first2=Hong|last2=Hua|first3=Bo|last3=Sun|first4=Qiaokun|last4=Dai|first5=Jinzhou|last5=Luo|first6=Xiaoqiang|last6=Pan|first7=Ziwei|last7=Liu|title=Diversification of heterotrophic protists at the eve of Cambrian explosion|journal=Global and Planetary Change|volume=203|date=August 2021|article-number=103545|doi=10.1016/j.gloplacha.2021.103545|bibcode=2021GPC...20303545M }}</ref>

As oxygen levels rose, crown eukaryotes may have outcompeted stem eukaryotes, expanding into oxygen-rich marine environments that supported an aerobic metabolism enabled by their mitochondria. Stem eukaryotes may have gone extinct due to competition and the extreme climatic changes of the Cryogenian glaciations (720–635 Mya) and subsequent global warming, cementing the dominance of crown eukaryotes which began to appear abundantly in this era, fueled by the proliferation of algae.<ref name="Javaux-2007" /><ref name="Brocks-2023" /> After the Gaskiers glaciation of the Late Ediacaran (ca. 579 Mya), fossils of heterotrophic protists diversify further. Some fossils at 548 Mya, similar to vase-shaped microfossils, are interpreted as the oldest traces of foraminifers (e.g., ''Protolagena''),<ref name="Chai-2021">{{cite journal|first1=Shu|last1=Chai|first2=Hong|last2=Hua|first3=Jinjie|last3=Ren|first4=Qiaokun|last4=Dai|first5=Zaihang|last5=Cui|title=Vase-shaped microfossils from the late Ediacaran Dengying Formation of Ningqiang, South China: taxonomy, preservation and biological affinity|journal=Precambrian Research|volume=352|date=January 2021|article-number=105968|doi=10.1016/j.precamres.2020.105968|bibcode=2021PreR..35205968C }}</ref> but their foraminiferal affinity is doubtful.<ref name="Xiao-2021" />

<gallery class="center" mode="nolines" heights="110px" widths="150px" style="font-size:90%"> Vase-shaped microfossil, Neoproterozoic Kwagunt Formation.jpg|A vase-shaped microfossil c. 742 Mya resembling modern testate amoebae<ref>{{cite journal|last1=Morais|first1=Luana|last2=Fairchild|first2=Thomas Rich|last3=Lahr|first3=Daniel J.G.|last4=Rudnitzki|first4=Isaac D.|last5=Schopf|first5=J. William|last6=Garcia|first6=Amanda K.|last7=Kudryavtsev|first7=Anatoliy B.|last8=Romero|first8=Guilherme R.|display-authors=5|title=Carbonaceous and siliceous Neoproterozoic vase-shaped microfossils (Urucum Formation, Brazil) and the question of early protistan biomineralization|journal=Journal of Paleontology|volume=91|issue=3|date=2017|doi=10.1017/jpa.2017.16|pages=393–406|doi-access=free}}</ref> Proterocladus antiquus, Nanfen Formation, Xihe Group (North China).jpg|''Proterocladus'', the oldest fossil of green algae Bangiomorpha pubescens, Hunting Formation (Sumerset Island, Nunavut, Canada).jpg|''Bangiomorpha'', the oldest fossil of red algae </gallery>

=== Phanerozoic diversifications and major events ===

At the very start of the Paleozoic era, the first definitive fossils of radiolarian<ref name="Chang-2018">{{cite journal |last1=Chang |first1=Shan |last2=Feng |first2=Qinglai |last3=Zhang |first3=Lei |title=New Siliceous Microfossils from the Terreneuvian Yanjiahe Formation, South China: The Possible Earliest Radiolarian Fossil Record |journal=Journal of Earth Science |date=14 August 2018 |volume=29 |issue=4 |pages=912–919 |doi=10.1007/s12583-017-0960-0|bibcode=2018JEaSc..29..912C |s2cid=134890245 }}</ref><ref name="Zhang-2019">{{cite journal |last1=Zhang |first1=Ke |last2=Feng |first2=Qing-Lai |title=Early Cambrian radiolarians and sponge spicules from the Niujiaohe Formation in South China |journal=Palaeoworld |date=September 2019 |volume=28 |issue=3 |pages=234–242 |doi=10.1016/j.palwor.2019.04.001|s2cid=146452469 }}</ref><ref name="Maletz-2017">{{cite journal |last1=Maletz |first1=Jörg |title=The identification of putative Lower Cambrian Radiolaria |journal=Revue de Micropaléontologie |date=June 2017 |volume=60 |issue=2 |pages=233–240 |doi=10.1016/j.revmic.2017.04.001|bibcode=2017RvMic..60..233M }}</ref> and foraminiferal<ref name="Pawlowski-2003">{{cite journal|last1=Pawlowski|first1=Jan|last2=Holzmann|first2=Maria|last3=Berney|first3=Cédric|last4=Fahrni|first4=José|last5=Gooday|first5=Andrew J.|last6=Cedhagen|first6=Thomas|last7=Habura|first7=Andrea|last8=Bowser|first8=Samuel S.|date=2003|title=The evolution of early Foraminifera|journal=Proceedings of the National Academy of Sciences|volume=100|issue=20|pages=11494–11498|doi=10.1073/pnas.2035132100|doi-access=free |pmid=14504394 |pmc=208786|bibcode=2003PNAS..10011494P }}</ref><ref name="Xiao-2021" /> shells are found, alongside the first small shelly fauna.<ref name="Braun-2007">{{the Rise and Fall of the Ediacaran Biota |author=A. Braun |author2=J. Chen |author3=D. Waloszek |author4=A. Maas |chapter=First Early Cambrian Radiolaria |pages=143–149 |doi=10.1144/SP286.10}}</ref> Following the Cambrian explosion of animals, the Precambrian microbe-dominated ecosystems were replaced by primarily benthic and nekto-benthic communities, with most marine organisms limited to the depths of shallow water environments.<ref name="Servais-2016">{{cite journal|first1=Thomas|last1=Servais|first2=Vincent|last2=Perrier|first3=Taniel|last3=Danelian|first4=Christian|last4=Klug|first5=Ronald|last5=Martin|first6=Axel|last6=Munnecke|first7=Hendrik|last7=Nowak|first8=Alexander|last8=Nützel|first9=Thijs R.A.|last9=Vandenbroucke|first10=Mark|last10=Williams|first11=Christian M.Ø.|last11=Rasmussen|title=The onset of the 'Ordovician Plankton Revolution' in the late Cambrian|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|volume=458|date=15 September 2016|pages=12–28|doi=10.1016/j.palaeo.2015.11.003|bibcode=2016PPP...458...12S }}</ref> Mirroring the animal radiation, there was a radiation of phytoplanktonic protists (acritarchs)<ref name="Vecoli-2004">{{cite journal|first1=Marco|last1=Vecoli|first2=Alain|last2=Le Hérissé|title=Biostratigraphy, taxonomic diversity and patterns of morphological evolution of Ordovician acritarchs (organic-walled microphytoplankton) from the northern Gondwana margin in relation to palaeoclimatic and palaeogeographic changes|journal=Earth-Science Reviews|volume=67|date=October 2004|pages=267–311|doi=10.1016/j.earscirev.2004.03.002|issue=3–4|bibcode=2004ESRv...67..267V }}</ref> around 520–510 Ma, followed by a decrease in diversity around 500 Ma.<ref>{{cite journal|first1=Hendrik|last1=Nowak|first2=Thomas|last2=Servais|first3=Claude|last3=Monnet|first4=Stewart G.|last4=Molyneux|first5=Thijs R.A.|last5=Vandenbroucke|title=Phytoplankton dynamics from the Cambrian Explosion to the onset of the Great Ordovician Biodiversification Event: A review of Cambrian acritarch diversity|journal=Earth-Science Reviews|volume=151|date=December 2015|pages=117–131|doi=10.1016/j.earscirev.2015.09.005|bibcode=2015ESRv..151..117N |hdl=20.500.12210/34278|url=http://nora.nerc.ac.uk/id/eprint/514043/1/Nowak%20et%20al%20-%20Cambrian%20Phytoplankton_FINAL%20TEXT.pdf |hdl-access=free}}</ref> The surviving acritarchs expanded in diversity and morphological innovation<ref name="Vecoli-2004" /> due to a decrease in predation from benthic animals, which suffered extinction due to various proposed environmental factors such as anoxia.<ref name="Zhang-2023">{{Cite journal |last1=Zhang |first1=Lei |last2=Algeo |first2=Thomas J. |last3=Zhao |first3=Laishi |last4=Dahl |first4=Tais W. |last5=Chen |first5=Zhong-Qiang |last6=Zhang |first6=Zihu |last7=Poulton |first7=Simon W. |last8=Hughes |first8=Nigel C. |last9=Gou |first9=Xueqing |last10=Li |first10=Chao |date=2023-05-12 |title=Environmental and trilobite diversity changes during the middle-late Cambrian SPICE event |journal=Geological Society of America Bulletin |doi=10.1130/b36421.1 |issn=0016-7606}}</ref> Both phytoplankton and zooplankton (e.g., radiolarians) flourished, as signaled by an increase of organic carbon buried in the sediment known as the SPICE event (~497 Mya).<ref name="Servais-2016" /><ref name="Zhang-2023" /> This abundant biomass supported a second animal radiation known as the Great Ordovician Biodiversification Event. This period is also known as the 'Ordovician Plankton Revolution' due to the significant diversification of planktonic protists.<ref name="Servais-2016" /> Starting in the middle Ordovician, the earliest fossils of eugelnids (''Moyeria'') appear.<ref name="Strother-2020">{{cite journal|last1=Strother|first1=Paul K.|last2=Taylor|first2=Wilson A.|last3=van de Schootbrugge|first3=Bas|last4=Leander|first4=Brian S.|last5=Wellman|first5=Charles H.|title=Pellicle ultrastructure demonstrates that ''Moyeria'' is a fossil euglenid|journal=Palynology|date=2020|volume=44|issue=3|pages=461–471|doi=10.1080/01916122.2019.1625457|doi-access=free|bibcode=2020Paly...44..461S }}</ref>

In the Devonian period, the first fossils of freshwater arcellinid testate amoebae are found (e.g., ''Palaeoleptochlamys'', ''Cangweulla''),<ref>{{cite journal|first1=Kai|last1=Wang|first2=Hong-He|last2=Xu|first3=Bing-Cai|last3=Liu|first4=Jiao|last4=Bai|first5=Yao|last5=Wang|first6=Peng|last6=Tang|first7=Jian-Feng|last7=Lu|first8=Yi|last8=Wang|title=Shallow-marine testate amoebae with internal structures from the Lower Devonian of China|journal=iScience|date=19 May 2023|volume=26|issue=5|article-number=106678|doi=10.1016/j.isci.2023.106678|doi-access=free|pmid=37182111 |pmc=10173733|bibcode=2023iSci...26j6678W }}</ref> as well as various types of freshwater green algae, including charophytes, volvocaceans and desmids,<ref>{{cite journal|first1=Michael|last1=Krings|title=Algae from the Lower Devonian Rhynie chert: Populations of a probable saccoderm desmid (Mesotaeniaceae, Zygnematales) preserved in a microbial mat|journal=Review of Palaeobotany and Palynology|volume=304|date=September 2022|article-number=104697|doi=10.1016/j.revpalbo.2022.104697|bibcode=2022RPaPa.30404697K }}</ref> and some fossils that might represent glaucophytes.<ref>{{cite journal|first1=Michael|last1=Krings|first2=Hans|last2=Kerp|title=A tiny parasite of unicellular microorganisms from the Lower Devonian Rhynie and Windyfield cherts, Scotland|journal=Review of Palaeobotany and Palynology|volume=271|date=December 2019|article-number=104106|doi=10.1016/j.revpalbo.2019.104106|bibcode=2019RPaPa.27104106K}}</ref> Some benthic foraminifera acquired the ability of calcifying,<ref name="Suzuki-2015">{{cite book|chapter=Oldest Fossil Records of Marine Protists and the Geologic History Toward the Establishment of the Modern-Type Marine Protist World|first1=Noritoshi|last1=Suzuki|first2=Masahiro|last2=Oba|publisher=Springer Japan|date=2015|doi=10.1007/978-4-431-55130-0_15|pages=359–394|title=Marine Protists: Diversity and Dynamics|editor-first1=Susumu|editor-last1=Ohtsuka|editor-first2=Toshinobu|editor-last2=Suzaki|editor-first3=Takeo|editor-last3=Horiguchi|editor-first4=Noritoshi|editor-last4=Suzuki|editor-first5=Fabrice|editor-last5=Not|isbn=978-4-431-55130-0}}</ref> and particularly the giant fusulinids became the dominant fossilizable protists. This period also includes the molecular origin of haptophytes (~310 Mya) and silicoflagellates (397–382 Mya), which did not leave fossil traces until later in the Mesozoic. After the Late Devonian extinction (372 Mya), nassellarian-like radiolarians appeared for the first time, with a unique body plan among marine protists.<ref name="Suzuki-2015" />

During the Carboniferous period, no new fossilizable protists originated despite the major environmental changes. However, radiolarian diversity and productivity increased, causing the accumulation of large amounts of biosiliceous sediment (chert) worldwide until the Early Cretaceous.<ref name="Wever-2007">{{cite conference|first1=Patrick|last1=Wever|first2=Luis|last2=O'Dogherty|first3=Spela|last3=Gorican|date=2007|title=The plankton turnover at the Permo-Triassic boundary, emphasis on radiolarians|editor-first1=Peter O.|editor-last1=Baumgartner|editor-first2=Jonathan C.|editor-last2=Aitchison|editor-first3=Patrick|editor-last3=De Wever|editor-first4=Sarah-Jane|editor-last4=Jackett|book-title=Radiolaria: Siliceous Plankton through Time|conference=10th International Meeting of Radiolarian Palaeontologists|series=Swiss Journal of Geosciences Supplement|volume=2|publisher=Birkhäuser Verlag|location=Basel|doi=10.1007/978-3-7643-8344-2_4|pages=49–62}}</ref><ref>{{cite book|editor-last1=Spörli|editor-first1=K. B.|editor-last2=Takemura|editor-first2=A.|editor-last3=Hori|editor-first3=R. S.|title=The oceanic Permian/Triassic boundary sequence at Arrow Rocks (Oruatemanu), Northland, New Zealand|series=GNS Science Monograph|volume=24|pages=45–67|chapter=The depositional environment of the Induan (Early Triassic) biosiliceous sequence (units 2B and 3 of the Oruatemanu Formation), Arrow Rocks, New Zealand|first1=Noritoshi|last1=Suzuki|first2=Yoshiaki|last2=Aita|first3=Satoshi|last3=Yamakita|first4=Yoshihito|last4=Kamata|first5=Atsushi|last5=Takemura|first6=Toru|last6=Fujiki|first7=Kaoru|last7=Ogane|first8=Toyosaburo|last8=Sakai|first9=Rie S.|last9=Hori|location=Lower Hutt, New Zealand|publisher=GNS Science|date=2007|isbn=978-0-478-09919-5|chapter-url=https://www.researchgate.net/publication/280321398}}</ref><ref>{{cite journal|title=Discovery of Lower Permian radiolarian and conodont faunas from the bedded chert of the Chanthaburi area along the Sra Kaeo suture zone, eastern Thailand|first1=Doungrutai|last1=Saesaengseerung|first2=Sachiko|last2=Agematsu|first3=Katsuo|last3=Sashida|first4=Apsorn|last4=Sardsud|journal=Paleontological Research|volume=13|issue=2|pages=119–138|date=30 June 2009|doi=10.2517/1342-8144-13.2.119|bibcode=2009PalRe..13..119S |doi-access=free}}</ref> Around the Capitanian mass extinction event (262–259 Mya) of the Permian period, coccolithophores genetically diverged from the rest of haptophytes, possibly as a response to a reduction in atmospheric oxygen, and there was a faunal turnover from larger to smaller fusulinids.<ref name="Suzuki-2015" /> Spumellarian radiolarians appear in the latest Permian.<ref name="Wever-2007" />

The Permian-Triassic extinction event (~251.9 Mya) caused the extinction of many radiolarians, which manifests as a gap in the chert record. The Triassic period saw the acceleration of radiolarian diversity<ref name="Wever-2007"/> and the appearance of several groups of calcaerous nannofossils, including dinocysts, the oldest identifiable coccolithophore ''Crucirhabdus minutus'', and the oldest fossils of Phaeodaria.<ref name="Suzuki-2015"/> There's a variety of protozoa, including soft-bodied ciliates, and filamentous algae found in amber from the Late Triassic (220–230 Ma).<ref name="Poinar-1993">{{cite journal|first1=George O.|last1=Poinar|first2=Benjamin M.|last2=Waggoner|first3=Ulf-Christian|last3=Bauer|title=Terrestrial Soft-Bodied Protists and Other Microorganisms in Triassic Amber|journal=Science|volume=259|pages=222–224|date=1993|issue=5092 |doi=10.1126/science.259.5092.222|pmid=17790989 |bibcode=1993Sci...259..222P }}</ref>

Around the Early–Middle Jurassic, after the global Toarcian Oceanic Anoxic Event there was a diversification of dinoflagellates and coccolithophores, in both species and abundance. This interval also saw the completion of a symbiosis between Acantharia radiolarians and lineages of ''Phaeocystis'' haptophytes, as well as the appearance of planktonic foraminifera.<ref name="Suzuki-2015"/> The period of low atmospheric oxygen ends in the Aptian-Albian boundary during the Early Cretaceous, and the first fossils of diatoms and silicoflagellates appear.<ref name="Suzuki-2015"/> Samples of amber from around 100 Ma contain the oldest fossil records of apicomplexans (particularly malarian agents and gregarines), trypanosomes,<ref name="Leung-2017">{{cite journal|last1=Leung|first1=Tommy L. F.|date=2017|title=Fossils of parasites: what can the fossil record tell us about the evolution of parasitism?|journal=Biological Reviews|volume=92|issue=1 |pages=410–430|doi=10.1111/brv.12238|pmid=26538112 }}</ref> and metamonads—particularly mutualistic parabasalids of cockroaches, representing the earliest record of mutualism between protists and animals.<ref name="Poinar-2009">{{cite journal|first1=George O.|last1=Poinar|title=Description of an early Cretaceous termite (Isoptera: Kalotermitidae) and its associated intestinal protozoa, with comments on their co-evolution|journal=Parasites & Vectors|volume=2|number=12|date=18 February 2009|article-number=12 |doi=10.1186/1756-3305-2-12|doi-access=free |pmid=19226475 |pmc=2669471 |bibcode=2009PVec....2...12P }}</ref><ref name="Poinar-2009b">{{cite journal|title=Early Cretaceous protist flagellates (Parabasalia: Hypermastigia: Oxymonada) of cockroaches (Insecta: Blattaria) in Burmese amber|first1=George|last1=Poinar|journal=Cretaceous Research|volume=30|issue=5|date=October 2009|pages=1066–1072|doi=10.1016/j.cretres.2009.03.008|bibcode=2009CrRes..30.1066P }}</ref>

Across the Mesozoic era, coccolithophores, dinoflagellates and later diatoms became the dominating eukaryotic producers in oceans until today, as opposed to cyanobacteria and green algae which dominated earlier. Their diversification caused an accelerated transfer of primary production into higher trophic levels, which in turn caused the animal "Mesozoic marine revolution", characterized by the appearance of widespread predation among most invertebrate phyla.<ref>{{cite journal|title=A bottom-up perspective on ecosystem change in Mesozoic oceans|first1=Andrew H.|last1=Knoll|first2=Michael J.|last2=Follows|date=26 October 2016|doi=10.1098/rspb.2016.1755|volume=283|issue=1841|pmid=27798303|journal=Proceedings of the Royal Society B: Biological Sciences|doi-access=free|pmc=5095382 |bibcode=2016PBioS.28361755K }}</ref>

The Cenozoic era began with another extinction event (~66 Ma) that caused the replacement of mesozoic forms of dinoflagellates, foraminifers, coccolithophores, and silicoflagellates with forms that dominate marine habitats today. Right after this event, putative ebridians begin appearing in the fossil record (e.g., ''Ammodochium''), but the oldest reliable ebridian fossils belong to the upper middle Eocene (42–33.7 Ma).<ref name="Suzuki-2015"/> Around this time, the oldest fossils of synurids appear (~49–40 Ma).<ref name="Brown-2010">{{cite journal|last1=Brown|first1=Joseph W.|last2=Sorhannus|first2=Ulf | title = A Molecular Genetic Timescale for the Diversification of Autotrophic Stramenopiles (Ochrophyta): Substantive Underestimation of Putative Fossil Ages | journal = PLOS ONE | date = 2010 | volume = 5 | issue = 9 | article-number = e12759 | doi = 10.1371/journal.pone.0012759 | doi-access = free| pmid = 20862282 | pmc = 2940848 | bibcode = 2010PLoSO...512759B }}</ref> Following the Middle Eocene Climatic Optimum (~40 Ma), diatoms became the dominant agents of marine silicon precipitation as opposed to radiolarians, and the fossil record shows the first raphid diatoms and collodarians.<ref name="Suzuki-2015"/>

== Footnotes == {{Notelist|25em}}

== References == {{Reflist}}

== External links == {{Wikispecies|Protista}} {{Wikispecies|Protoctista}} {{Commons category|Protista}} * [https://eukmap.unieuk.net/ UniEuk Taxonomy App] * [http://tolweb.org/Eukaryotes/3 Tree of Life: Eukaryotes] * Tsukii, Y. (1996). ''Protist Information Server'' (database of protist images). Laboratory of Biology, Hosei University. [http://protist.i.hosei.ac.jp/ Protist Information Server]. Updated: March 22, 2016.

{{Organisms et al.}} {{Life on Earth}} {{Eukaryota}} {{Protozoa protist}} {{Taxonbar|from=Q10892}} {{Authority control}}

Category:Protists Category:Obsolete eukaryote taxa Category:Paraphyletic groups