# Unicellular organism

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Organism that consists of only one cell

Unicellular organism Valonia ventricosa, a species of alga with a diameter that ranges typically from 1 to 4 centimetres (0.4 to 1.6 in), is among the largest unicellular species

A **unicellular organism**, also known as a **single-celled organism**, is an [organism](/source/Organism) that consists of a single [cell](/source/Cell_(biology)), unlike a [multicellular organism](/source/Multicellular_organism) that consists of multiple cells. Organisms fall into two general categories: [prokaryotic](/source/Prokaryotic) organisms and [eukaryotic](/source/Eukaryotic) organisms. Most prokaryotes are unicellular and are classified into [bacteria](/source/Bacteria) and [archaea](/source/Archaea). Many eukaryotes are multicellular, but some are unicellular such as [protozoa](/source/Protozoa), unicellular [algae](/source/Algae), and unicellular [fungi](/source/Fungi). Unicellular organisms are thought to be the oldest form of life, with early organisms emerging 3.5–3.8 billion years ago.[1][2]

Although some prokaryotes live in [colonies](/source/Colony_(biology)), they are not specialised cells with differing functions. These organisms live together, and each cell must carry out all life processes to survive. In contrast, even the simplest multicellular organisms have cells that depend on each other to survive.

Most multicellular organisms have a unicellular life-cycle stage. [Gametes](/source/Gamete), for example, are reproductive unicells for multicellular organisms.[3]

Some organisms are partially unicellular, like *[Dictyostelium discoideum](/source/Dictyostelium_discoideum)*. Additionally, unicellular organisms can be [multinucleate](/source/Multinucleate), like *[Caulerpa](/source/Caulerpa)*, *[Plasmodium](/source/Plasmodium)*, and [Myxogastria](/source/Myxogastria).

## Evolutionary hypothesis

Life timeline This box: view talk edit −4500 — – — – −4000 — – — – −3500 — – — – −3000 — – — – −2500 — – — – −2000 — – — – −1500 — – — – −1000 — – — – −500 — – — – 0 — Water Single-celled life Photosynthesis Multicellular life Plants Arthropods Molluscs Flowers Dinosaurs Mammals Birds Primates Hadean Archean Proterozoic Phanerozoic ← Earth formed ← Earliest water ← LUCA ← Earliest fossils ← Atmospheric oxygen ← Sexual reproduction ← Earliest fungi ← Neoproterozoic oxygenation event ← Ediacaran biota ← Cambrian explosion ← Earliest tetrapods ← Earliest hominoid (million years ago)

The [origin of life](/source/Origin_of_life) is largely still a mystery. Primitive [protocells](/source/Protocell) are thought to be the precursors to today's unicellular organisms.

In one theory, known as the [RNA world hypothesis](/source/RNA_world), early RNA molecules would have been the basis for catalyzing organic chemical reactions and self-replication.[4]

Compartmentalization was necessary for chemical reactions to be more likely as well as to differentiate reactions with the external environment. For example, an early RNA replicator [ribozyme](/source/Ribozyme) may have replicated other replicator ribozymes of different RNA sequences if not kept separate.[5] Such hypothetic[*[clarification needed](https://en.wikipedia.org/wiki/Wikipedia:Please_clarify)*] cells with an RNA genome instead of the usual DNA genome are called '[ribocells](/source/Ribocells)' or 'ribocytes'.[4]

When [amphiphiles](/source/Amphiphile) like [lipids](/source/Lipid) are placed in water, the hydrophobic tails aggregate to form [micelles](/source/Micelle) and [vesicles](/source/Vesicle_(biology_and_chemistry)), with the hydrophilic ends facing outwards.[6][5] Primitive cells likely used self-assembling fatty-acid vesicles to separate chemical reactions and the environment.[5] Because of their simplicity and ability to self-assemble in water, it is likely that these simple [membranes](/source/Lipid_bilayer) predated other forms of early biological molecules.[6]

## Prokaryotes

Prokaryotes lack membrane-bound organelles, such as [mitochondria](/source/Mitochondrion) or a [nucleus](/source/Cell_nucleus).[7] Instead, most prokaryotes have an irregular region that contains DNA, known as the [nucleoid](/source/Nucleoid).[8] Most prokaryotes have a single, circular [chromosome](/source/Chromosome), which is in contrast to eukaryotes, which typically have linear chromosomes.[9] Nutritionally, prokaryotes have the ability to utilize a wide range of organic and inorganic material for use in metabolism, including sulfur, cellulose, ammonia, or nitrite.[10] Prokaryotes are relatively ubiquitous in the environment and some (known as extremophiles) thrive in extreme environments.[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

### Bacteria

Modern [stromatolites](/source/Stromatolite) in Shark Bay, Western Australia. It can take a century for a stromatolite to grow 5 cm.[11]

**Bacteria in a capsule**

Bacteria are one of the world's oldest forms of life, and are found virtually everywhere on Earth.[10] Many common bacteria have [plasmids](/source/Plasmid), which are short, circular, self-replicating DNA molecules that are separate from the bacterial chromosome.[12] Plasmids can carry genes responsible for novel abilities, of current critical importance being antibiotic resistance.[13] Bacteria predominantly reproduce asexually through a process called [binary fission](/source/Fission_(biology)). However, about 80 different species can undergo a sexual process referred to as natural [genetic transformation](/source/Transformation_(genetics)).[14] Transformation is a bacterial process for transferring DNA from one cell to another, and is apparently an adaptation for [repairing DNA](/source/DNA_repair) damage in the recipient cell.[15] In addition, plasmids can be exchanged through the use of a [pilus](/source/Pilus) in a process known as [conjugation](/source/Bacterial_conjugation).[13]

The photosynthetic [cyanobacteria](/source/Cyanobacteria) are arguably the most successful bacteria, and changed the early atmosphere of the Earth by oxygenating it.[16] [Stromatolites](/source/Stromatolite), structures made up of layers of [calcium carbonate](/source/Calcium_carbonate) and trapped sediment left over from cyanobacteria and associated community bacteria, left behind extensive fossil records.[16][17] The existence of stromatolites gives an excellent record as to the development of cyanobacteria, which are represented across the [Archaean](/source/Archean) (4 billion to 2.5 billion years ago), [Proterozoic](/source/Proterozoic) (2.5 billion to 540 million years ago), and [Phanerozoic](/source/Phanerozoic) (540 million years ago to present day) eons.[17] Much of the fossilized stromatolites of the world can be found in [Western Australia](/source/Western_Australia).[17] There, some of the oldest stromatolites have been found, some dating back to about 3,430 million years ago.[17]

Clonal [aging](/source/Ageing) occurs naturally in [bacteria](/source/Bacteria), and is apparently due to the accumulation of damage that can happen even in the absence of external stressors.[18]

### Archaea

A bottom-dwelling community found deep in the European Arctic.[19]

Hydrothermal vents release heat and [hydrogen sulfide](/source/Hydrogen_sulfide), allowing extremophiles to survive using [chemolithotrophic](/source/Chemolithotroph) growth.[20] Archaea are generally similar in appearance to bacteria, hence their original classification as bacteria, but have significant molecular differences most notably in their membrane structure and ribosomal RNA.[21][22] By sequencing the ribosomal RNA, it was found that the Archaea most likely split from bacteria and were the precursors to modern eukaryotes, and are actually more phylogenetically related to eukaryotes.[22] As their name suggests, Archaea comes from a Greek word *archaios,* meaning original, ancient, or primitive.[23]

Some archaea inhabit the most biologically inhospitable environments on earth, and this is believed to in some ways mimic the early, harsh conditions that life was likely exposed to[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]. Examples of these Archaean [extremophiles](/source/Extremophile) are as follows:

- [Thermophiles](/source/Thermophile), optimum growth temperature of 50 °C-110 °C, including the genera *[Pyrobaculum](/source/Pyrobaculum)*, *[Pyrodictium](/source/Pyrodictium)*, *[Pyrococcus](/source/Pyrococcus)*, *[Thermus aquaticus](/source/Thermus_aquaticus)* and *[Melanopyrus](https://en.wikipedia.org/w/index.php?title=Melanopyrus&action=edit&redlink=1).*[24]

- [Psychrophiles](/source/Psychrophile), optimum growth temperature of less than 15 °C, including the genera *[Methanogenium](/source/Methanogenium)* and *[Halorubrum](/source/Halorubrum).*[24]

- [Alkaliphiles](/source/Alkaliphile), optimum growth pH of greater than 8, including the genus *[Natronomonas](/source/Natronomonas)*.[24][25]

- [Acidophiles](/source/Acidophile), optimum growth pH of less than 3, including the genera *[Sulfolobus](/source/Sulfolobus) and [Picrophilus](/source/Picrophilus)*.[24][26]

- [Piezophiles](/source/Barophile), (also known as [barophiles](/source/Piezophile)), prefer high pressure up to 130 MPa, such as deep ocean environments, including the genera *[Methanococcus](/source/Methanococcus)* and *[Pyrococcus](/source/Pyrococcus)*.[24]

- [Halophiles](/source/Halophile), grow optimally in high salt concentrations between 0.2 M and 5.2 M [NaCl](/source/Sodium_chloride), including the genera *[Haloarcula](/source/Haloarcula)*, *[Haloferax](/source/Haloferax)*, *[Halococcus](/source/Halococcus)*.[24][27]

[Methanogens](/source/Methanogen) are a significant subset of archaea and include many extremophiles, but are also ubiquitous in wetland environments as well as the ruminant and hindgut of animals.[28] This process utilizes hydrogen to reduce carbon dioxide into methane, releasing energy into the usable form of [adenosine triphosphate](/source/Adenosine_triphosphate).[28] They are the only known organisms capable of producing methane.[29] Under stressful environmental conditions that cause [DNA damage](/source/DNA_damage_(naturally_occurring)), some species of archaea aggregate and transfer DNA between cells.[30] The function of this transfer appears to be to replace damaged DNA sequence information in the recipient cell by undamaged sequence information from the donor cell.[31]

## Eukaryotes

Eukaryotic cells contain membrane bound organelles. Some examples include mitochondria, a nucleus, or the Golgi apparatus. Prokaryotic cells probably transitioned into eukaryotic cells between 2.0 and 1.4 billion years ago.[32] This was an important step in evolution. In contrast to prokaryotes, eukaryotes reproduce by using [mitosis](/source/Mitosis) and [meiosis](/source/Meiosis). Sex appears to be a ubiquitous and ancient, and inherent attribute of [eukaryotic](/source/Eukaryote) life.[33] Meiosis, a true sexual process, allows for efficient [recombinational](/source/Genetic_recombination) repair of DNA damage [15] and a greater range of genetic diversity by combining the DNA of the parents followed by [recombination](/source/Genetic_recombination).[32] Metabolic functions in eukaryotes are more specialized as well by sectioning specific processes into organelles.[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

The [endosymbiotic theory](/source/Symbiogenesis) holds that mitochondria and chloroplasts have bacterial origins. Both organelles contain their own sets of DNA and have bacteria-like ribosomes. It is likely that modern mitochondria were once a species similar to *[Rickettsia](/source/Rickettsia)*, with the parasitic ability to enter a cell.[34] However, if the bacteria were capable of respiration, it would have been beneficial for the larger cell to allow the parasite to live in return for energy and detoxification of oxygen.[34] Chloroplasts probably became symbionts through a similar set of events, and are most likely descendants of cyanobacteria.[35] While not all eukaryotes have mitochondria or chloroplasts, mitochondria are found in most eukaryotes, and chloroplasts are found in all plants and algae. Photosynthesis and respiration are essentially the reverse of one another, and the advent of respiration coupled with photosynthesis enabled much greater access to energy than [fermentation](/source/Fermentation) alone.[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

### Protozoa

*Paramecium tetraurelia*, a ciliate, with oral groove visible

[Protozoa](/source/Protozoa) are largely defined by their method of locomotion, including [flagella](/source/Flagellum), [cilia](/source/Cilium), and [pseudopodia](/source/Pseudopodia).[36] While there has been considerable debate on the classification of protozoa caused by their sheer diversity, in one system there are currently seven phyla recognized under the kingdom Protozoa: [Euglenozoa](/source/Euglenozoa), [Amoebozoa](/source/Amoebozoa), [Choanozoa *sensu* Cavalier-Smith](/source/Choanozoa), [Loukozoa](/source/Loukozoa), [Percolozoa](/source/Percolozoa), [Microsporidia](/source/Microsporidia) and [Sulcozoa](/source/Sulcozoa).[37][38] Protozoa, like plants and animals, can be considered heterotrophs or autotrophs.[34] Autotrophs like *[Euglena](/source/Euglena)* are capable of producing their energy using photosynthesis, while heterotrophic protozoa consume food by either funneling it through a mouth-like gullet or engulfing it with pseudopods, a form of [phagocytosis](/source/Phagocytosis).[34] While protozoa reproduce mainly asexually, some protozoa are capable of sexual reproduction.[34] [Protozoa](/source/Protozoa) with sexual capability include the pathogenic species *[Plasmodium falciparum](/source/Plasmodium_falciparum)*, *[Toxoplasma gondii](/source/Toxoplasma_gondii)*, *[Trypanosoma brucei](/source/Trypanosoma_brucei)*, *[Giardia duodenalis](/source/Giardia_duodenalis)* and *[Leishmania](/source/Leishmania)* species.[15]

[Ciliophora](/source/Ciliate), or ciliates, are a group of protists that utilize cilia for locomotion. Examples include *[Paramecium](/source/Paramecium)*, [*Stentors*](/source/Stentor_(protozoa)), and *[Vorticella](/source/Vorticella)*.[39] Ciliates are widely abundant in almost all environments where water can be found, and the cilia beat rhythmically in order to propel the organism.[40] Many ciliates have [trichocysts](/source/Trichocyst), which are spear-like organelles that can be discharged to catch prey, anchor themselves, or for defense.[41][42] Ciliates are also capable of sexual reproduction, and utilize two nuclei unique to ciliates: a [macronucleus](/source/Macronucleus) for normal metabolic control and a separate [micronucleus](/source/Micronucleus) that undergoes meiosis.[41] Examples of such ciliates are *[Paramecium](/source/Paramecium)* and *[Tetrahymena](/source/Tetrahymena)* that likely employ meiotic recombination for repairing DNA damage acquired under stressful conditions.[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*]

The Amebozoa utilize pseudopodia and cytoplasmic flow to move in their environment. *[Entamoeba histolytica](/source/Entamoeba_histolytica)* is the cause of amebic dysentery.[43] *[Entamoeba histolytica](/source/Entamoeba_histolytica)* appears to be capable of [meiosis](/source/Meiosis).[44]

### Unicellular algae

A scanning electron microscope image of a [diatom](/source/Diatom)

Unicellular algae are plant-like autotrophs and contain [chlorophyll](/source/Chlorophyll).[45] They include groups that have both multicellular and unicellular species:

- [Euglenophyta](/source/Euglenid), flagellated, mostly unicellular algae that occur often in fresh water.[45] In contrast to most other algae, they lack cell walls and can be [mixotrophic](/source/Mixotroph) (both autotrophic and heterotrophic).[45] An example is *[Euglena gracilis](/source/Euglena_gracilis)*.

- [Chlorophyta](/source/Chlorophyta) (green algae), mostly unicellular algae found in fresh water.[45] The chlorophyta are of particular importance because they are believed to be most closely related to the evolution of land plants.[46]

- [Diatoms](/source/Diatom), unicellular algae that have siliceous cell walls.[47] They are the most abundant form of algae in the ocean, although they can be found in fresh water as well.[47] They account for about 40% of the world's primary marine production, and produce about 25% of the world's oxygen.[48] Diatoms are very diverse, and comprise about 100,000 species.[48]

- [Dinoflagellates](/source/Dinoflagellate), unicellular flagellated algae, with some that are armored with [cellulose](/source/Cellulose).[49] Dinoflagellates can be mixotrophic, and are the algae responsible for [red tide](/source/Red_tide).[46] Some dinoflagellates, like *[Pyrocystis fusiformis](/source/Pyrocystis_fusiformis)*, are capable of [bioluminescence](/source/Bioluminescence).[50]

### Unicellular fungi

Transmission electron microscope image of budding *[Ogataea polymorpha](/source/Ogataea_polymorpha)*

Unicellular fungi include the [yeasts](/source/Yeast). Fungi are found in most habitats, although most are found on land.[51] Yeasts reproduce through mitosis, and many use a process called [budding](/source/Budding), where most of the [cytoplasm](/source/Cytoplasm) is held by the mother cell.[51] *[Saccharomyces cerevisiae](/source/Saccharomyces_cerevisiae)* ferments carbohydrates into carbon dioxide and alcohol, and is used in the making of beer and bread.[52] *S. cerevisiae* is also an important model organism, since it is a eukaryotic organism that is easy to grow. It has been used to research [cancer](/source/Cancer) and [neurodegenerative diseases](/source/Neurodegenerative_disease) as well as to understand the [cell cycle](/source/Cell_cycle).[53][54] Furthermore, research using *[S. cerevisiae](/source/Saccharomyces_cerevisiae)* has played a central role in understanding the mechanism of meiotic [recombination](/source/Genetic_recombination) and the adaptive function of [meiosis](/source/Meiosis). [Candida spp](/source/Candida_(fungus)). are responsible for [candidiasis](/source/Candidiasis), causing infections of the mouth and/or throat (known as thrush) and vagina (commonly called yeast infection).[55]

## Macroscopic unicellular organisms

Most unicellular organisms are of [microscopic](/source/Microscopic_scale) size and are thus classified as [microorganisms](/source/Microorganism). However, some unicellular protists and bacteria are [macroscopic](/source/Macroscopic_scale) and visible to the naked eye.[56] Examples include:

- *[Brefeldia maxima](/source/Brefeldia_maxima)*, a [slime mold](/source/Slime_mold), examples have been reported up to a centimetre thick with a surface area of over a square metre and weighed up to around 20 kg[57]

- [Xenophyophores](/source/Xenophyophore), protozoans of the phylum [Foraminifera](/source/Foraminifera), are the largest examples known, with *[Syringammina fragilissima](/source/Syringammina_fragilissima)* achieving a diameter of up to 20 cm (7.9 in)[58]

- *[Nummulite](/source/Nummulite)*, foraminiferans

- *[Valonia ventricosa](/source/Valonia_ventricosa)*, an [alga](/source/Alga) of the class [Chlorophyceae](/source/Chlorophyceae), can reach a diameter of 1 to 4 cm (0.4 to 2 in)[59][60]

- *[Acetabularia](/source/Acetabularia)*, algae

- *[Caulerpa](/source/Caulerpa)*, algae,[61][*[unreliable source?](https://en.wikipedia.org/wiki/Wikipedia:Reliable_sources)*] may grow to 3 metres long[62]

- *[Gromia sphaerica](/source/Gromia_sphaerica)*, amoeba, 5 to 38 mm (0.2 to 1 in)[62]

- *[Thiomargarita magnifica](/source/Thiomargarita_magnifica)* is the largest bacterium, reaching a length of up to 20 mm

- *[Epulopiscium fishelsoni](/source/Epulopiscium_fishelsoni)*, a bacterium

- *[Stentor](/source/Stentor_(ciliate))*, ciliates nicknamed trumpet animalcules

- *[Bursaria](/source/Bursaria_(ciliate))*, largest colpodean ciliates.

## See also

- [Abiogenesis](/source/Abiogenesis)

- [Asexual reproduction](/source/Asexual_reproduction)

- [Colonial organism](/source/Colonial_organism)

- [Individuality in biology](/source/Individuality#Biology)

- [Largest organisms](/source/Largest_organisms)

- [Modularity in biology](/source/Modularity#Modularity_in_biology)

- [Multicellular organism](/source/Multicellular_organism)

- [Sexual reproduction](/source/Sexual_reproduction)

- [Superorganism](/source/Superorganism)

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Adapted from the Wikipedia article [Unicellular organism](https://en.wikipedia.org/wiki/Unicellular_organism) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Unicellular_organism?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.