Micractinium

{{Short description|Genus of algae}} {{Automatic taxobox | image = Micractinium pusillum EPA.jpg | image_caption = ''Micractinium pusillum'' | taxon = Micractinium | authority = Fresenius, 1858 | type_species = ''Micractinium pusillum'' | type_species_authority = Fresenius | subdivision_ranks = Species | subdivision = See text. | synonyms = * ''Diacanthos'' {{au|Korshikov}}<ref name=AlgaeBase>{{AlgaeBase genus|id=43430|title=''Diacanthos'' O.Korshikov, 1953 |access-date=2025-01-13}}</ref> }}

'''''Micractinium''''' is a genus of microscopic green algae in the family Chlorellaceae.<ref name="Algaebase"/> Species of the genus ''Micractinium'' are found as phytoplankton, and are commonly found in freshwater habitats around the world.<ref name=Bicudo_and_Menezes2006/> A few species are found as endosymbionts of ciliates.<ref name=Krivina/> There is increasing interest in ''Micractinium'' due to its high growth rate and lipid production.<ref name=KTB2022/>

==Description== Cells of ''Micractinium'' are spherical to ellipsoidal, and are solitary or stuck together in colonies. Colonies can contain up to 128, occasionally 256 cells,<ref name=Bicudo_and_Menezes2006>{{cite book | title= Gêneros de Algas de Águas Continentais do Brasil: chave para identificação e descrições | edition=2 | year=2006 | first1=Carlos E. M. |last1=Bicudo | first2=Mariângela | last2= Menezes | publisher= RiMa Editora | pages=508 | isbn= 857656064X }}</ref> and are polyhedral or pyramidal in shape.<ref name=Krivina/> Most<ref name=Krivina>{{cite journal | doi=10.1111/pre.12469 | title=New species ''Micractinium kostikovii'' (Chlorellaceae, Trebouxiophyceae) from Russia | year=2022 | last1=Krivina | first1=Elena | last2=Temraleeva | first2=Anna | last3=Sinetova | first3=Maria | journal=Phycological Research | volume=70 | issue=1 | pages=22–34 | bibcode=2022PhycR..70...22K | s2cid=239494013 }}</ref> species generally produce long narrow bristles that taper from base to tip. Cells have a single cup-shaped chloroplast with a single pyrenoid.<ref name=Bicudo_and_Menezes2006/> Bristles consist of protein and lack cellulosic fibers; they are produced after the cell wall.<ref>{{cite journal|doi=10.1016/j.protis.2006.05.006 |title=Genotype versus Phenotype Variability in Chlorella and Micractinium (Chlorophyta, Trebouxiophyceae) |date=2006 |last1=Luo |first1=Wei |last2=Pflugmacher |first2=Stephan |last3=Pröschold |first3=Thomas |last4=Walz |first4=Norbert |last5=Krienitz |first5=Lothar |journal=Protist |volume=157 |issue=3 |pages=315–333 |pmid=16843061 }}</ref>

''Micractinium'' reproduces asexually via the formation of autospores (usually four per cell), which are released from by a rupture in the parental cell wall. Autospores generally remain attached to each other to form colonies, but occasionally detach and develop into single cells; when this happens, the number and arrangement of spines can differ from the typical morphology.<ref>{{cite journal|last1=Hindák|first1=F.|date=1980|title=Three new species of green algae (Chlorophyceae)|journal=Preslia|volume=52|issue=4|pages=289–298|url=https://www.preslia.cz/article/11123}}</ref> In one species, ''Micractinium pusillum'', oogamy has been reported, but this needs reinvestigation.<ref name=KTB2022/>

==Ecology== Members of the genus ''Micractinium'' are common in a variety of freshwater to brackish habitats, and can tolerate a wide array of temperatures, ranging from 0°C in Antarctica to hot springs over 70°C. They play an important role in the ecosystem, as primary producers and producers of oxygen. They also purify the water they inhabit via accumulation and mineralization of pollutants.<ref name=KTB2022>{{cite journal|doi=10.18699/VJGB-22-11 |title=Species delimitation and microalgal cryptic diversity analysis of the genus Micractinium (Chlorophyta) |date=2022 |last1=Krivina |first1=E. S. |last2=Temraleeva |first2=A. D. |last3=Bukin |first3=Yu. S. |journal=Vavilov Journal of Genetics and Breeding |volume=26 |issue=1 |pages=74–85 |pmid=35342860 |pmc=8894098 }}</ref> Some species, such as ''Micractinium conductrix'' are obligate endosymbionts of ciliates; these species cannot produce grow without vitamin B1 and vitamin B12 and thus obtain them from their hosts.<ref name=KTB2022/>

==Taxonomy== The genus ''Micractinium'' was formerly placed in the family Micractiniaceae, characterized by sexual reproduction without the production of zoospores, and colonies covered with bristles. Molecular phylogenetic studies showed that Micractiniaceae was polyphyletic and that ''Micractinium'' was closely related to ''Chlorella''.<ref name=ProsPitsDari20/> Accordingly, ''Micractinium'' has been transferred to the family Chlorellaceae.<ref name="Algaebase"/>

Both ''Chlorella'' and ''Micractinium'' sporopollenin in their cell walls, and have pyrenoids that are transversed by thylakoid membranes and covered in a starch sheath. Traditionally, ''Micractinium'' has been distinguished from ''Chlorella'' on the basis of forming colonies and bristles. However, ''Micractinium'' sometimes does not produce bristles and colonies—it appears to be an inducible defense against grazers such as rotifers. Additionally, some species of ''Micractinium'' do not produce bristles, such as those that are endosymbiotic within other organisms.<ref name=Krivina/>

The genus ''Diacanthos'', distinguished by having exactly two spines on opposite sides of the cell, is a synonym of ''Micractinium''.<ref name=AlgaeBase/>

===Species=== {{As of|2022|March}}, AlgaeBase accepted the following species:<ref name="Algaebase"/> *''Micractinium appendiculatum'' Korshikov *''Micractinium belenophorum'' (Korshikov) T.Proschold, C.Block, W.Luo & L.Kreinitz *''Micractinium bornhemiense'' (W.Conrad) Korshikov *''Micractinium conductrix'' (K.Brandt) Pröschold & Darienko *''Micractinium conococcoides'' T.Hortobagyi *''Micractinium crassisetum'' Hortobágyi *''Micractinium depressum'' C.-C.Jao & Ling *''Micractinium elongatum'' (H.J.Carter) Hegewald & Schnepf *''Micractinium extremum'' Hortobágyi *''Micractinium inermum'' R.Hoshina & Y.Fujiwara *''Micractinium kostikovii'' E.Krivina & A.Temraleeva *''Micractinium parvisetum'' Walton *''Micractinium parvum'' Hindák *''Micractinium pusillum'' Fresenius *''Micractinium quadrisetum'' (Lemmermann) G.M.Smith *''Micractinium simplicissimum'' H.Chae, H.-G. Choi & J.H.Kim *''Micractinium singularis'' H.Chae, H.-G.Choi & J.H.Kim *''Micractinium strigoniense'' T.Hortobagyi *''Micractinium tetrahymenae'' Pröschold, Pitsch & Darienko *''Micractinium valkanovii'' Vodenicarov *''Micractinium variabile'' H.Chae, H.-G.Choi & J.H.Kim

Species have traditionally been defined based on morphological characteristics, such as cell shape, colony shape, and number and size of bristles.<ref name=KomarekFott1983>{{cite book | title= Chlorophyceae (Grünalgen), Ordnung Chlorococcales | last1=Komárek | first1=J. | last2=Fott | first2=B. | date=1983 | publisher= E. Schweizerbart'sche Verlagsbuchhandlung | series=Das Phytoplankton des Süßwassers | language=German | pages=1044 }}</ref> However, ''Micractinium'' displays a large amount of phenotypic plasticity, such as producing or not producing bristles; this makes species delimitation based on morphological alone unreliable.<ref>{{cite journal|doi=10.3390/d12050200|doi-access=free |title=Micractinium tetrahymenae (Trebouxiophyceae, Chlorophyta), a New Endosymbiont Isolated from Ciliates |date=2020 |last1=Pröschold |first1=Thomas |last2=Pitsch |first2=Gianna |last3=Darienko |first3=Tatyana |journal=Diversity |volume=12 |issue=5 |page=200 |bibcode=2020Diver..12..200P }}</ref> This means there is a high amount of cryptic diversity within ''Micractinium''.<ref name=KTB2022/>

==References== {{Reflist|refs= <ref name="Algaebase">{{cite web | url = http://www.algaebase.org/browse/taxonomy/?id=#6969 | publisher = AlgaeBase World-wide electronic publication, National University of Ireland, Galway | access-date = 2022-03-02 | title = Genus: ''Micractinium'' taxonomy browser |author1=Guiry, M.D. |author2=Guiry, G.M. |name-list-style=amp }}</ref>

<ref name=ProsPitsDari20>{{Citation |mode=cs1 |last1=Pröschold |first1=Thomas |last2=Pitsch |first2=Gianna |last3=Darienko |first3=Tatyana |date=2020 |title=''Micractinium tetrahymenae'' (Trebouxiophyceae, Chlorophyta), a New Endosymbiont Isolated from Ciliates |journal=Diversity |volume=12 |issue=5 |page=200 |doi=10.3390/d12050200 |name-list-style=amp |doi-access=free |bibcode=2020Diver..12..200P }}</ref> }}

Category:Trebouxiophyceae genera Category:Chlorellaceae Category:Freshwater algae