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Micractinium
	Micractinium pusillum
Scientific classification
Clade:	Viridiplantae
Division:	Chlorophyta
Class:	Trebouxiophyceae
Order:	Chlorellales
Family:	Chlorellaceae
Genus:	Micractinium; Fresinius, 1858
Type species
	Micractinium pusillum;
Species
	See text.

Micractinium is a genus of green algae in the family Chlorellaceae.^[1] Species of the genus Micractinium are common in freshwater habitats. A few species are found as endosymbionts of protozoa, such as Micractinium conductrix and Micractinium tetrahymenae.^[2]

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,^[3] and are polyhedral or pyramidal in shape.^[2] Most^[2] species generally produce long narrow bristles that taper from base to tip. Cells have a single cup-shaped chloroplast with a single pyrenoid.^[3]

Micractinium is closely related, and similar to, the genus Chlorella. Both lack 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 never produce bristles, such as those that are endosymbiotic within other organisms.^[2]

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.^[4] Accordingly, Micractinium has been transferred to the family Chlorellaceae.^[1]

Species

As of March 2022^[update], AlgaeBase accepted the following species:^[1]

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

References

^ ^a ^b ^c Guiry, M.D. & Guiry, G.M. "Genus: Micractinium taxonomy browser". AlgaeBase World-wide electronic publication, National University of Ireland, Galway. Retrieved 2022-03-02.
^ ^a ^b ^c ^d Krivina, Elena; Temraleeva, Anna; Sinetova, Maria (2022). "New species Micractinium kostikovii (Chlorellaceae, Trebouxiophyceae) from Russia". Phycological Research. 70: 22–34. doi:10.1111/pre.12469. S2CID 239494013.
^ ^a ^b Bicudo, Carlos E. M.; Menezes, Mariângela (2006). Gêneros de Algas de Águas Continentais do Brasil: chave para identificação e descrições (2 ed.). RiMa Editora. p. 508. ISBN 857656064X.
^ Pröschold, Thomas; Pitsch, Gianna & Darienko, Tatyana (2020). "Micractinium tetrahymenae (Trebouxiophyceae, Chlorophyta), a New Endosymbiont Isolated from Ciliates". Diversity. 12 (5): 200. doi:10.3390/d12050200.