Micractinium

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]

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]