Micractinium pusillum (Fresenius,1858). Usually found in four-celled colonies (only three of the four cells of this colony are seen here). Each cell has two to seven long, delicate setae. Each cell has one cup-shaped plastid with a pyrenoid. 18S ribosomal RNA gene studies place this genus in the Trebouxiophyceae. One of the distinguishing features of trebouxiophytes is metacentric mitosis in which centrioles are located near the metaphase plate of the chromosomes rather than at the spindle poles. Other characteristics include formation of microtubules parallel to the dividing cell wall during cytokinesis (phycoplast). The similar genus, Errerella, has only one spine per cell.Collected from a freshwater pond near Boise, Idaho. July 2005. Phase contrast.
