British distribution (genus): Widespread, very common.
A large genus of several hundred species, mostly in freshwater but some in marine plankton and a number in soil. Different species can tolerate oligotrophic to eutrophic conditions, or even organic pollution. Terrestrial species can be resistant to dessication. C. nivalis is a famous cause of "red snow" in arctic or alpine locations and is known from late snow beds in the Cairgorm mountains in Scotland.
C. eugametos and C. reinhardtii are important experimental organisms used for a range of studies including molecular genetics and physiology.
Photograph of material in culture, 1998, species unnamed, possibly C. reinhardtii.