Picoplanktonic Choricystis species (Chlorococcales, Chlorophyta)and problems surrounding the morphologically similar 'Nannochloris-like algae'

The morphology, ultrastructure, and 18S rRNA gene sequences of two strains of picoplanktonic chlorophytes isolated from the same lake were compared with a culture of Choricystis minor (Skuja)Fott (Chlorococcales, Chlorophyta)from the culture collection in Göttingen, BRD. It was concluded that the isolated strains were attributable to this species, one of the most common representatives of green picoplankton in freshwater ecosystems. This autosporine alga is often confused with taxa of the genus Nannochloris Naumann (Ulotrichales, Chlorophyta)propagating by binary cell division. Taxonomical problems caused by the misunderstanding of 'Nannochloris-like algae' are discussed. The 18S rRNA phylogenetic tree shows the heterogeneous systematics of picoplanktonic chlorophytes, e.g. Nanochlorum eucaryotum Wilhelm et al. (Chlorococcales, Chlorophyta)and Choricystis minor. These genera belong to different clusters of the phylogenetic tree. Choricystis hindakii Tell should be reduced to synonymy with C. minor.