Ultrastructure, cell wall composition, and phylogenetic position of Pseudodictyosphaerium jurisii (Chlorococcales, Chlorophyta) including a comparison with other picoplanktonic green algae

A pure culture of a common colonial freshwater picoplanktonic chlorophyte, Pseudodictyosphaerium jurisii, was isolated from Lake Stechlin (Brandenburg , Germany). This strain was compared with solitary picochlorophytes with re spect to morphology, ultrastructure, and the biochemical composition of the cell walls. Its phylogenetic position and relationships were determined by analysis of 18S rDNA sequence data. It is similar in cell morphology and u ltrastructure to Choricystis minor, Mychonastes homosphaera, and the marine picoplanktonic alga Nanochlorum eucaryotum. In their ultrastructure, bioch emistry of the cell wall, and 18S rDNA, these picoplanktonic algae are clea rly different from Chlorella species, and these features indicate a close r elationship between Pseudodictyosphaerium and Mychonastes. In contrast to N annochloris, which propagates by binary fission, Pseudodictyosphaerium, Cho ricystis, and Mychonastes multiply by autosporulation. This study provides further evidence that picoplanktonic chlorophytes evolved independently in several lineages among the Chlorophyceae and Trebouxiophyceae.