Taxonomic reassessment of the genus Mychonastes (Chlorophyceae, Chlorophyta) including the description of eight new species

The generic concept of the chlorophycean green algae Mychonastes (preferably living solitary) and Pseudodictyosphaerium (preferably living in colonies) is evaluated by means of small-subunit and internal transcribed spacer (ITS) rRNA gene sequence analyses and microscopical observations. Members of these genera belong to the most common chlorophycean algae of the picoplankton and small-sized nanoplankton of fresh and brackish waters. The morphological difference solitary vs colonial life form is not reflected by molecular phylogenetic data, and it is therefore systematically irrelevant. Consequently, all taxa of the two genera are combined under the generic name Mychonastes. The genus Mychonastes is emended. The synonymy of Mychonastes homosphaera (formerly Chlorella homosphaera) and Chlorella minutissima is confirmed. Eight new species are described from inland waters of Europe, Africa, Asia and South America. Besides certain morphological criteria such as shape and size of the cells and appearance and organization of the mucilaginous strands among the cells of colonies, the main diacritic features are of genetic nature. Comparing the secondary structure of ITS-2, compensatory base changes are found to discriminate the new species. The species of Mychonastes form a monophyletic clade within the Sphaeropleales sensu lato.