Description of four new species of <i>Mychonastes</i> (Chlorophyceae) from Japanese freshwater habitats based on molecular and morphological data

SUMMARY Species of the green algal genus Mychonastes have been recognized as ubiquitous ultrasmall components of freshwater communities worldwide, mainly based on molecular data. However, detailed taxonomic identification of Japanese members of Mychonastes has not yet been conducted. We taxonomically identified species of 13 Mychonastes strains in the Microbial Culture Collection at the National Institute for Environmental Studies originating from Japan based on molecular data and comparative morphology. The strains were classified into 10 species, including four new species, Mychonastes ariiae Nozaki sp. nov. (NIES‐2900), Mychonastes takamurae Nozaki sp. nov. (NIES‐2334, 2336), Mychonastes kasaiae Nozaki sp. nov. (NIES‐3743) and Mychonastes gigas Nozaki sp. nov. (NIES‐2340). Ultrastructural features of the cell walls and chloroplasts of these four species were consistent with the generic concept of Mychonastes . The four new species are distinguishable from other species of Mychonastes based on differences in nuclear ribosomal DNA (rDNA) internal transcribed spacer (ITS) sequences and morphological features including the mucilaginous matrix surrounding the cells, mode of rupture of the mother cell wall during daughter cell release, formation of large colonies in liquid culture and colony morphology on agar plates. The evolution of Mychonastes species is discussed based on phylogenetic inferences obtained using a combined dataset for four chloroplast genes ( psaB , rbcL , psbC and atpB ) and rDNA ITS sequences.