Selenastrum densum sp. nov., a new member of the genus Selenastrum (Selenastraceae, Chlorophyceae, Chlorophyta)

The genus Selenastrum comprises common coccoid green algae found in diverse habitats worldwide. This genus has a complex taxonomic history, but recent applications of molecular phylogenetic methods have removed some members with similar morphologies from Selenastrum and established new genera. However, due to the lack of available molecular sequences and isolates, the species diversity of the genus has not been fully explored. We conducted a detailed examination of the taxonomy of genus Selenastrum based on 11 new strains collected from China. The multi-disciplinary study utilized morphology, ultrastructure, and phylogeny based on multiple molecular markers, and ITS-2 secondary structure. All the included strains of genus Selenastrum were clustered into two distinct clades. The members of one clade were similar morphologically to the type species Selenastrum bibraianum, whereas the strains of the other clade had distinctly different colony structures. Based on the latter clade’s larger colony cell numbers and independent phylogenetic position, we proposed that it as a new species of the genus Selenastrum, namely Selenastrum densum sp. nov. Additional taxa sampling and molecular data will help to discover additional new species and clarify the validity of morphological taxonomic characteristics within the genus Selenastrum.