Taxonomic revision of Chloromonas nivalis (Volvocales, Chlorophyceae) strains, with the new description of two snow-inhabiting Chloromonas species

Chloromonas nivalis (Volvocales, Chlorophyceae) is considered a cosmopolitan species of a snow-inhabiting microalga because cysts morphologically identifiable as zygotes of the species are distributed worldwide. However, recent molecular data demonstrated that field-collected cysts identified as the zygotes consist of multiple species. Recently, we demonstrated that species identification of snow-inhabiting Chloromonas species is possible based on light and electron microscopy of asexual life cycles in strains and molecular phylogenetic analyses. Vegetative cells without eyespots and of inverted-teardrop shape have been reported once in North American material of C. nivalis; however, strains with such vegetative cells in snow-inhabiting species of Chloromonas have not been examined taxonomically in detail. Here, we used light and transmission electron microscopy together with molecular analyses of multiple DNA sequences to examine several C. nivalis strains. The morphological data demonstrated that one North American strain could be identified as C. nivalis, whereas three other strains should be re-classified as C. hoshawii sp. nov. and C. remiasii sp. nov. based on vegetative cell morphology, the number of zoospores within the parental cell wall during asexual reproduction, and whether cell aggregates (resulting from repeated divisions of daughter cells retained within a parental cell wall) were observed in the culture. This taxonomic treatment was supported by multigene phylogeny and comparative molecular analyses that included a rapidly evolving DNA region. Our molecular phylogenetic analyses also demonstrated that the North American strain of C. nivalis was phylogenetically separated from the Austrian and Japanese specimens previously identified as C. nivalis based on zygote morphology.