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

Abstract

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.

Chloromonas svalbardensis n. sp. with Insights into the Phylogroup Chloromonadinia (Chlorophyceae)

“…Matsuzaki et al. ). Chloromonas svalbardensis exhibits similar characteristics as the Chloromonas type species, Cr.…”

“…nivalis veils multiple species and extended molecular phylogenetic investigations could considerably reshape its taxonomic status and biogeography (Matsuzaki et al. , ). In addition, here we suggest that the genus Chloromonas would be understood just as a part of clade 1 (= core Chloromonas ; Fig.…”

“…The formation of cell aggregates resulting from the repeated divisions of daughter cells within a parental cell wall is currently considered as a diagnostic feature in delineating cold-adapted Chloromonas species (e.g. Matsuzaki et al 2018). Chloromonas svalbardensis exhibits similar characteristics as the Chloromonas type species, Cr.…”

“…For example, species within Chloromonas clade 1 have a massive parietal cup-or urn-shaped chloroplast with a number of continuously connected lobes (the "reticulata" type as already discussed above). Clade 2 bears chloroplast mostly composed of parietal bands/platelets or disks which may or may not be connected to each other (Hoham et al 2006;Matsuzaki et al 2014Matsuzaki et al , 2018 while clade 3 exhibit asteroid chloroplast morphology (Pr€ oschold et al 2001). Furthermore, majority of Chloromonas species within clade 1 have eyespot (with an exception of Cr.…”

“…On the other hand, the currently described species Cr. remiasii Matsuzaki, Nozaki and Kawachi possesses an ellipsoidal to elongated D-shaped eyespot (Matsuzaki et al 2018). For comparison, members of clade 3 have both an eyespot and a central pyrenoid (Pr€ oschold et al 2001).…”