Chloromonas arctica sp. nov., a psychrotolerant alga from snow in the High Arctic (Chlamydomonadales, Chlorophyta)

With the advent of molecular phylogenetic methods, it has become possible to assess the bioversity of snow algae more accurately. In this study, we focused on a morphological, ultrastructural and taxonomic description of a new Chloromonas-like alga isolated from snow in the High Arctic (Svalbard). Light and transmission electron microscopy revealed broad ellipsoidal or ellipsoidal-cylindrical, occasionally spherical cells with a chloroplast without a pyrenoid, an inconspicuous eyespot and a papilla. The size difference and the aforementioned morphological traits clearly distinguished the alga from its closest counterparts within the genus Chloromonas. Moreover, we were able to cultivate the alga at both 5 and 20 °C, revealing the psychrotolerant nature of the strain. Phylogenetic analyses of the plastid rbcL and nuclear 18S rRNA gene showed that the alga is nested within a clade containing a number of psychrotolerant strains within the Chloromonadinia phylogroup (Chlorophyceae). In the rbcL phylogeny, the alga formed an independent lineage, sister to the freshwater species Chloromonas paraserbinowii. Comparisons of secondary structure models of a highly variable ITS2 rDNA marker showed support for a distinct species identity for the new strain. The ITS2 secondary structure of the new isolate differed from the closest matches 'Chlamydomonas' gerloffii and Choloromonas reticulata by three and five compensatory base changes, respectively. Considering the morphological and molecular differences from its closest relatives, a new psychrotolerant species from the Arctic, Choromonas arctica sp. nov., is proposed.