Rosetta gen. nov. (Chlorophyta): Resolving the identity of red snow algal rosettes.

Thick-walled rosette-like snow algae were long thought to be a life stage of various other species of snow algae. Rosette-like cells have not been cultured, but by manually isolating cells from 38 field samples in southern British Columbia, we assigned a variety of rosette morphologies to DNA sequence. Phylogenetic analysis of Rubisco large-subunit (rbcL) gene, ribosomal internal transcribed spacer 2 (ITS2) rRNA region, and 18S rRNA gene revealed that the rosette-like cells form a new clade within the phylogroup Chloromonadinia. Based on these data, we designate a new genus, Rosetta, which comprises five novel species: R. castellata, R. floranivea, R. stellaria, R. rubriterra, and R. papavera. In a survey of 762 snow samples from British Columbia, we observed R. floranivea exclusively on snow overlying high-elevation glaciers, whereas R. castellata was observed at lower elevations, near the tree line. The other three species were rarely observed. Spherical red cells enveloped in a thin translucent sac were conspecific with Rosetta, possibly a developmental stage. These results highlight the unexplored diversity among snow algae and emphasize the utility of single-cell isolation to advance the centuries-old problem of disentangling life stages and cryptic species.

The underlying green biciliate morphology of the orange snow alga Sanguina aurantia.

In the summer, blooms of microalgae appear on alpine and polar snowfields, creating expanses of red snow sometimes called 'watermelon snow'1. These blooms are attracting research attention because they decrease snow albedo, thereby accelerating the effects of global warming on snowmelt2. Currently, meltwater from alpine snowfields provides one-sixth of the world's population with water for drinking, agriculture, and the generation of hydroelectric power3. Each spring, the surface of new snow is colonized by microscopic organisms from unknown sources. One possibility is that when the melt begins, ciliated cells swim up from the substrate below to populate the snow surface. However, Sanguina, a cosmopolitan genus that frequently dominates high-alpine and arctic blooms4,5, are thick-walled, red or orange in colour, and immotile. Here, we describe a culture of motile green biciliate cells isolated from a sample of red snow. Using cross-referenced Bayesian and maximum-likelihood phylogenetic methods for two genetic markers, ITS2 and rbcL, we establish the green biciliate as belonging to the genus Sanguina. Compensatory-base-change analysis of ITS2 rRNA structure delimits the green culture as S. aurantia, conspecific with individual, thick-walled immotile orange cells, picked from field samples collected in British Columbia and Svalbard. Using single cells was invaluable for comparing sequences derived from thick-walled red and orange Sanguina cells, which do not exist in culture, with the cultured green biciliates.