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. [ABSTRACT FROM AUTHOR]