Snow alga Sanguina aurantia as revealed through de novo genome assembly and annotation.

To thrive on melting alpine and polar snow, some Chlorophytes produce an abundance of astaxanthin, causing red blooms, often dominated by genus Sanguina. The red cells have not been cultured, but we recently grew a green biciliate conspecific with Sanguina aurantia from a sample of watermelon snow. This culture provided source material for Oxford Nanopore Technology and Illumina sequencing. Our assembly pipeline exemplifies the value of a hybrid long- and short-read approach for the complexities of working with a culture grown from a field sample. Using bioinformatic tools we separated assembled contigs into two genomic pools based on a difference in GC content (57.5% and 55.1%). We present the data as two assemblies of S. aurantia variants but explore other possibilities. High-throughput chromatin conformation capture analysis (Hi-C sequencing) was used to scaffold the assemblies into a 96 MB genome designated 'A' and a 102 MB genome designated 'B'. Both assemblies are highly contiguous: genome A consists of 38 scaffolds with an N50 of 5.4 Mb while genome B has 50 scaffolds with an N50 of 6.4 Mb. RNA-sequencing was used to improve gene annotation.

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.