Diversity of heterotrophic picoeukaryotes in the ice of the Kandalaksha Gulf (White Sea, Russia) based on rRNA gene high-throughput sequencing

Contemporary climate change in the Arctic is causing the reduction of the ice habitat. This process induces rearrangements in the community composition of ice-dwelling microbial eukaryotes, with heterotrophic picoeukaryotes being one of the least studied groups. Here, we report the results of a DNA metabarcoding investigation of heterotrophic picoeukaryote diversity in the ice of the Kandalaksha Gulf of the White Sea by Illumina high-throughput sequencing of the 18S rRNA V4 gene region. In total, 121 operational taxonomic units (OTUs) belonging to heterotrophic protists were revealed. The communities of heterotrophic picoeukaryotes in first-year ice were represented by seven eukaryotic domains (Stramenopiles, Alveolata, Rhizaria, Cryptista, Haptista, Apusozoa, Opisthokonta) and within 15 phyla. Rhizaria was the most dominant domain accounting for 48% of the total relative read abundance and included only Cercozoa. The taxonomic composition of heterotrophic picoeukaryotes was analyzed in detail with attention to rare and important microbial eukaryotes and unusual finds in sea ice habitats, such as the parasitic Perkinsea. Unknown Cercozoa clade was revealed. We have demonstrated that the White Sea heterotrophic picoeukaryote communities are diverse but insufficiently studied. Only 39% of OTUs were classified down to the order, family, or genus level, and only 11% of OTUs were classified to the genus level. This demonstrates that many unsequenced unicellular eukaryotes are found in sea ice and highlights some limitations of the V4 18S rRNA gene metabarcoding approach—the incompleteness of databases (lack of reference sequences) and shortness of the V4 region (inability to classify OTUs to species level).