Protist community structure in Atlantic Arctic through the year as revealed by high throughput sequencing.

Unicellular protists, including microalgae are main suppliers of photosynthetic products that higher trophic levels depend upon. Yet, the Arctic protist community contains a vast unknown diversity and many of the described species are poorly known. Diversity and dynamics throughout the polar night and at great depths remains particularly understudied due to limited access. In the MicroPolar project we asked “Who are present and when during the year?”, “How is the community structured by depth and geography?” and “What are the environmental driving forces for the observed distribution?” Sampling cruises to North and West of Svalbard were carried out five times during 2014 (January, March, May, August and November). Samples were collected at 3-6 stations and four depths (1-1000m). Fifty litres were divided into four size fractions from micro- to picoplankton. The 18S V4 region of nuclear rDNA and the 16S V9 region of chloroplast rDNA and rRNA were high throughput sequenced with Illumina. We obtained c. 35 000 Operational Taxonomic Units (OTUs) clustered at 98% similarity. The protist community composition differed significantly between the dark and light seasons, and between surface water and beyond the photic zone. Also OTU richness varied significantly with depth and season. OTU richness was significantly higher in the deeper layers all months except March. The May samples had lowest richness, whereas August had highest. Both in terms of OTU richness and proportional read abundance Dinophyta was dominating in most samples, except in the May samples representing the spring bloom, where diatoms dominated. The 200 most frequent OTUs were detected all seasons and in both surface and deeper layers. The majority of these were assigned to defined clades consisting of environmental sequences only, such as MAST-clades and Dino-Group clades. This highlights the need for more reference DNA sequences from cultures. Several abundant and frequent OTUs matched well-known Arctic species, such as Chaetoceros gelidus (socialis), Phaeocystis pouchetii, Gyrodinium fusiforme and Micromonas pusilla. Interestingly, OTUs assigned to Picobiliphyta were also among the most ubiquitous OTUs. Picobiliphyta were first described in 2007 in particular from samples from the Arctic Ocean, and all are thought to be heterotrophic. As global climate change is expected to be most severe in the Arctic and may affect microbial communities and biogeochemical cycles more seriously in this region than anywhere else, this study is important to be able to assess the impact of climate change in the Arctic at the microbial level.