The protist assemblage in the central Arctic Ocean is scarcely surveyed despite them being the major primary producers. Elucidating their response to changing environmental variables requires an a priori analysis of their current diversity, including abundant and rare species. In late summer 2011, samples were collected during the ARK-XXVI/3 expedition (RV Polarstern) to study Arctic protist community structures, by implementation of automated ribosomal intergenic spacer analysis (ARISA) and 454-pyrosequencing. Protist assemblages were related to the hydrology and environmental variables (temperature, salinity, ice coverage, nitrate, phosphate, and silicate). The abundant (a parts per thousand yen1 %) biosphere and rare (< 1 %) biosphere were considered separately in the diversity analysis in order to reveal their mutual relationships. A relation between hydrology and protist community structure was highly supported by ARISA and partially by 454-pyrosequencing. Sea ice showed a stronger influence on the local community structure than nutrient availability, making statements on the water mass influence more difficult. Dinoflagellates (Syndiniales), chlorophytes (Micromonas spp.), and haptophytes (Phaeocystis spp.) were important contributors to the abundant biosphere, while other dinoflagellates and stramenopiles dominated the rare biosphere. No significant correlation was found between the abundant and rare biosphere. However, relative contributions of major taxonomic groups revealed an unexpected stable community structure within the rare biosphere, indicating a potential constant protist reservoir. This study provides a first molecular survey of protist diversity in the central Arctic Ocean, focusing on the diversity and distribution of abundant and rare protists according to the environmental conditions, and can serve as baseline for future analysis.
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