A Winter-to-Summer Transition of Bacterial and Archaeal Communities in Arctic Sea Ice

The Arctic is warming 2–3 times faster than the global average, leading to a decrease in Arctic sea ice extent, thickness, and associated changes in sea ice structure. These changes impact sea ice habitat properties and the ice-associated ecosystems. Sea-ice algal blooms provide various algal-derived carbon sources for the bacterial and archaeal communities within the sea ice. Here, we detail the transition of these communities from winter through spring to early summer during the Norwegian young sea ICE (N-ICE2015) expedition. The winter community was dominated by the archaeon <i>Candidatus</i> Nitrosopumilus and bacteria belonging to the <i>Gammaproteobacteria</i> (<i>Colwellia</i>, <i>Kangiellaceae,</i> and <i>Nitrinocolaceae</i>), indicating that nitrogen-based metabolisms, particularly ammonia oxidation to nitrite by <i>Cand.</i> Nitrosopumilus was prevalent. At the onset of the vernal sea-ice algae bloom, the community shifted to the dominance of <i>Gammaproteobacteria</i> (<i>Kangiellaceae, Nitrinocolaceae</i>) and <i>Bacteroidia</i> (<i>Polaribacter</i>), while <i>Cand.</i> Nitrosopumilus almost disappeared. The bioinformatically predicted carbohydrate-active enzymes increased during spring and summer, indicating that sea-ice algae-derived carbon sources are a strong driver of bacterial and archaeal community succession in Arctic sea ice during the change of seasons. This implies a succession from a nitrogen metabolism-based winter community to an algal-derived carbon metabolism-based spring/ summer community.