Nitrogen and sulfur metabolisms encoded in prokaryotic communities associated with sea ice algae

Sea ice habitats harbour seasonally abundant microalgal communities, which can be highly productive in the spring when the availability of light increases. An active, bloom-associated prokaryotic community relies on these microalgae for their organic carbon requirements, however an analysis of the encoded metabolic pathways within them is lacking. Hence, our understanding of biogeochemical cycling within sea ice remains incomplete. Here, we generated metagenomic assembled genomes from the bottom of first-year sea ice in northwestern Hudson Bay, during a spring diatom bloom. We show that the prokaryotic community had the metabolic potential to degrade algal derived dimethylsulphoniopropionate and oxidise sulfur. Facultative anaerobic metabolisms, specifically, dissimilatory nitrate reduction and denitrification were also prevalent here, suggesting some sea ice prokaryotes are metabolically capable of adapting to fluctuating oxygen levels during algal bloom conditions. Such denitrification could be an important loss of fixed-N₂ in the changing Arctic marine system.