Sediments as Potential Sources of Non-Cyanobacterial Diazotrophs in Arctic Sea Ice and Seawater.

Although nitrogen fixation has recently been recognized in the Arctic Ocean, the biogeographical patterns of diazotrophs in this region remain poorly resolved, leaving a critical gap in understanding of environmental nitrogen inputs. To address these uncertainties, we examined diazotrophic and overall prokaryotic communities in first-year land-fast sea ice, seawater, and sediments of the Pacific Arctic to identify habitat-specific patterns. Diazotrophic communities in sea ice and sediments exhibited stable structures across all samples, with both habitats consistently dominated by the anaerobic phylum Thermodesulfobacteriota (particularly Desulfuromonadia) and other sulfate-reducing lineages. In contrast, seawater communities were spatially heterogeneous, with some samples showing elevated abundances of Thermodesulfobacteriota. Prokaryotic communities showed marked differences among habitats, with Thermodesulfobacteriota representing a major group in sediments. β-diversity and amplicon sequence variant (ASV)-sharing analyses revealed extensive overlap between sea ice and sediment diazotrophs. Given that sea ice is a transient habitat, these findings suggest that diazotrophs in sea ice originate from underlying sediments. Moreover, the Thermodesulfobacteriota ASVs detected in seawater were identical to those in sediments, indicating likely resuspension. Collectively, our results highlight sediments as key reservoirs of diazotrophs in the Pacific Arctic, particularly on the continental shelf.