Prokaryotic Community Structures in the Bering and Chukchi Seas

Bacteria are essential to marine ecosystems as they remineralize organic matter, cycle carbon, nitrogen, and other key elements, and support higher trophic levels through food webs. The Bering and Chukchi Seas off the coast of Alaska are among the most productive regions in the world, with intense seasonal algal blooms, yet their bacterial communities remain poorly characterized, and the region is warming rapidly as heat transport from the Pacific Ocean increases. Here, we used 16S rRNA gene amplicon sequencing of 181 samples collected on 0.2 {micro}m filters at three depths (surface, middle, and bottom) across 62 stations during the Arctic Shelf Growth, Advection, Respiration and Deposition Rate Experiments (ASGARD) cruise in June 2017, when sea surface temperatures were anomalously warm. Hierarchical clustering of amplicon sequence variant (ASV) relative abundances resolved 11 sample clusters in three divisions, all dominated by Alphaproteobacteria, Gammaproteobacteria, and Bacteroidia. Community structure was related to water mass, salinity, and bloom stage, but not to depth. Clusters ranged from Gammaproteobacteria-dominant, Bacteroidia-poor assemblages in warm and cool Shelf Water (Division A) to Bacteroidia-rich assemblages spanning pre- to post-bloom conditions (Divisions B and C). In distance-based redundancy analysis, temperature, salinity, dissolved oxygen, nitrate, and fluorescence together explained 14.8% of the variation in community composition, with salinity the strongest contributor. These results indicate that water-mass origin, physicochemical conditions, and bloom stage jointly shape bacterial communities, and that changes in temperature, salinity, and the magnitude and timing of blooms could substantially alter bacterial communities of this Pacific gateway to the Arctic.