Particle-Associated Prokaryotic Community Structures in the Bering and Chukchi Seas

Free-living and particle-associated bacteria play distinct roles in the cycling of organic matter, but how these lifestyles are partitioned in the highly productive, rapidly warming Bering and Chukchi Seas is largely unknown. During the Arctic Shelf Growth, Advection, Respiration and Deposition Rate Experiments (ASGARD) cruise in June 2017, a period of anomalously warm sea surface temperatures, we collected 78 samples at three depths (surface, middle, and bottom) at 12 process stations and sequentially filtered them onto 20 µm, 3 µm, and 0.2 µm filters. Using 16S rRNA gene amplicon sequencing and hierarchical clustering of amplicon sequence variant (ASV) relative abundances, we resolved four sample clusters. Although pore size was not used to build the clusters, communities separated into a free-living cluster associated mainly with Anadyr Water (FL1; 0.2 µm fraction) and three particle-associated clusters (PA1–PA3; 3 and 20 µm fractions), several of which were enriched in Bacteroidia. Alphaproteobacteria, Gammaproteobacteria, and Bacteroidia were present in all size fractions, with Gammaproteobacteria most biased toward the free-living fraction. Community structure did not differ with depth, consistent with the shallow, weakly stratified shelf, and distance-based redundancy analysis identified salinity as the sole significant environmental factor distinguishing communities. These results suggest that lifestyle and water-mass origin, rather than depth, structure size-fractionated bacterial communities on the Pacific Arctic shelf, and that freshening and changes in bloom dynamics could reshape particle-associated communities in this region.