Community composition, co-occurrence, and environmental drivers of bacterioplankton community in surface and 50-m water layers in the subarctic North Pacific

The Western Subarctic Gyre(WSG) is one of the two gyre-systems in the subarctic North Pacific known for high nutrient and low-chlorophyll waters. However, the bacterioplankton in marine water of this area, either in terms of the taxonomic composition or functional structure, remains relatively unexplored. A total of 22 sampling sites from two water layers(surface water, SW and 50-m layer water,FW) were collected in this area. The physiochemical parameters of waters, Synechococcus, and bacterial density, as well as the bacterioplankton community composition and distribution pattern, were analyzed.The nutrient concentrations of DIN, DIP, and DSi, Chl-a concentration, and the average abundance of heterobacteria in FW were higher than those in SW. However, temperature and the average abundance of Synechococcus and pico-eukaryotes were higher in SW. A total of 3 269 OTUs were assigned, and 2 123OTUs were commonly shared; moreover, similar alpha diversity patterns were observed in both SW and FW. The bacterioplankton community showed significantly obvious correlation with salinity, DIP, DIN,and Chl a in both SW and FW. Proteobacteria, Cyanobacteria, Bacteroidota, Actinobacteriota, and Firmicutes were the main phyla while Synechococcus＿CC9902, Psychrobacter, and Sulfitobacter were the dominant genera in each sampling site. Most correlations that happened between the OTUs in the cooccurrence network were positive and inter-module. Higher edges and graph density were found in SW,indicating that more correlations occurred, and the community was more complex in SW. This study provided novel knowledge on the bacterioplankton community structure and the correlation characteristics in WSG.