Controls on microbial community composition in hadal trench sediments

Despite recent advances in hadal research, our understanding of this environment is still in its infancy. Previous studies on hadal sediments already indicate the existence of an intrinsic microbial community, that is clearly distinct from adjacent abyssal environments. Factors governing this dissimilarity include biogeochemical differences, depositional regimes, and adaptation to extreme pressure. To explore how such factors affect hadal microbial community composition we collected over 400 sediment samples at high vertical resolution from 10 locations in the Kermadec and Atacama trenches and analyzed community composition based on deep sequencing of the V4-V5 region of the 16S rRNA gene. These results were compared to parallel detailed biogeochemical investigations. The analysis revealed distinct microbial communities in each of the trenches, but these communities were still more similar to each other than to their adjacent abyssal plain counterparts. Strikingly, although coring locations along the trench axis were separated by hundreds of kilometers, the communities showed higher similarity between locations for a given sediment horizon than across the sediment horizons at a given location, though horizons were only centimeters apart. The parallel downcore variation in community composition between locations occurred despite an irregular depositional regime where mass wasting events cause irregular depth profiles of organic carbon and microbial abundance. Ultimately, our results indicate that diagenetic gradients and the resulting biogeochemical zonation play a dominant role in shaping the overall community composition here.