Uniform selective pressures within redox zones drive gradual changes in microbial community composition in hadal sediments.

Microbial communities in marine sediments are highly diverse, yet the processes that give rise to this complexity are unclear. It has been proposed that benthic microbial communities must be continuously re-seeded from the water column because dispersal within the sediment is severely limited. Previous studies consistently report that the composition of the microbial community gradually changes with sediment depth. However, the relative contributions of the processes that underlie these compositional gradients have not been determined, and it is unknown whether microbial dispersal is indeed too slow to outpace burial. Here, we applied ecological statistical frameworks to 16S rRNA gene amplicon-based community composition data from Atacama Trench sediments to investigate the links between biogeochemistry, burial, and microbial community assembly processes. We confirm that dispersal limitation affects microbial communities and find that gradual changes in community composition are driven by selective pressures that change abruptly across the discrete boundaries between redox zones rather than along continuous biogeochemical gradients, while selective pressures are uniform within each zone. The gradual changes in community composition over centimetres of depth within a zone hence reflects a decades-long response to the abruptly changing selective pressures.