Depth‐Dependent Dynamics and Assembly Mechanisms of Bacterial Communities in the Eastern Indian Ocean

Abstract Bacteria play a crucial role in the biogeochemical cycling of the oligotrophic ocean, which constitutes over 50% of the ocean surface and is expected to expand globally due to climate change. Although substantial data exist on bacterial diversity and structure in various environments, their biogeographic patterns, co‐occurrence, and assembly processes—especially in vital oligotrophic regions like the Eastern Indian Ocean (EIO)—remain poorly understood. This study employed 16S rRNA gene high‐throughput sequencing and multiple statistical analyses to investigate the spatial dynamics and assembly mechanisms of bacterial communities across a 2,100 km transect in the EIO, sampling at depths of 5, 25, 50 m, DCM, 100, 150, and 200 m during the intermonsoon (March–May 2022) period. The results indicated that bacterial community diversity, structure, and taxonomic composition varied more significantly with depth than latitude. Additionally, community diversity positively correlated with depth. Distance‐decay analysis revealed that community similarity was influenced by both depth and environmental factors. Mantel test showed that all environmental factors except for NH 4 + and salinity significantly impacted the decay relationship between bacterial community similarity and environmental distance. Null model analysis revealed that stochastic processes (particularly drift) predominantly shaped the assembly of bacterial communities, with their contribution declining with depth. Furthermore, the analysis of co‐occurrence network of bacterial communities in different water layers suggested that biotic interactions (e.g., mutualism) play a significant role in balancing determinism and stochasticity. These findings provide valuable insights into characteristics of bacterial communities in the EIO and enhance our understanding of the mechanisms that sustain bacterial diversity in extreme oligotrophic environments.