Dissolved oxygen gradients regulate microbial community assembly, metabolic function, and biogeochemical cycling in the southern Indian Ocean

Dissolved oxygen (DO) is a fundamental determinant in assessing the integrity and resilience of marine ecosystems. However, the influence of DO fluctuations on microbial community assembly, metabolic potentials, and interactions remain unresolved from the open ocean surface to the seafloor, especially in the Ninety East Ridge (NER) region of the southern Indian Ocean. Here, DO profiles from 30 to 1000 m water depths at six stations proximate to the NER were characterized. Three stations exhibited significantly lower DO concentrations at mesopelagic-to-bathypelagic depths, which was potentially modulated by the local topography. Comparative analyses of microbial taxonomic diversity and metabolic potential between lower DO and normal DO stations were conducted by integrating 16S rRNA gene sequencing and metagenomic profiling. Our results show distinct DO-driven shifts, with reduced metabolic diversity and enhanced heterotrophic pathways under lower DO. Co-occurrence network topology found prevalent positive interactions in normal DO communities versus more negative relationships under lower DO. Notably, sulfur oxidation and denitrification were tightly coupled, especially at depths with lower DO, with sulfur-oxidizing denitrifiers identified as key players mediating nitrogen and sulfur transformations. This study found that the NER’s topography shapes oxygen availability, which in turn structures microbial ecological strategies and biogeochemical transformations in the deep ocean.