Abstract
Historically, our understanding of bacterial ecology in the Indian Ocean has been limited to regional studies that place emphasis on community structure and function within oxygen‐minimum zones. Thus, bacterial community dynamics across the wider Indian Ocean are largely undescribed. As part of Bio‐GO‐SHIP, we sequenced the 16S rRNA gene from 465 samples collected on sections I07N and I09N. We found that (1) there were 23 distinct bioregions within the Indian Ocean, (2) the southeastern gyre had the largest gradient in bacterial alpha‐diversity, (3) the Indian Ocean surface microbiome was primarily composed of a core set of taxa, and (4) bioregions were characterized by transitions in physical and geochemical conditions. Overall, we showed that bacterial community structure spatially delineated the surface Indian Ocean and that these microbially defined regions were reflective of subtle ocean physical and geochemical gradients. Therefore, incorporating metrics of in situ microbial communities into marine ecological regions traditionally defined by remote sensing will improve our ability to delineate warm, oligotrophic regions.