Bio-GO-SHIP: Shifts in bacterial communities reveal subtle biogeochemical regimes across the Indian Ocean

Historically, our understanding of ecological responses to biogeochemical gradients and physical dynamics in the Indian Ocean has been limited to regional studies. Microbial communities represent in-situ biosensors that are sensitive to changes in the surface ocean. They can therefore be used to identify where subtle changes in the environment occur and to understand links between the ecology and surrounding environment. Here, we perform the largest study of microbial biodiversity in the Indian Ocean, using 505 DNA samples collected on GO-SHIP cruises I07N and I09N. This dataset spans a large geographic area, starting in the southern Indian Ocean gyre, crossing through the equatorial zone, and entering the Arabian Sea or the Bay of Bengal. We used 16S rRNA amplicon sequencing to identify transition points in bacterial community structure and to define ecological boundaries. We found that these boundaries aligned with shifts in geochemistry (e.g., nutrient availability) and/or physical dynamics (e.g., ocean fronts, eddies, and salinity), indicating fine-scale regional separation in biogeochemical functioning. Thus, our study demonstrates how using microbial communities provides an integrated approach for evaluating links between the ecology, geochemistry, and physical dynamics of the Indian Ocean.