Shifts in bacterial communities reveal subtle biogeochemical regimes of the central Indian Ocean

In microbial ecology studies, environmental factors are typically used to characterize and differentiate between communities. However, we can also use a reverse approach, whereby shifts in microbial community composition can reveal subtle changes in the underlying biogeochemical functioning of the region. To this end, we used 16S rRNA amplicon sequencing to identify variation in the biogeochemical functioning of the poorly characterized central Indian Ocean. In 2016, 230 DNA samples were collected from Western Australia to the Bay of Bengal on GO-SHIP line I09N. We found that the bacterial communities partitioned into seven distinct clusters. Some clusters could be directly associated with known biogeochemical regimes, including the colder Western Australian Current, the Bay of Bengal, and the core of the southern subtropical gyre. However, we also observed two distinct clusters that covered portions of the Bay of Bengal and equatorial regions. From the microbial community data, we hypothesize that these communities reflect regimes with reduced nutrient stress. Additionally, one cluster included samples from the Sri Lanka upwelling region and other regions, which may reflect communities adapted to deeper mixing, lower temperature, and little nutrient limitation. A final unique cluster was present south of the equator in conjunction with a shoaling nutricline. We hypothesize that this region is characterized by mild Fe-stress as well as nitrogen and Fe co-limitation, but the absence of certain microbial lineages suggests that this equatorial section is not a classical HNLC region with Liebig-style Fe-limitation. It would have been challenging to identify this regional separation in biogeochemical functioning from chemical and physical measurements alone. Thus, our study demonstrates how shifts and similarities in microbial communities provide an integrated approach for evaluating links between the biological, biogeochemical, and physical dynamics of the Indian Ocean.