High prokaryotic diversity in the oxygen minimum zone of the Bay of Bengal: implications for nutrient cycling

Abstract

Primary productivity in the Bay of Bengal remains relatively low throughout the year, yet it harbors the fourth most intense oxygen minimum zone (OMZ) in the ocean. Despite the unique characteristics of the OMZ in the Bay, including trace oxygen levels, the microbial community and its biogeochemical role remain understudied. We investigated prokaryotic diversity in the euphotic zone and the OMZ of the Bay of Bengal. Alpha diversity was significantly higher in the OMZ compared to the surface waters. Community structures varied significantly between the coastal and the open ocean and within the OMZ across different oxygen levels. Among bacterial phyla, Proteobacteria dominated, followed by Cyanobacteria and Actinobacteria, while Thermoplasmata dominated among Archaea. Cyanobacteria, Actinobacteria, and Bacteroidota predominated in the euphotic zone. In contrast, higher abundances of Marinimicrobiota, Marine Group B, Crenarchaeota, Planctomycetota, Chloroflexi, Verrucomicrobiota, Nitrospinota, and Acidobacteriota were observed in the OMZ. At the genus level, Prochlorococcus, Candidatus (Ca.) Actinomarina, clade Ia, and SAR86 dominated in the euphotic zone, whereas SAR324, SAR406 clade, Ca. Nitrosopumilaceae, and SUP05 were prevalent in the OMZ. A positive correlation between cyanobacterial abundances and nitrogen fixation rates emphasized their potential role in diazotrophy. SUP05 likely contributes to sulfur oxidation and the reduction of nitrate and nitrite in the OMZ. Predictive functional gene analysis suggests that denitrifiers, anammox bacteria, and sulfur oxidizers prevailed in the OMZ. Our study highlights distinct prokaryotic diversity across the water column and their contributions to the nitrogen and sulfur cycling in the Bay of Bengal.