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. The Bay of Bengal (BoB) has long stood as a biogeochemical enigma, withsubsurface waters containing extremely low, but persistent, concentrationsof oxygen in the nanomolar range which – for some, yet unconstrained, reason –are prevented from becoming anoxic. One reason for this may be the lowproductivity of the BoB waters due to nutrient limitation and the resultinglack of respiration of organic material at intermediate waters. Thus, theparameters determining primary production are key in understanding whatprevents the BoB from developing anoxia. Primary productivity in the sunlitsurface layers of tropical oceans is mostly limited by the supply ofreactive nitrogen through upwelling, riverine flux, atmospheric deposition,and biological dinitrogen ( N2 ) fixation. In the BoB, a stablestratification limits nutrient supply via upwelling in the open waters, andriverine or atmospheric fluxes have been shown to support only less than one-quarter of the nitrogen for primary production. This leaves a largeuncertainty for most of the BoB's nitrogen input, suggesting a potentialrole of N2 fixation in those waters. Here, we present a survey of N2 fixation and carbon fixation in the BoBduring the winter monsoon season. We detected a community of N2 fixerscomparable to other oxygen minimum zone (OMZ) regions, with only a fewcyanobacterial clades and a broad diversity of non-phototrophic N2 fixers present throughout the water column (samples collected between 10and 560  m water depth). While similar communities of N2 fixers wereshown to actively fix N2 in other OMZs, N2 fixation rates werebelow the detection limit in our samples covering the water column betweenthe deep chlorophyll maximum and the OMZ. Consistent with this, no N2 fixation signal was visible in δ15N signatures. We suggest thatthe absence of N2 fixation may be a consequence of a micronutrientlimitation or of an O2 sensitivity of the OMZ diazotrophs in the BoB.Exploring how the onset of N2 fixation by cyanobacteria compared tonon-phototrophic N2 fixers would impact on OMZ O2 concentrations,a simple model exercise was carried out. We observed that both photic-zone-based and OMZ-based N2 fixation are very sensitive to evenminimal changes in water column stratification, with stronger mixingincreasing organic matter production and export, which can exhaustremaining O2 traces in the BoB.

No nitrogen fixation in the Bay of Bengal?