No nitrogen fixation in the Bay of Bengal?

The Bay of Bengal (BoB) has long stood as a bio-geochemical enigma, with subsurface waters containing extremely low, but persistent, concentrations of oxygen in the nanomolar range which - for some, yet unconstrained, reason - are prevented from becoming anoxic. One reason for this may be the low productivity of the BoB waters due to nutrient limitation and the resulting lack of respiration of organic material at intermediate waters. Thus, the parameters determining primary production are key in understanding what prevents the BoB from developing anoxia. Primary productivity in the sunlit surface layers of tropical oceans is mostly limited by the supply of reactive nitrogen through upwelling, riverine flux, atmospheric deposition, and biological dinitrogen (N-2) fixation. In the BoB, a stable stratification limits nutrient supply via upwelling in the open waters, and riverine or atmospheric fluxes have been shown to support only less than one-quarter of the nitrogen for primary production. This leaves a large uncertainty for most of the BoB's nitrogen input, suggesting a potential role of N-2 fixation in those waters. Here, we present a survey of N-2 fixation and carbon fixation in the BoB during the winter monsoon season. We detected a community of N-2 fixers comparable to other oxygen minimum zone (OMZ) regions, with only a few cyanobacterial clades and a broad diversity of non-phototrophic N-2 fixers present throughout the water column (samples collected between 10 and 560m water depth). While similar communities of N-2 fixers were shown to actively fix N-2 in other OMZs, N-2 fixation rates were below the detection limit in our samples covering the water column between the deep chlorophyll maximum and the OMZ. Consistent with this, no N-2 fixation signal was visible in delta N-15 signatures. We suggest that the absence of N-2 fixation may be a consequence of a micronutrient limitation or of an O-2 sensitivity of the OMZ diazotrophs in the BoB. Exploring how the onset of N-2 fixation by cyanobacteria compared to non-phototrophic N-2 fixers would impact on OMZ O-2 concentrations, a simple model exercise was carried out. We observed that both photic-zone-based and OMZ-based N-2 fixation are very sensitive to even minimal changes in water column stratification, with stronger mixing increasing organic matter production and export, which can exhaust remaining O-2 traces in the BoB.