The Bay of Bengal: An Enigmatic Diazotrophic Niche

Biological dinitrogen (N<inf>2</inf>) fixation is meagerly explored in the Bay of Bengal (Bay). Stratified, warm, oligotrophic (but relatively high iron and phosphate) and oxygen minimum zone (OMZ) waters of the Bay might be a niche for diazotrophs. Therefore, we conducted N<inf>2</inf> fixation rate measurements during the spring inter-monsoon in the euphotic zone, the OMZ and below the OMZ down to 1,500 m depth near the coastal and in the central Bay. We further assessed primary production and cyanobacterial community composition along with their potential environmental controlling parameters. N<inf>2</inf> fixation rates in the euphotic zone were low (0.02–0.38 nmol N L⁻¹ d⁻¹) and their contribution to primary production was small (<2%). Despite conducive conditions for diazotrophy in the Bay, the reason for the relatively low euphotic zone and OMZ N<inf>2</inf> fixation rates remained unclear and enigmatic. Interestingly, significantly higher N<inf>2</inf> fixation rates occurred below the OMZ (>600 m depth), ranging from 0.06 to 0.11 nmol N L⁻¹ d⁻¹ where oxygen concentrations ranged between 0.5 and 1.6 mL L⁻¹, rather than within the OMZ where rates ranged from 0.02 to 0.08 nmol N L⁻¹ d⁻¹ and oxygen concentrations were ≤0.5 mL L⁻¹. Euphotic zone N<inf>2</inf> fixation showed seasonality in the Bay with increasing rates from spring to summer season, perhaps owing to increasing Fe flux as the summer monsoon approaches its peak.