The Bay of Bengal: An Enigmatic Diazotrophic Niche

Biological dinitrogen (N-2) 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-2 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-2 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-2 fixation rates remained unclear and enigmatic. Interestingly, significantly higher N-2 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-2 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. Plain Language Summary The growth of marine phytoplankton is primarily limited by the reactive forms of nitrogen. Dinitrogen-fixing organisms (termed "diazotrophs") supplement this need by providing a natural fertilizer-ammonium. Diazotrophs prefer phosphate and iron-rich, but reactive nitrogen-deficit waters. Therefore, the oligotrophic Bay of Bengal (the northeastern Indian Ocean) with excess phosphate relative to reactive nitrogen and high iron input could be a potential niche for diazotrophy. However, our results indicate that N-2 fixation rates were low and supported less than 2% of organic matter formation in the Bay of Bengal during the spring inter-monsoon. Surprisingly, N-2 fixation rates were higher below the oxygen minimum zone than within it. Thus, our study provides additional evidence that N-2 fixation is feasible (low but persistent) in dark marine places with abundant oxygen in conjunction with surface ocean.