No nitrogen fixation in the Bay of Bengal?

Abstract

The Bay of Bengal (BoB) has long stood as a biogeochemical 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 560 m 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 δ 15 N signatures. We sug-gest 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.

Dinitrogen fixation rates in the Bay of Bengal during summer monsoon

“…The N 2 fixation rates observed in our study are higher than the recently reported rates in the Bay for January 2014 (Löscher et al 2020) where the rates at DCM were below detection limit though they found the clusters of N 2 fixing microbes. Löscher et al (2020) targeted oxygen minimum zones for heterotrophic N 2 fixation rates, while most of the N 2 fixation occurs in the surface waters. Carbon uptake rates in their study ranged between 286-1855 nmol C l −1 d −1 at the depth of DCM whereas in our study, it varied from 1.4 to 282.2 nM C hr −1 (or 17 to 3386 nM C d −1 ) in the Sunlit layer of the Bay.…”

“…(more abundant) and unicellular diazotrophic cyanobacteria along with proteobacteria (mainly alpha-, beta-, and gamma-proteobacteria) with higher abundance in the equatorial region. During winter monsoon, Löscher et al (2020) observed Synechococcus and Trichodesmium with no evidence of unicellular diazotrophic cyanobacteria. Diatoms-associated diazotrophs have also been reported during summer monsoon for same sampling stations as ours (Bhaskar et al 2007).…”

A comparison of bacterial communities from OMZ sediments in the Arabian Sea and the Bay of Bengal reveals major differences in nitrogen turnover and carbon recycling potential

“…In contrast, in the BoB-OMZ has been reported less intense than the AS-OMZ (Paulmier, 2009) with DO concentrations still present in the nanomolar range (Bristow et al, 2017). Nitrogen loss has been described as rather insignificant and limited by substrate availability resulting from low organic matter supply by primary production (Bristow et al, 2017;Löscher et al, 2020). The sequence of electron acceptor utilization in such an environment, generally follow the thermodynamic energy yield (Froelich et al, 1979).…”

Optimality-based non-Redfield plankton–ecosystem model (OPEM v1.1) in UVic-ESCM 2.9 – Part 1: Implementation and model behaviour

“…13. Unfortunately, our model simulations differ most strongly in the Indian Ocean, for which no data exist in the MAREDAT database of Luo et al (2012). One of the problems we face regarding N 2 fixation is that our UVic simulations do not include benthic denitrification and hence miss the dominant oceanic fixed-N loss term (e.g.…”

“…Estimates of Chl (MODIS Aqua, level 3; https://oceancolor.gsfc.nasa.gov/l3/, last access: 11 November 2019; Hu et al, 2012), particulate organic carbon, and net primary and community production (POC, NPP, and NCP;Westberry et al, 2008;Li and Cassar, 2016) are based on satellite data. In situ N 2 -fixation data are from MARine Ecosystem biomass DATa (MAREDAT) (Luo et al, 2012). 3 , and PO 3− 4 concentrations.…”