No nitrogen fixation in the Bay of Bengal?

Wiebke Mohr,Carolin R Löscher,Donald E Canfield,Hermann W Bange

doi:10.5194/bg-17-851-2020

Abstract

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 (N2) 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 N2 fixation in those waters. Here, we present a survey of N2 fixation and carbon fixation in the BoB during the winter monsoon season. We detected a community of N2 fixers comparable to other oxygen minimum zone (OMZ) regions, with only a few cyanobacterial clades and a broad diversity of non-phototrophic N2 fixers present throughout the water column (samples collected between 10 and 560 m water depth). While similar communities of N2 fixers were shown to actively fix N2 in other OMZs, N2 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 N2 fixation signal was visible in δ15N signatures. We suggest that the absence of N2 fixation may be a consequence of a micronutrient limitation or of an O2 sensitivity of the OMZ diazotrophs in the BoB. Exploring how the onset of N2 fixation by cyanobacteria compared to non-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 even minimal changes in water column stratification, with stronger mixing increasing organic matter production and export, which can exhaust remaining O2 traces in the BoB.

Highlights

Primary production in large areas of the surface ocean is limited by the availability of fixed nitrogen (Moore et al, 2013)
We suggest that the absence of N2 fixation may be a consequence of a micronutrient limitation or of an O2 sensitivity of the oxygen minimum zone (OMZ) diazotrophs in the Bay of Bengal (BoB)
Consistent with previous descriptions of primary producers at our study site (Loisel et al, 2013) and with satellite imaging (Fig. S1 in the Supplement), we identified cyanobacteria related to Synechococcus and Prochlorococcus as the most abundant primary producers in our metagenome from the BoB deep chlorophyll maximum (DCM), accounting for 3.3 % of operational taxonomic units (OTUs), while eukaryotic phytoplankton accounted for only 0.3 % of OTUs (Table S1 in the Supplement)

Summary

Introduction

Primary production in large areas of the surface ocean is limited by the availability of fixed nitrogen (Moore et al, 2013). The development of novel molecular tools revealed that non-cyanobacterial N2 fixers are widely distributed throughout ocean waters (Farnelid et al, 2011, 2013; Fernandez et al, 2011; Luo et al, 2012; Riemann et al, 2010; Zehr et al, 1998) and sediments (Fulweiler et al, 2007; Andersson et al, 2014; Bertics et al, 2013; Gier et al, 2017, 2016) Their quantitative importance for global N2 fixation, is not yet clear.

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Results

Conclusion