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N2 production rates limited by nitrite availability in the Bay of Bengal oxygen minimum zone

Abstract

A third or more of the fixed nitrogen lost from the oceans as N2 is removed by anaerobic microbial processes in open ocean oxygen minimum zones. These zones have expanded over the past decades, and further anthropogenically induced expansion could accelerate nitrogen loss. However, in the Bay of Bengal there has been no indication of nitrogen loss, although oxygen levels are below the detection level of conventional methods (1 to 2 μM). Here we quantify the abundance of microbial genes associated with N2 production, measure nitrogen transformations in incubations of sampled seawater with isotopically labelled nitrogen compounds and analyse geochemical signatures of these processes in the water column. We find that the Bay of Bengal supports denitrifier and anammox microbial populations, mediating low, but significant N loss. Yet, unlike other oxygen minimum zones, our measurements using a highly sensitive oxygen sensor demonstrate that the Bay of Bengal has persistent concentrations of oxygen in the 10 to 200 nM range. We propose that this oxygen supports nitrite oxidation, thereby restricting the nitrite available for anammox or denitrification. If these traces of oxygen were removed, nitrogen loss in the Bay of Bengal oxygen minimum zone waters could accelerate to global significance.

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Figure 1: Station locations and oxygen data for the Bay of Bengal.
Figure 2: Abundance of bacterial 16S rRNA and selected functional genes in the Bay of Bengal.
Figure 3: Depth profiles and oxygen regulation experiments of N transformations in the Bay of Bengal.
Figure 4: Geochemical indicators of nitrogen cycling in the Bay of Bengal OMZ waters.

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Acknowledgements

We thank the captain and crew of the ORV Sagar Kanya for their support during sampling. We are grateful to A. Glud, G. Klockgether, J. Larkum, L. Piepgras and P. Sørensen for technical and analytical assistance. This study was supported by the Danish National Research Foundation (DNRF53), European Research Council ‘Oxygen’ grant (267233), and the Max Planck Society. We gratefully acknowledge the Ministry of Earth Sciences (MoES), INDIA for funding the research through the SIBER (INDIA) project GAP2425 and for making RV Sagar Kanya available for this work. J.D. was supported by the Sonderforschungsbereiche (SFB754) GEOMAR, Kiel and C.M.C. by a scholarship from the Natural Sciences and Engineering Research Council of Canada (NSERC).

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L.A.B., M.L., M.G., R.N.G., M.M.M.K., G.L., S.W.A.N., N.P.R., B.T. and D.E.C. designed the study; L.A.B., C.M.C., M.L., J.D., M.F., G.L., J.M. and D.E.C. performed experiments; L.A.B., C.M.C., M.L., M.A.A., M.F., G.L., B.T., A.H.T. and D.E.C. analysed data; A.P. provided nutrient data; L.A.B., C.M.C., M.L., R.N.G., G.L., B.T. and D.E.C. wrote the manuscript with input from all co-authors.

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Correspondence to D. E. Canfield.

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Bristow, L., Callbeck, C., Larsen, M. et al. N2 production rates limited by nitrite availability in the Bay of Bengal oxygen minimum zone. Nature Geosci 10, 24–29 (2017). https://doi.org/10.1038/ngeo2847

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