Primary production in over half of the world’s oceans is limited by fixed nitrogen availability. The main loss term from the fixed nitrogen inventory is the production of dinitrogen gas (N2) by heterotrophic denitrification or the more recently discovered autotrophic process, anaerobic ammonia oxidation (anammox). Oceanic oxygen minimum zones (OMZ) are responsible for about 35% of oceanic N2 production and up to half of that occurs in the Arabian Sea1. Although denitrification was long thought to be the only loss term, it has recently been argued that anammox alone is responsible for fixed nitrogen loss in the OMZs2,3,4. Here we measure denitrification and anammox rates and quantify the abundance of denitrifying and anammox bacteria in the OMZ regions of the Eastern Tropical South Pacific and the Arabian Sea. We find that denitrification rather than anammox dominates the N2 loss term in the Arabian Sea, the largest and most intense OMZ in the world ocean. In seven of eight experiments in the Arabian Sea denitrification is responsible for 87–99% of the total N2 production. The dominance of denitrification is reproducible using two independent isotope incubation methods. In contrast, anammox is dominant in the Eastern Tropical South Pacific OMZ, as detected using one of the isotope incubation methods, as previously reported3,5. The abundance of denitrifying bacteria always exceeded that of anammox bacteria by up to 7- and 19-fold in the Eastern Tropical South Pacific and Arabian Sea, respectively. Geographic and temporal variability in carbon supply may be responsible for the different contributions of denitrification and anammox in these two OMZs. The large contribution of denitrification to N2 loss in the Arabian Sea indicates the global significance of denitrification to the oceanic nitrogen budget.
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W. Naqvi helped make it possible to schedule the RV Roger Revelle to work in the Arabian Sea in 2007. We thank the master and crews of the RV Knorr and RV Roger Revelle, and J. Moffett who was chief scientist on the Knorr. This work was supported by NSF grants to B.B.W., A.H.D. and A.J.
Author Contributions B.B.W., A.H.D. and A.J. designed the Arabian Sea experiments and participated in all the fieldwork; J.J.R. designed the ETSP and Arabian Sea exetainer experiments and performed them in the ETSP; S.E.B. and B.B.W. performed the exetainer experiments in the Arabian Sea; J.J.R. made the exetainer mass spec measurements for both Arabian Sea and ETSP; B.X.C. and A.H.D. made the mass spectrometer measurements for the bag experiments; B.B.W., A.H.D., A.J., S.E.B. and B.X.C. carried out the bag experiments in the Arabian Sea; A.J. performed the Q-PCR experiments; H.N. and A.P. provided the nitrite and nitrate data for Fig. 1a and b; B.B.W. wrote the paper with input from A.H.D., A.J., S.E.B., B.X.C. and J.J.R.
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Ward, B., Devol, A., Rich, J. et al. Denitrification as the dominant nitrogen loss process in the Arabian Sea. Nature 461, 78–81 (2009) doi:10.1038/nature08276
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