Program     Nitrogen cycling in the ocean: From genes to ecosystems and from the past to the future       0930 Nitrite oxidation exceeds reduction and fixed nitrogen loss in anoxic Pacific waters  (Invited) Monday 7th @ 0930-0950, Multifunction Hall Andrew R. Babbin* , Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Carolyn Buchwald, Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Department of Oceanography, Dalhousie University, Halifax, Nova Scotia B3H 4R2 Canada François M. M. Morel, Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA Presenter Email: babbin@mit.edu Although the microbial oxidation of nitrite (NO2–) has been known for over a century, studies of the pathways and microbes involved have generally proceeded under the assumption that nitrite oxidation to nitrate requires dioxygen (O2). Fundamentally, however, dioxygen is not required as the oxygen atom added to the NO2– to form nitrate (NO3–) derives from water in aerobic cultures. Anaerobic NO2– oxidation until now has been conclusively shown only for anammox bacteria, albeit only as a limited sink for NO2– in their metabolism. Here, using direct experimental techniques optimized for anoxic conditions, we show that anaerobic NO2– oxidation is substantial, widespread, and consistent across the oxygen deficient zones of the eastern tropical Pacific Ocean. We hypothesize this reaction occurs with an oxidant other than O2. Additionally, anaerobic NO2– oxidation rates are up to an order of magnitude larger than simultaneous N2 production rates for which these zones are known, and cannot be explained by anammox rates alone. Higher rates of NO2– oxidation over reduction in anoxic waters are paradoxical but help resolve the balance of anammox and denitrification rates and the calculation of the global fixed nitrogen loss budget.

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