LAUSR

Abstract The diversity of nitrogen-based dissimilatory metabolisms in anoxic waters continues to increase with additional studies to the marine oxygen deficient zones (ODZs). 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). Anaerobic NO2– oxidation until now has been conclusively shown only for anammox bacteria, albeit only as a limited sink for NO2– in their metabolism compared to the NO2– reduced to N2. Here, using direct experimental techniques optimized for replicating in situ anoxic conditions, we show that NO2– oxidation is substantial, widespread, and consistent across the ODZs of the eastern tropical Pacific Ocean. Regardless of the specific oxidant, 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 to explain how anammox rates can be enhanced over denitrification in shallow anoxic waters (σθ