Nitrite oxidation exceeds reduction and fixed nitrogen loss in anoxic Pacific waters

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 (σ_θ < 26.4) at the edge of the ODZs but not within the ODZ core. Furthermore, nitrite oxidation may be the key to reconciliation of the perceived imbalance of the global fixed nitrogen loss budget.