Constraining oxygen consumption by nitrite oxidation in oceanic oxygen minimum zones

Oceanic oxygen minimum zones (OMZs) are globally significant sites of biogeochemical cycling where microorganisms deplete dissolved oxygen (DO) to concentrations <20 {micro}M. Amid intense competition for DO in these metabolically challenging environments, aerobic nitrite oxidation may consume significant amounts of DO and help maintain low DO concentrations, but this remains unquantified. Using parallel measurements of oxygen consumption rates and 15N-nitrite oxidation rates applied to both water column profiles and oxygen manipulation experiments, we show that the contribution of nitrite oxidation to overall DO consumption systematically increases as DO declines below 2 {micro}M. Nitrite oxidation could account for all DO consumption only under DO concentrations <393 nM found in and below the secondary chlorophyll maximum. These patterns were consistent across sampling stations and experiments, reflecting coupling between nitrate reduction and nitrite-oxidizing Nitrospina with high oxygen affinity (based on isotopic and omic data). Collectively our results provide new insight into the contribution of nitrite oxidation to the maintenance and biogeochemical dynamics of OMZs.