Abstract
Oceanic oxygen minimum zones (OMZs) occur where microorganisms deplete dissolved oxygen (DO) to exceptionally low levels, and are globally significant sites of biogeochemical cycling. Amid the intense competition for DO and other substrates occurring in these metabolically challenging environments, aerobic nitrite oxidation may consume significant amounts of DO, but this has not been examined comprehensively. Using parallel measurements of oxygen consumption rates and 15N-nitrite oxidation rates applied to water column profiles and to oxygen manipulation experiments, we show that nitrite oxidation is a substantial sink for DO in the ocean’s largest OMZ. The contribution of nitrite oxidation to overall DO consumption increased at low DO concentrations, tracking gradients and variations within and across multiple stations in the eastern tropical North Pacific Ocean. Oxygen manipulation experiments produced highly consistent effects, with nitrite oxidation responsible for progressively more DO consumption (up to 97%) as DO was experimentally decreased. Natural abundance stable isotope data indicated coupling of nitrite oxidation and nitrate reduction, while 16S rRNA and metagenome sequencing revealed that Nitrospina ecotypes possessing high-affinity cytochrome oxidase genes were prevalent and active within the OMZ. Collectively, our results demonstrate that nitrite oxidation consumes significant amounts of DO, and that this proportion increases as DO declines—indicating that nitrite oxidation is critically important to the formation and maintenance of OMZs.
Significance Oceanic oxygen minimum zones (OMZs) are naturally-occuring regions of low oxygen found in select areas of the ocean. Lack of dissolved oxygen has important implications for both the distribution of marine organisms and global biogeochemical cycles, yet we have a limited understanding of how oxygen is depleted to such low levels. Here we comprehensively quantify the contribution of nitrite oxidation to oxygen depletion in the ocean’s largest OMZ. We observed highly consistent patterns across depth profiles, and in multiple experiments where we manipulated oxygen concentrations, finding that nitrite oxidation consumes progressively more oxygen at lower oxygen concentrations. Our findings demonstrate that nitrite oxidation plays a pivotal role in exhausting oxygen to the low levels found in OMZs.
Competing Interest Statement
The authors have declared no competing interest.