Nitrite oxidation in the upper water column and oxygen minimum zone of the eastern tropical North Pacific Ocean

Abstract Nitrogen (N) is an essential nutrient in the sea and its distribution is controlled by microorganisms. Within the N cycle, nitrite (NO2−) has a central role because its intermediate redox state allows both oxidation and reduction, and so it may be used by several coupled and/or competing microbial processes. In the upper water column and oxygen minimum zone (OMZ) of the eastern tropical North Pacific Ocean (ETNP), we investigated aerobic NO2− oxidation, and its relationship to ammonia (NH3) oxidation, using rate measurements, quantification of NO2−-oxidizing bacteria via quantitative PCR (QPCR), and pyrosequencing. 15NO2− oxidation rates typically exhibited two subsurface maxima at six stations sampled: one located below the euphotic zone and beneath NH3 oxidation rate maxima, and another within the OMZ. 15NO2− oxidation rates were highest where dissolved oxygen concentrations were <5 μm, where NO2− accumulated, and when nitrate (NO3−) reductase genes were expressed; they are likely sustained by NO3− reduction at these depths. QPCR and pyrosequencing data were strongly correlated (r2=0.79), and indicated that Nitrospina bacteria numbered up to 9.25% of bacterial communities. Different Nitrospina groups were distributed across different depth ranges, suggesting significant ecological diversity within Nitrospina as a whole. Across the data set, 15NO2− oxidation rates were decoupled from 15NH4+ oxidation rates, but correlated with Nitrospina (r2=0.246, P<0.05) and NO2− concentrations (r2=0.276, P<0.05). Our findings suggest that Nitrospina have a quantitatively important role in NO2− oxidation and N cycling in the ETNP, and provide new insight into their ecology and interactions with other N-cycling processes in this biogeochemically important region of the ocean.