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Nitrite oxidation is an essential step in transformations of fixed nitrogen. The physiology of nitrite oxidizing bacteria (NOB) implies that the rates of nitrite oxidation should be controlled by concentration of their substrate, nitrite, and the terminal electron acceptor, oxygen. The sensitivities of nitrite oxidation to oxygen and nitrite concentrations were investigated using N-15 tracer incubations in the Eastern Tropical North Pacific. Nitrite stimulated nitrite oxidation under low in situ nitrite conditions, following Michaelis-Menten kinetics, indicating that nitrite was the limiting substrate. The nitrite half-saturation constant (K-s = 0.254 +/- 0.161 mu M) was 1-3 orders of magnitude lower than in cultivated NOB, indicating higher affinity of marine NOB for nitrite. The highest rates of nitrite oxidation were measured in the oxygen depleted zone (ODZ), and were partially inhibited by additions of oxygen. This oxygen sensitivity suggests that ODZ specialist NOB, adapted to low-oxygen conditions, are responsible for apparently anaerobic nitrite oxidation.

Plain Language Summary Nitrite is a key intermediate in the biogeochemistry of low-oxygen marine environments, including the loss of fixed nitrogen as dinitrogen gas and nitrous oxide. Nitrate reduction to nitrite coupled to reoxidation of nitrite to nitrate has been proposed as a cycle that can preserve bioavailable nitrogen in oxygen minimum zones. This cycle implies that nitrite oxidation occurs in the absence, or near absence, of oxygen. Nitrite oxidation is considered to be an obligately aerobic process, although it has been reported from anoxic waters. Here we report on the regulation of nitrite oxidation by oxygen and nitrite in natural assemblages from the oxygen minimum zone of the Eastern Tropical North Pacific. We show that natural assemblages have very high affinity for nitrite and that oxygen actually inhibits nitrite oxidation in anoxic samples. These findings have implications for the marine nitrogen budget now and in future scenarios of changing ocean conditions.