All about Nitrite: Exploring Nitrite Sources and Sinks in the Eastern Tropical North Pacific Oxygen Minimum Zone

Abstract. Oxygen minimum zones (OMZs), due to their large volumes of perennially deoxygenated waters, are critical regions for understanding how the interplay between anaerobic and aerobic nitrogen (N) cycling microbial pathways affects the marine N budget. Here we present a suite of measurements of the most significant OMZ N cycling rates, which all involve nitrite (NO₂^–) as a product, reactant, or intermediate, in the Eastern Tropical North Pacific (ETNP) OMZ. These measurements and comparisons to data from previously published OMZ cruises present additional evidence that NO₃^– reduction is the predominant OMZ N flux, followed by NO₂^– oxidation back to NO₃^–. The combined rates of both of these N recycling processes were observed to be much greater (up to nearly 200x) than the combined rates of the N loss processes of anammox and denitrification, especially in waters near the anoxic / oxic interface. We also show that NO₂^– oxidation can occur in functionally anoxic incubations, measurements that further strengthen the case for truly anaerobic NO₂^– oxidation. We also evaluate the possibility that NO₂^– dismutation provides the oxidative power for anaerobic NO₂^– oxidation. Although almost all treatments returned little evidence for dismutation (as based on product inhibition, substrate stimulation, and stoichiometric hypotheses), results from one treatment under conditions closest to in situ NO₂^– values may support the occurrence of NO₂^– dismutation. The partitioning of N loss between anammox and denitrification differed widely from stoichiometric predictions of at most 29 % anammox; in fact, N loss rates at many depths consisted entirely of anammox. Through investigating the magnitudes of NO₃^– reduction and NO₂^– oxidation, testing for anaerobic NO₂^– oxidation, examining the possibility of NO₂^– dismutation, and further documenting the balance of N loss processes, these new data shed light on many open questions in OMZ N cycling research.