Nitrite Oxidation in Oxygen Minimum Zones: Mechanisms and Implications

Abstract

Oxygen minimum zones (OMZs) are regions of the world’s ocean that are permanently depleted in dissolved oxygen. Nitrite oxidation, traditionally considered an obligately aerobic process, has been observed in the anoxic core of these OMZs challenging longstanding assumptions about nitrogen cycling in low-oxygen environments. This paper investigates nitrite oxidation within the Eastern Tropical South Pacific (ETSP) oxygen minimum zone through ¹⁵NO₂⁻ tracer incubations on samples collected at three stations spanning from the oxygen minimum zone’s anoxic core to oxygenated waters outside of the OMZ. Experiments tested nitrite oxidation rates under anoxic, low oxygen (1 μM O2) and oxygen-spike treatments designed to mimic transient lateral oxygen intrusions. Results revealed unexpectedly high nitrite oxidation rates under anoxic treatment in samples from the anoxic core of the OMZ (92.52 ± 19.32 nM/day at station PS3 depth 230 m), with little to no activity under oxic or spike conditions. These findings contradict the hypothesis that episodic oxygen availability sustains nitrite oxidation in OMZs, suggesting instead that some nitrite-oxidizing bacteria (NOB) possess a previously unrecognized anaerobic metabolism. This discovery supports growing evidence of niche-adapted NOB within OMZs, distinct from their aerobic counterparts. Potential mechanisms include nitrite dismutation or alternate oxidants, though further work is needed to test these theories. As OMZs expand under climate change, understanding these alternative pathways is critical to predicting nitrogen cycle dynamics and fixed nitrogen loss in the ocean.