Oxygen sensitivity of anammox and coupled N-cycle processes in oxygen minimum zones

Nutrient measurements indicate that 30-50% of the total nitrogen (N) loss in the ocean occurs in oxygen minimum zones (OMZs). This pelagic N-removal takes place within only ~0.1% of the ocean volume, hence moderate variations in the extent of OMZs due to global warming may have a large impact on the global N-cycle. We examined the effect of oxygen (O ₂) on anammox, NH ₃ oxidation and NO ₃ ^- reduction in ¹⁵N-labeling experiments with varying O ₂ concentrations (0-25 μmol L ^-1) in the Namibian and Peruvian OMZs. Our results show that O ₂ is a major controlling factor for anammox activity in OMZ waters. Based on our O ₂ assays we estimate the upper limit for anammox to be ~20 μmol L ^-1. In contrast, NH ₃ oxidation to NO ₂ ^- and NO ₃ ^- reduction to NO ₂ ^- as the main NH ₄ ⁺ and NO ₂ ^- sources for anammox were only moderately affected by changing O ₂ concentrations. Intriguingly, aerobic NH ₃ oxidation was active at non-detectable concentrations of O ₂, while anaerobic NO ₃ ^- reduction was fully active up to at least 25 μmol L ^-1 O ₂. Hence, aerobic and anaerobic N-cycle pathways in OMZs can co-occur over a larger range of O ₂ concentrations than previously assumed. The zone where N-loss can occur is primarily controlled by the O ₂-sensitivity of anammox itself, and not by any effects of O ₂ on the tightly coupled pathways of aerobic NH ₃ oxidation and NO ₃ ^- reduction. With anammox bacteria in the marine environment being active at O ₂ levels ~20 times higher than those known to inhibit their cultured counterparts, the oceanic volume potentially acting as a N-sink increases tenfold. The predicted expansion of OMZs may enlarge this volume even further. Our study provides the first robust estimates of O ₂ sensitivities for processes directly and indirectly connected with N-loss. These are essential to assess the effects of ocean de-oxygenation on oceanic N-cycling.