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 similar to 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, NH3 oxidation and NO3- reduction in N-15-labeling experiments with varying O-2 concentrations (0-25 mu mol 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 similar to 20 mu mol L-1. In contrast, NH3 oxidation to NO2- and NO3- reduction to NO2- as the main NH4+ and NO2- sources for anammox were only moderately affected by changing O-2 concentrations. Intriguingly, aerobic NH3 oxidation was active at non-detectable concentrations of O-2, while anaerobic NO3- reduction was fully active up to at least 25 mu mol 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 NH3 oxidation and NO3- reduction. With anammox bacteria in the marine environment being active at O-2 levels similar to 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.