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

Abstract

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. © 2011 Kalvelage et al.