Ammonium and nitrite oxidation at nanomolar oxygen concentrations in oxygen minimum zone waters

A major percentage of fixed nitrogen (N) loss in the oceans occurs within nitrite-rich oxygen minimum zones (OMZs) via denitrification and anammox. It remains unclear to what extent ammonium and nitrite oxidation co-occur, either supplying or competing for substrates involved in nitrogen loss in the OMZ core. Assessment of the oxygen (O-2) sensitivity of these processes down to the O-2 concentrations present in the OMZ core (< 10 nmol.L-1) is therefore essential for understanding and modeling nitrogen loss in OMZs. We determined rates of ammonium and nitrite oxidation in the seasonal OMZ off Concepcion, Chile at manipulated O-2 levels between 5 nmol.L-1 and 20 mu mol.L-1. Rates of both processes were detectable in the low nanomolar range (5-33 nmol.L-1 O-2), but demonstrated a strong dependence on O-2 concentrations with apparent half-saturation constants (K(m)s) of 333 +/- 130 nmol.L-1 O-2 for ammonium oxidation and 778 +/- 168 nmol.L-1 O-2 for nitrite oxidation assuming one-component Michaelis-Menten kinetics. Nitrite oxidation rates, however, were better described with a two-component Michaelis-Menten model, indicating a high-affinity component with a Km of just a few nanomolar. As the communities of ammonium and nitrite oxidizers were similar to other OMZs, these kinetics should apply across OMZ systems. The high O-2 affinities imply that ammonium and nitrite oxidation can occur within the OMZ core whenever O-2 is supplied, for example, by episodic intrusions. These processes therefore compete with anammox and denitrification for ammonium and nitrite, thereby exerting an important control over nitrogen loss.