Abstract
The ocean is a net source of N(2)O, a potent greenhouse gas and ozone-depleting agent. However, the removal of N(2)O via microbial N(2)O consumption is poorly constrained and rate measurements have been restricted to anoxic waters. Here we expand N(2)O consumption measurements from anoxic zones to the sharp oxygen gradient above them, and experimentally determine kinetic parameters in both oxic and anoxic seawater for the first time. We find that the substrate affinity, O(2) tolerance, and community composition of N(2)O-consuming microbes in oxic waters differ from those in the underlying anoxic layers. Kinetic parameters determined here are used to model in situ N(2)O production and consumption rates. Estimated in situ rates differ from measured rates, confirming the necessity to consider kinetics when predicting N(2)O cycling. Microbes from the oxic layer consume N(2)O under anoxic conditions at a much faster rate than microbes from anoxic zones. These experimental results are in keeping with model results which indicate that N(2)O consumption likely takes place above the oxygen deficient zone (ODZ). Thus, the dynamic layer with steep O(2) and N(2)O gradients right above the ODZ is a previously ignored potential gatekeeper of N(2)O and should be accounted for in the marine N(2)O budget.