Microbial NO consumption in and above marine NO production hotspots.

The ocean is a net source of NO, a potent greenhouse gas and ozone-depleting agent. However, the removal of NO via microbial NO consumption is poorly constrained and rate measurements have been restricted to anoxic waters. Here we expand NO 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 tolerance, and community composition of NO-consuming microbes in oxic waters differ from those in the underlying anoxic layers. Kinetic parameters determined here are used to model in situ NO production and consumption rates. Estimated in situ rates differ from measured rates, confirming the necessity to consider kinetics when predicting NO cycling. Microbes from the oxic layer consume NO 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 NO consumption likely takes place above the oxygen deficient zone (ODZ). Thus, the dynamic layer with steep O and NO gradients right above the ODZ is a previously ignored potential gatekeeper of NO and should be accounted for in the marine NO budget.