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Abstract. Oxygen minimum zones (OMZs), due to their large volumes of perennially deoxygenated waters, are critical regions for understanding how the interplay between anaerobic and aerobic nitrogen (N) cycling microbial pathways affects the marine N budget. Here, we present a suite of measurements of the most significant OMZ N cycling rates, which all involve nitrite (NO2-) as a product, reactant, or intermediate, in the eastern tropical North Pacific (ETNP) OMZ. These measurements and comparisons to data from previously published OMZ cruisespresent additional evidence that NO3- reduction is the predominant OMZ N flux, followed by NO2- oxidation back to NO3-. The combined rates of both of these N recycling processes were observed to be much greater (up to nearly 200 times) thanthe combined rates of the N loss processes of anammox and denitrification, especially in waters near the anoxic–oxic interface. We also showthat NO2- oxidation can occur when O2 is maintained near 1 nM by a continuous-purge system, NO2-oxidation and O2 measurements that further strengthen the case for truly anaerobic NO2- oxidation. We also evaluate thepossibility that NO2- dismutation provides the oxidative power for anaerobic NO2- oxidation. The partitioning ofN loss between anammox and denitrification differed widely from stoichiometric predictions of at most 29 % anammox; in fact,N loss rates at many depths were entirely due to anammox. Our new NO3- reduction, NO2- oxidation, dismutation, andN loss data shed light on many open questions in OMZ N cycling research, especially the possibility of truly anaerobicNO2- oxidation.

 

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. 

Estuaries emit a large but highly uncertain amount of Nitrous oxide (N₂O) into the atmosphere. To better understand N₂O cycling processes in estuaries, we provide the first direct observations of N₂O consumption in the seasonally anoxic Chesapeake Bay, the largest estuary in the United States. N₂O consumption rates in anoxic waters reached up to 3.3 nmol L⁻¹ d⁻¹but were generally undetectable in oxygenated waters. However, N₂O consumption rates were substantially enhanced when the oxygen concentration was experimentally decreased in initially oxygenated samples, indicating the potential of N₂O consumption in oxygenated environments, for example, surface waters. These potential N₂O consumption rates followed Michaelis‐Menten kinetics as a function of increasing N₂O substrate concentration. N₂O‐consuming microbes that predominantly contained the clade II nitrous oxide reductase gene were detected throughout the water column. These new observations of environmental controls on N₂O consumption will benefit the modeling of N₂O cycling and help to constrain the estuarine N₂O flux.