Abstract
The ocean is a net source of N2O, a potent greenhouse gas and ozone-depleting agent. However, the removal of N2O via microbial N2O consumption is poorly constrained and rate measurements have been restricted to anoxic waters. Here we expand N2O 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, O2 tolerance, and community composition of N2O-consuming microbes in oxic waters differ from those in the underlying anoxic layers. Kinetic parameters determined here are used to model in situ N2O production and consumption rates. Estimated in situ rates differ from measured rates, confirming the necessity to consider kinetics when predicting N2O cycling. Microbes from the oxic layer consume N2O 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 N2O consumption likely takes place above the oxygen deficient zone (ODZ). Thus, the dynamic layer with steep O2 and N2O gradients right above the ODZ is a previously ignored potential gatekeeper of N2O and should be accounted for in the marine N2O budget.
Highlights
Nitrous oxide (N2O) is a greenhouse gas with about 300 times greater radiative forcing per mole than carbon dioxide, it is the dominant ozone-depleting agent emitted in the 21st century [1]
Anoxic incubations were amended with standard additions of (15N)2O tracer with a final concentration of 50 nM at stations PS1, PS2 and PS3
Measured rates in oxygen deficient waters were on the same order of magnitude as previously measured rates in the Eastern Tropical North Pacific (ETNP), but lower than rates at one coastal station in that study [6], indicating high variability of N2O cycling in the coastal regions as previously suggested [17]
Summary
Nitrous oxide (N2O) is a greenhouse gas with about 300 times greater radiative forcing per mole than carbon dioxide, it is the dominant ozone-depleting agent emitted in the 21st century [1]. The oxic surface layer and the oxycline of OMZs above the ODZ could be of vital importance in regulating N2O emissions if N2O consumption occurs there. Recent detection of nosZ genes and transcripts in oxic seawater [14, 15] implies the potential for N2O consumption there. Direct rate measurements are required to determine whether this microbial potential results in N2O consumption. N2O-consuming microbes that contain only nosZ (i.e., none of the other genes in the complete denitrification pathway) are of particular interest, because their activity results in net N2O consumption. Based on the analysis of 652 draft or complete microbial genomes with one or more dentification genes, nosZ-only microbes are overrepresented among these isolates from the ocean compared to other ecosystems [16]. O2 tolerance and substrate kinetics of N2O consumption were determined and used to estimate in situ N2O consumption and production rates, which reflect the in situ conditions more accurately than directly using measured rates from incubation experiments without correcting for substrate additions
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