Nitrous oxide as a function of oxygen and archaeal gene abundance in the North Pacific.

Oceanic oxygen minimum zones are strong sources of the potent greenhouse gas N2 O but its microbial source is unclear. We characterized an exponential response in N2 O production to decreasing oxygen between 1 and 30 μmol O2  l-1 within and below the oxycline using 15 NO2 - , a relationship that held along a 550 km offshore transect in the North Pacific. Differences in the overall magnitude of N2 O production were accounted for by archaeal functional gene abundance. A one-dimensional (1D) model, parameterized with our experimentally derived exponential terms, accurately reproduces N2 O profiles in the top 350 m of water column and, together with a strong 45 N2 O signature indicated neither canonical nor nitrifier-denitrification production while statistical modelling supported production by archaea, possibly via hybrid N2 O formation. Further, with just archaeal N2 O production, we could balance high-resolution estimates of sea-to-air N2 O exchange. Hence, a significant source of N2 O, previously described as leakage from bacterial ammonium oxidation, is better described by low-oxygen archaeal production at the oxygen minimum zone's margins.