The ocean is a major source of atmospheric nitrous oxide (N₂O), a powerful greenhouse gas and ozone destroyer, and oxygen (O₂)-depleted marine waters are hotspots of N₂O accumulation and emission. N₂O is both a product of and a substrate for microbial metabolism, but we lack a clear understanding of which microbes and bio(geo)chemical mechanisms are involved, and how these are controlled by environmental factors. Filling this knowledge gap is essential, as O₂-depleted regions are expanding and increasing fluxes of reactive nitrogen (N) from land to sea stimulate marine N cycling, which may further enhance N₂O accumulation.

Looking across low oxygen waters in coastal to open ocean settings (including Mariager Fjord, Denmark; Saanich Inlet, British Columbia, and the eastern tropical North Pacific – ETNP) we disentangled the contributions of denitrification and nitrification to N₂O production using both ¹⁵N-labeled substrates and ¹⁸O₂. Denitrification was shown to the dominant source of N₂O, however, the data suggested that microbes reduced the ¹⁵N-nitrate in an atypical “closed” pathway without freely exchangeable intermediates. Surprisingly, no variability in the N₂O production rate via this pathway was observed over a manipulated oxygen range of 0.1 to 15 µM.

N₂O consumption was not detectable in oxycline waters but increased steeply below the oxic-anoxic interface along with the accumulation of H₂S at coastal sites. Consistent with this distribution, the process was highly sensitive to O₂, with 50% inhibition at ~150 nM O₂ added, and was stimulated by low amounts of H₂S (≤ 5 µM) while higher H₂S concentrations were inhibitory. Environmental controls need to be assessed across spatial and temporal scales if we are to build a foundation for the prediction of N₂O emissions in a changing world.