Acidification Stimulates N ₂ O Production by Oceanic Nitrifying Bacteria

Abstract Recent studies have shown changes in the production rates of nitrous oxide (N 2 O) in aerobic seawater in response to ocean acidification (OA). Understanding how N 2 O production responds to OA is crucial because N 2 O is a strong greenhouse gas and stratospheric ozone‐depleting substance emitted from the ocean. The pH dependence of N 2 O production rates on marine bacteria Nitrosococcus oceani strain NS58, one of the ammonia oxidizing bacteria that are relevant to nitrification occurring in eutrophic seawater, was investigated under several dissolved oxygen (DO) conditions. We also measured abundance ratios of N 2 O molecules substituted with rare stable isotopes (isotopocules) to distinguish the two major pathways of N 2 O production by nitrifiers: NH 2 OH oxidation and NO 2 − reduction. The ammonium oxidation rate () and N 2 O production rate () calculated respectively from the temporal change of the product concentrations were 4–34 × 10 −15 mol h −1 cell −1 and 1–15 × 10 −17 mol h −1 cell −1 . When compared in the stable phase ( t = 44–76 hr), decreased concomitantly with decreasing DO, also exhibiting a slight increase in acidified water. In contrast, was highest at 35% DO (air saturation), showing a 5%–60% increase by acidification (pH 7.7 vs. 8.0) depending on DO. Isotopocule ratios showed an increased contribution from NO 2 − reduction over NH 2 OH oxidation under 35% and 3% DO, but its pH dependence was negligible except under 3% DO. These results suggest that OA increases N 2 O emission in particular from eutrophic seawater and that both N 2 O production pathways can be stimulated to the same degree.