Ammonia oxidation by aerobic methanotrophs as a source of marine nitrous oxide

Aerobic methanotrophs encode a hydroxylamine oxidoreductase, which facilitates the oxidation of ammonia to nitrite or nitric oxide, potentially leading to nitrous oxide production. Aerobic methane oxidation has been documented in shallow marine waters or the water column of the open ocean. However, little is known about the distribution pattern of marine aerobic methanotrophs containing hydroxylamine oxidoreductase and their contribution to marine nitrous oxide emissions. Here, by analyzing global marine metagenomes, we show that hydroxylamine oxidoreductase-containing aerobic methanotrophs were widely distributed across diverse marine habitats, with higher abundances in methane seep systems and estuary regions than in other environments. Among these, aerobic methanotrophs belonging to Gammaproteobacteria were the most widely distributed and abundant functional group. We also identified a second order within Gammaproteobacteria (Ga0077536) potentially capable of aerobic methanotrophy, and a complete repertoire of denitrification genes in a gammaproteobacterial methanotroph, expanding the phylogenetic and functional diversity of marine aerobic methanotrophs. By using enrichments of estuarine methanotrophs in combination with 15N stable isotope tracing and metatranscriptomic analysis, we indicate that marine aerobic methanotrophs take part in ammonia oxidation and nitrous oxide production. The ammonia oxidation can persist for ~6 days, and the nitrous oxide produced is at least partially derived from the hydroxylamine oxidation. Given the prevalence of denitrification genes in aerobic methanotrophs, methane oxidation may also be coupled to NOx- reduction under anoxic marine conditions, potentially contributing to nitrous oxide production. The intrinsic nature of aerobic methanotrophs could partially offset the mitigation of global warming achieved through the methane consumption.