Widespread detoxifying NO reductases impart a distinct isotopic fingerprint on N(2)O under anoxia

Nitrous oxide (N(2)O), a potent greenhouse gas, can be generated by compositionally complex microbial populations in diverse contexts. Accurately tracking the dominant biological sources of N(2)O has the potential to improve our understanding of N(2)O fluxes from soils as well as inform the diagnosis of human infections. Isotopic “Site Preference” (SP) values have been used towards this end, as bacterial and fungal nitric oxide reductases produce N(2)O with different isotopic fingerprints. Here we show that flavohemoglobin, a hitherto biogeochemically neglected yet widely distributed detoxifying bacterial NO reductase, imparts a distinct SP value onto N(2)O under anoxic conditions that correlates with typical environmental N(2)O SP measurements. We suggest a new framework to guide the attribution of N(2)O biological sources in nature and disease.