A novel bacterial protein family that catalyses nitrous oxide reduction.

Nitrous oxide (NO), a driver of global warming and climate change, has reached unprecedented concentrations in Earth's atmosphere. Current NO sources outpace NO sinks, emphasizing the need for comprehensive understanding of processes that consume NO. Microbes that express the enzyme NO reductase (NOR) convert NO to climate change-neutral dinitrogen (N). Known NORs belong to the canonical clade I and clade II NosZ reductases and are considered key enzymes for NO reduction. Here we report a previously unrecognized protein family with a role in NO reduction, clade III lactonase-type NOR (L-NOR), which diverges in sequence from canonical NosZ but conserves three-dimensional protein structural features. Integrated physiological, metagenomic, proteomic and structural modelling studies demonstrate that L-NORs catalyse NO reduction. L-NOR genes occur in several phyla, predominantly in uncultured taxa with broad geographic distribution. Our findings expand the known diversity of NORs and implicate previously unrecognized taxa (for example, Nitrospinota) in NO consumption. The expansion of NOR diversity and the identification of a novel type of catalyst for NO reduction advances the understanding of NO sinks, has implications for greenhouse gas emission and climate change modelling, and expands opportunities for innovative biotechnologies aimed at curbing NO emissions.