A novel bacterial protein family that catalyses nitrous oxide reduction

Nitrous oxide (N₂O), a driver of global warming and climate change, has reached unprecedented concentrations in Earth's atmosphere¹. Current N₂O sources outpace N₂O sinks, emphasizing the need for comprehensive understanding of processes that consume N₂O. Microbes that express the enzyme N₂O reductase (N₂OR) convert N₂O to climate change-neutral dinitrogen (N₂). Known N₂ORs belong to the canonical clade I and clade II NosZ reductases and are considered key enzymes for N₂O reduction^2-4. Here we report a previously unrecognized protein family with a role in N₂O reduction, clade III lactonase-type N₂OR (L-N₂OR), 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-N₂ORs catalyse N₂O reduction. L-N₂OR genes occur in several phyla, predominantly in uncultured taxa with broad geographic distribution. Our findings expand the known diversity of N₂ORs and implicate previously unrecognized taxa (for example, Nitrospinota) in N₂O consumption. The expansion of N₂OR diversity and the identification of a novel type of catalyst for N₂O reduction advances the understanding of N₂O sinks, has implications for greenhouse gas emission and climate change modelling, and expands opportunities for innovative biotechnologies aimed at curbing N₂O emissions^5,6.