A pathway for biological methane production using bacterial iron-only nitrogenase

Methane (CH ₄ ) is a potent greenhouse gas that is released from fossil fuels and is also produced by microbial activity, with at least one billion tonnes of CH ₄ being formed and consumed by microorganisms in a single year ¹ . Complex methanogenesis pathways used by archaea are the main route for bioconversion of carbon dioxide (CO ₂ ) to CH ₄ in nature ^2-4 . Here, we report that wild-type iron-iron (Fe-only) nitrogenase from the bacterium Rhodopseudomonas palustris reduces CO ₂ simultaneously with nitrogen gas (N ₂ ) and protons to yield CH ₄ , ammonia (NH ₃ ) and hydrogen gas (H ₂ ) in a single enzymatic step. The amount of CH ₄ produced by purified Fe-only nitrogenase was low compared to its other products, but CH ₄ production by this enzyme in R. palustris was sufficient to support the growth of an obligate CH ₄ -utilizing Methylomonas strain when the two microorganisms were grown in co-culture, with oxygen (O ₂ ) added at intervals. Other nitrogen-fixing bacteria that we tested also formed CH ₄ when expressing Fe-only nitrogenase, suggesting that this is a general property of this enzyme. The genomes of 9% of diverse nitrogen-fixing microorganisms from a range of environments encode Fe-only nitrogenase. Our data suggest that active Fe-only nitrogenase, present in diverse microorganisms, contributes CH ₄ that could shape microbial community interactions.