Molecular mechanism of anaerobic ammonium oxidation

Two distinct microbial processes, denitrification and anaerobic ammonium oxidation (anammox), are responsible for the release of fixed nitrogen as dinitrogen gas (N₂) to the atmosphere. Denitrification has been studied for over 100 years and its intermediates and enzymes are well known. Even though anammox is a key biogeochemical process of equal importance, its molecular mechanism is unknown, but it was proposed to proceed through hydrazine (N₂H₄). Here we show that N₂H₄ is produced from the anammox substrates ammonium and nitrite and that nitric oxide (NO) is the direct precursor of N₂H₄. We resolved the genes and proteins central to anammox metabolism and purified the key enzymes that catalyse N₂H₄ synthesis and its oxidation to N₂. These results present a new biochemical reaction forging an N-N bond and fill a lacuna in our understanding of the biochemical synthesis of the N₂ in the atmosphere. Furthermore, they reinforce the role of nitric oxide in the evolution of the nitrogen cycle.