Dirammox (direct ammonia oxidation) to nitrogen (N₂): discovery, current status, and perspectives

Microbial ammonia oxidation plays an important role in nitrogen (N₂) cycling in natural and man-made systems. Heterotrophic microorganisms that oxidize ammonia were observed more than a century ago; however, the underlying molecular mechanism of ammonia oxidation is still mysterious. Dirammox (direct ammonia oxidation to N₂) is a newly described heterotrophic ammonia oxidation process in which ammonia or its organic amine is oxidized into hydroxylamine and then directly converted to N₂ gas without the involvement of nitrite and nitrate. As demonstrated with Alcaligenes species, the conversion of ammonia to hydroxylamine is mediated by the dnf genes, and hydroxylamine conversion to N₂ is considered both a biotic and abiotic process. Dirammox is different from the N₂-producing processes of nitrification-denitrification and anaerobic ammonia oxidation (anammox), in which nitrite or nitrate is involved. Here, we review the discovery of dirammox, progress toward understanding its genetics, biochemistry, physiology, and ecology, and future perspectives and directions.