Anaerobic ammonium oxidation (anammox) is a bacterium mediated process that is responsible for the release of dinitrogen gas (N2) into the atmosphere; however, the molecular mechanism underlying this process was unknown. In silico studies of the bacterium 'Candidatus Kuenenia stuttgartiensis' had proposed that anammox occurs through three redox reactions that involve the conversion of nitrite (NO2) to nitric oxide (NO), of NO to hydrazine (N2H4) and of N2H4 to N2. This model has now been confirmed experimentally by this study, which also identified the key enzymes involved in each step. The authors found that this bacterium could produce NO from NO2 in culture, potentially mediated by its nitrite:nitride oxide reductase. Furthermore, incubation of NO with ammonium (NH4) gave rise to N2H4 in a reaction catalysed by a hydrazine synthase encoded in the kuste2859–kuste2861 gene cluster. Finally, N2H4 was oxidized to N2, and this reaction was found to be mediated by the hydrazine dehydrogenase kustc0694 (also known as Hao).