A possible role of nitric oxide (NO) or nitroxyl (HNO) in anammox was proposed by Hooper et al. by way of condensation of NO or HNO and ammonium on an enzyme related to the ammonium monooxygenase family. The formed hydrazine or imine could subsequently be converted by the enzyme hydroxylamine oxidase to dinitrogen gas, and the reducing equivalents produced in the reaction are required to combine NO or HNO and ammonium or to reduce nitrite to NO. Environmental genomics analysis of the species "Candidatus" "Kuenenia stuttgartiensis", through a slightly different and complementary metabolism mechanism, suggested NO to be the intermediate instead of hydroxylamine (Figure 4). However, this hypothesis also agreed that hydrazine was an important intermediate in the process. In this pathway (Figure 4), there are two enzymes unique to anammox bacteria: hydrazine synthase (hzs) and hydrazine dehydrogenase (hdh). The HZS produces hydrazine from nitric oxide and ammonium, and HDH transfer the electrons from hydrazine to ferredoxin. Few new genes, such as some known fatty acid biosynthesis and S-adenosylmethionine radical enzyme genes, containing domains involved in electron transfer and catalysis have been detected. Anammox microorganisms can also directly couple NO reduction to ammonia oxidation, without the need for nitrite supply.