Anammox 1,2 , a process of anaerobic ammonium oxidation, is an innovative technological advancement in the removal of ammonia nitrogen in waste water. This new process combines ammonia and nitrite directly into dinitrogen gas 3 . The anammox reaction can be represented as − + + 2 4 NO NH = N 2 + 2H 2 O. This reaction is carried out by anammox bacteria be-longing to the planctomycete group. Bac-teria capable of anaerobically oxidizing ammonia were not identified earlier and were known as 'lithotrophs missing from 4 . Subsequently, this missing lith-otroph was discovered 1 and identified as a new autotrophic member of the order Planctomycetales. It appears that they might be the most primitive group of bacteria, at the very root of the bacterial tree. Two of the anammox bacteria have been named provisionally 5 : Candidatus 'Brocadia anammoxidans' and Candida-tus 'Kuenenia stuttgartiensis'. The former bacterium was responsible for anaerobic oxidation of ammonia observed in the Netherlands, while the latter organism has been shown to be responsible for ammonia oxidation anaerobically in sev-eral wastewater treatment plants in Ger-many and Switzerland. Interestingly the two species of the anammox bacteria which have been characterized are rarely found together in a single anammox reac-tor, they must have different, as-yet un-known, ecological niches 5 . It is well known that the biological nitrogen cycle plays an important part in the maintenance of nitrogen balance in global biosphere. The conventional bio-logical nitrogen cycle involves microbial fixation of nitrogen gas to ammonia sym-biotically and non-symbiotically which is subsequently converted to organic nitro-gen. The ammonia released from organi-cally bound nitrogen and from man-made activities, is biologically oxidized aero-bically to nitrite and then to nitrate. The resultant nitrate and nitrite are reduced to nitrogen gas by a denitrifying group of bacteria using some electron donors (or-ganic or inorganic compounds). It has been indicated 6 that anaerobic ammonia oxidation also could be a major biologi-cal activity to be included in the nitrogen cycle, thus necessitating its modification. Recently 7 it was discovered that anam-mox makes a significant (up to 70%) contribution to nitrogen cycling in the world's oceans. Figure 1 shows the involvement of anammox process in the biological nitrogen cycle including meta-bolic pathway for anaerobic ammonium oxidation. Initially anammox research 5 was fo-cused on the basic properties of the proc-ess and on providing evidence for its microbial nature and the principles of the nitrogen and carbon metabolism. It appears that the anammox process is based on energy conservation from an-oxic ammonium oxidation with nitrite as the electron acceptor and hydrazine and hydroxylamine as the intermediates. Carbon dioxide is used as the main car-bon source for growth. One of the key enzymes of anaerobic ammonium oxida-tion is hydroxylamine oxidoreductase 8 . Its importance is illustrated by the fact that it constitutes 10% of the total cell protein. It catalyses the oxidation of hy-drazine and hydroxylamine. The enzyme has been located in a membrane-boun-ded, 'organelle' named anammoxasome, in the cytoplasm of the anammox cells 9 . This 'organelle' appears to be the center of anaerobic ammonium oxidation. It is quite likely that its function could be containment of hydrazine. The anam-Figure 1. Involvement of anammox pro-cess in biological nitrogen cycle including metabolic pathway for anaerobic ammo-nium oxidation (adapted from refs 3 and 5).