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Abstract

The existence of anaerobic ammonium oxidation (anammox) was hypothesized based on nutrient profiles and thermodynamic calculations (5, 31, 44). It was first discovered about 1 decade ago (25) in a pilot plant treating wastewater from a yeast-producing company in Delft, The Netherlands. The anammox reaction is the oxidation of ammonium under anoxic conditions with nitrite as the electron acceptor and dinitrogen gas as the product. Hydroxylamine and hydrazine were identified as important intermediates (51). Due to their very low growth rates (doubling time in enrichments is at best 11 days) the cultivation of the anammox bacteria proved to be tedious and required very efficient biomass retention (41, 43). A physical purification of anammox organisms from enrichment cultures was achieved with percoll density centrifugation (42). The purified cells performed the anammox reaction after activation by hydrazine. Based on phylogenetic analysis, the discovered anammox organism branched deep in the Planctomycetes phylum (Fig. 1A and B, [42]) and was named “Candidatus Brocadia anammoxidans” (19).