The Metabolism of Anammox

Abstract

Abstract Anaerobic ammonium‐oxidizing (anammox) bacteria derive their energy for growth from the conversion of ammonium and nitrite into dinitrogen gas. The bacteria are the latest addition to the biogeochemical nitrogen cycle. The slowly growing micro‐organisms that belong to the Planctomycetes are characterized by a compartmentalized cell architecture featuring a central cell organelle, the ‘anammoxosome’. Membrane systems surrounding the different cellular compartments are composed of unique ‘ladderane’ lipid molecules. Although the metabolism has not been completely resolved, nitrogen formation seems to involve the intermediary formation of hydrazine, a most reactive and toxic compound. Anammox bacteria have been detected in many oxygen‐limited freshwater and marine ecosystems investigated. In the marine environment, over 50% of the nitrogen gas released is made by anammox bacteria. Application of the anammox process offers an attractive alternative to current wastewater treatment systems for the removal of fixed nitrogen. Key concepts Ammonium is converted under anaerobic conditions into dinitrogen gas by a newly discovered and specialized group of micro‐organisms, the anammox bacteria. Anammox bacteria are chemolithoautotrophic organisms that gain their energy for growth from the conversion of the inorganic substrates ammonium and nitrite. Cell carbon is derived from carbon dioxide. Anammox bacteria are characterised by a unique cell plan featuring a central cell organelle, the anammoxosome. The lipids from anammox bacteria are composed of highly unusual ‘ladderane’ molecules. The conversion of ammonium and nitrite into dinitrogen gas proceeds through the intermediary formation of the toxic and highly reactive hydrazine. Anammox bacteria synthesize adenosine triphosphate (ATP) by an electrochemical proton gradient most likely established across the anammoxosome membrane. In nature approximately 50% of the dinitrogen gas released into the atmosphere is made by anammox bacteria. Anammox bacteria offer an economically attractive and environmentally friendly alternative to current wastewater treatment processes for the removal of fixed nitrogen.