The Anammoxosome Organelle: The Power Plant of Anaerobic Ammonium-Oxidizing (Anammox) Bacteria

Anaerobic ammonium-oxidizing (anammox) bacteria are important players in both the environment and industry where they contribute substantially to nitrogen removal from natural and man-made (eco)systems. Apart from their ecological value, anammox bacteria are extremely interesting from both a physiological and cell biological perspective. Cells of anammox bacteria contain a major membrane-bound compartment, the anammoxosome, which is dedicated to the anammox reaction and energy conversion. The anammox reaction converts the substrates ammonium and nitrite to the product dinitrogen gas via two reactive intermediates: nitric oxide and hydrazine. First, nitrite is converted to nitric oxide by a nitrite reductase, nitric oxide and ammonium are then combined by hydrazine synthase to form hydrazine, and, finally, hydrazine is oxidized to dinitrogen gas by hydrazine dehydrogenase. The hydrazine synthase and hydrazine dehydrogenase enzymes are biochemical novelties. Electrons released from hydrazine oxidation are proposed to be shuttled to an electron transport chain in the anammoxosome membrane resulting in the establishment of a proton motive force and subsequent ATP synthesis. The anammoxosome compartment has a highly curved membrane and contains tubule-like structures and electron-dense, iron-containing particles. Finally, anammox bacteria contain unique ladderane membrane lipids that are postulated to render anammox membranes less permeable than conventional membranes. In this chapter, the cell biology, physiology and biochemistry of the anammox reaction and anammoxosome compartment are discussed.