ogous to the mitochondria of eukaryotes. This review will discuss the anammox cell plan in detail, with the main focus on the anammoxosome. The identity of the anammoxosome as a prokaryotic organelle and the importance of this organelle for anammox bacteria are discussed as well as challenges these bacteria face by having three independent cell compartments. Abstract Anammox bacteria convert ammonium and nitrite to dinitrogen gas under anaerobic conditions to obtain their energy for growth. The

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... anammox reaction was deemed impossible until its discovery in the early 1990s. Now, anammox bacteria are recognized as major players in the global nitrogen cycle and estimated to be responsible for up to 50% of the nitrogen in the air that we breathe. In addition, anammox bacteria are extremely valuable for wastewater treatment where they are applied for the removal of ammonium. Besides their importance in industry and the environment, anammox bacteria defy some basic biological concepts. Whereas most other bacteria have only one cell compartment, the cytoplasm, anammox bacteria have three independent cell compartments bounded by bilayer membranes, from out-to inside; the paryphoplasm, riboplasm and anammoxosome. The anammoxosome is the largest compartment of the anammox cell and is proposed to be dedicated to energy conservation. As such it would be anal-