The ultrastructure of the compartmentalized anaerobic ammonium-oxidizing bacteria is linked to their energy metabolism.

The most striking example of a complex prokaryotic intracytoplasmic organization can be found in the members of the phylum Planctomycetes. Among them are the anammox (anaerobic ammonium-oxidizing) bacteria, which possess a unique cell compartment with an unprecedented function in bacteria: the anammoxosome is a prokaryotic cell organelle evolved for energy metabolism. It is an independent entity, which is enclosed by a contiguous membrane. Several lines of evidence indicate its importance in the anammox reaction and the unusual subcellular organization may well be essential for the lifestyle of anammox bacteria. The present review summarizes our knowledge about the ultrastructure of anammox cells and the connection between the anammoxosome and the energy metabolism of the cell. In the future, much more research will be necessary to validate the current models and to answer questions on the functional cell biology of anammox bacteria.