Anammox, an abbreviation for "anaerobic ammonium oxidation", is a globally important microbial process of the nitrogen cycle that takes place in many natural environments. The bacteria mediating this process were identified in 1999, and were a great surprise for the scientific community. In the anammox reaction, nitrite and ammonium ions are converted directly into diatomic nitrogen and water. The bacteria that perform the anammox process are genera that belong to the bacterial phylum Planctomycetota. The anammox bacteria all possess one anammoxosome, a lipid bilayer membrane-bound compartment inside the cytoplasm in which the anammox process takes place. The anammoxosome membranes are rich in ladderane lipids; the presence of these lipids is so far unique in biology. "Anammox" is also the trademarked name for an anammox-based ammonium removal technology developed by the Delft University of Technology. Process background In this biological process, which is a comproportionation reaction, nitrite and ammonium ions are converted directly into diatomic nitrogen and water. + → + . Globally, this process may be responsible for 30–50% of the gas produced in the oceans. It is thus a major sink for fixed nitrogen and so limits oceanic primary productivity. The bacteria that perform the anammox process belong to the bacterial phylum Planctomycetota. Currently, five anammox genera have been discovered: Brocadia, Kuenenia, Anammoxoglobus, Jettenia (all fresh water species), and Scalindua (marine species). The anammox bacteria are characterized by several striking properties: They all possess one anammoxosome, a membrane bound compartment inside the cytoplasm which is the locus of anammox catabolism. Further, the membranes of these bacteria mainly consist of ladderane lipids so far unique in biology. Of special interest is the conversion to hydrazine (normally used as a high-energy rocket fuel, and poisonous to most living organisms) as an intermediate. A final striking feature of the organism is the extremely slow growth rate; the doubling time is anywhere from 7–22 days. The anammox bacteria are geared towards converting their substrates at very low concentrations; in other words, they have a very high affinity to their substrates ammonium and nitrite (sub-micromolar range). Anammox cells are packed with cytochrome c type proteins (≈30% of the protein complement), including the enzymes that perform the key catabolic reactions of the anammox process, making the cells remarkably red. The anammox process was originally found to occur only from 20 °C to 43 °C but more recently, anammox has been observed at temperatures from 36