Anammox
Anammox, an abbreviation for ANaerobic AMMonium OXidation, is a globally important microbial process of the nitrogen cycle. The bacteria mediating this process were identified in 1999, and at the time were a great surprise for the scientific community. It takes place in many natural environments and anammox is also the trademarked name for the anammox-based ammonium removal technology that was developed by the Delft University of Technology.
In this biological process, nitrite and ammonium ions are converted directly into diatomic nitrogen and water.
Globally, this process may be responsible for 30-50% of the N2 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 Planctomycetes. 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 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 °C to 52 °C in hot springs and 60 °C to 85 °C at hydrothermal vents located along the Mid-Atlantic Ridge.
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