Anammox

In the anammox reaction, specialized bacteria oxidize ammonium (NH4+) while reducing nitrite (NO2−), producing dinitrogen gas (N2). The reaction occurs without oxygen and also yields a smaller amount of nitrate in the commonly described pathway. Nitrite serves as an electron acceptor; it is not interchangeable with oxygen. The anammox bacteria are members of the bacterial group Planctomycetota and have unusual, internally compartmentalized cells. Their slow growth makes them difficult to enrich and study compared with many commonly cultivated bacteria.

The possibility of anaerobic ammonium oxidation was inferred from nitrogen transformations that were not fully explained by the then-known cycle. The process was demonstrated in a wastewater-treatment reactor in the Netherlands in the 1990s, and the responsible bacteria were subsequently identified. Anammox is now recognized in natural environments as well as treatment systems. In oxygen-minimum waters and some sediments, it is one of the microbial pathways that converts fixed nitrogen to N2, returning nitrogen to the atmosphere. Its contribution varies among locations and depends on local oxygen, substrate supply and interactions with other nitrogen-cycle processes.

Wastewater systems can pair partial nitrification with anammox. In the first stage, aerobic microbes convert some ammonium to nitrite; anammox bacteria then use that nitrite together with remaining ammonium. This approach can reduce the need to fully oxidize ammonium to nitrate before conventional denitrification, and it can lower the need for added organic carbon in suitable wastewater streams. Performance depends on reactor design and stable operating conditions, including keeping oxygen and nitrite within ranges compatible with the anammox community. Slow bacterial growth means start-up and recovery can take time.

The best-characterized form uses nitrite as the electron acceptor, while research has also examined other electron-accepting pathways. A 2020 study reported anammox coupled to extracellular electron transfer in laboratory experiments, including transfer to electrodes or graphene oxide. Such results broaden the proposed metabolic possibilities, but they should not be confused with the conventional ammonium–nitrite reaction or assumed to be routine in natural settings or treatment plants. Measurements in the environment also require care: detecting nitrogen loss alone does not necessarily identify which microbial pathway caused it.