Ammoniaoxidizing

Ammoniaoxidizing refers to the biological oxidation of ammonia (NH3) to nitrite (NO2−), the first step in nitrification within the nitrogen cycle. This oxidation is carried out primarily by ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA). The process is initiated by ammonia monooxygenase (AMO), which converts NH3 to hydroxylamine (NH2OH). Hydroxylamine oxidoreductase (HAO) and related enzymes then oxidize NH2OH to NO2−, with electrons transferred through the respiratory chain to generate energy. The net reaction converts ammonia and oxygen into nitrite, protons, and water, enabling autotrophic carbon fixation for growth.

Taxa involved include AOB such as Nitrosomonas and Nitrosospira, and AOA such as Nitrosopumilus and Candidatus Nitrosoarchaeum. Ammoniaoxidizing organisms inhabit a wide range of environments, including soils, freshwater and marine sediments, wastewater treatment systems, and engineered bioreactors. AOA frequently predominate in oligotrophic or acidic environments, whereas AOB are often more abundant where ammonia concentrations are higher. Both groups are chemolithoautotrophs, using CO2 as a carbon source and ammonia as an energy source.

Ecological role and significance: Ammonia oxidation initiates nitrification, converting reduced nitrogen to nitrite and enabling subsequent oxidation to nitrate. This process influences soil fertility, nitrogen cycling, and wastewater treatment efficiency, and can affect emissions of nitrous oxide, a potent greenhouse gas. Ammoniaoxidation is affected by pH, temperature, ammonia availability, salinity, and the presence of inhibitors or competing microbes.