Nitrifying bacteria

Nitrifying bacteria are chemolithotrophic organisms that include species of the genera Nitrosomonas, Nitrosococcus, Nitrobacter and Nitrococcus. These bacteria get their energy by the oxidation of inorganic nitrogen compounds. Types include ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). Many species of nitrifying bacteria have complex internal membrane systems that are the location for key enzymes in nitrification: ammonia monooxygenase (which oxidizes ammonia to hydroxylamine) and nitrite oxidoreductase (which oxidizes nitrite to nitrate).

Nitrifying bacteria are a narrow taxonomic group in the environment, and are found in highest numbers where considerable amounts of ammonia are present (areas with extensive protein decomposition, and sewage treatment plants). Nitrifying bacteria thrive in lakes and rivers streams with high inputs and outputs of sewage and wastewater and freshwater because of the high ammonia content.

Nitrification in nature is a two-step oxidation process of ammonium (NH₄⁺) or ammonia (NH₃) to nitrate (NO₃⁻) catalyzed by two ubiquitous bacterial groups. The first reaction is oxidation of ammonium to nitrite by ammonia oxidizing bacteria (AOB) represented by the "Nitrosomonas" genus. The second reaction is oxidation of nitrite (NO₂⁻) to nitrate by nitrite-oxidizing bacteria (NOB), represented by the "Nitrobacter" genus.

Ammonia oxidation in autotrophic nitrification is a complex process that requires several enzymes, proteins and presence of oxygen. The key enzymes, necessary to obtaining energy during oxidation of ammonia to nitrite are ammonia monooxygenase (AMO) and hydroxylamine oxidoreductase (HAO). First is a transmembrane copper protein which catalyzes the oxidation of ammonia to hydroxylamine (1.1) taking two electrons directly from the quinone pool. This reaction requires O₂. In the second step (1.2), a trimeric multiheme c-type HAO converts hydroxylamine into nitrite in the periplasm with production of four electrons. The stream of four electron are channelled through cytochrome c₅₅₄ to a membrane-bound cytochrome c₅₅₂. Two of the electrons are routed back to AMO, where they are used for the oxidation of ammonia (quinol pool). Rest two electrons are used to generate a proton motive force and reduce NAD(P) through reverse electron transport.

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