Nitrifying bacteria

Nitrifying bacteria are chemoautotrophic or chemolithotrophs depending on the genera (Nitrosomonas, Nitrosococcus, Nitrobacter, Nitrococcus) bacteria that grow by consuming inorganic nitrogen compounds. 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.

Ecology
Nitrifying bacteria are a narrow species in the environment, and are found in lowest 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 low ammonia content.

Oxidation of ammonia to nitrate
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 ammonium oxidizing bacteria (AOB) represented by the Nitrosomonas species. The second reaction is oxidation of nitrite (NO2-) to nitrate by nitrate-oxidizing bacteria (NOB), represented by the Nitrobacter species.

First step nitrification - molecular mechanism
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 ammonium to nitrite are ammonia monooxygenase (AMO) and hydroxylamine oxidoreductase (HAO). First is a transmembrane copper protein which catalyzes the oxidation of ammonium 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.


 * NH3 + O2 → + 3H+ + 2e&minus; (1)
 * NH3 + O2 + 2H+ + 2e&minus; → NH2OH + (1.1)
 * NH2OH + →  + 5H+ + 4e&minus; (1.2)

Second step nitrification - molecular mechanism
Nitrite produced in the first step of autotrophic nitrification is oxidized to nitrate by nitrite oxidoreductase (NXR)(2). It is a membrane-associated iron-sulfur molybdoprotein, and is part of an electron transfer chain which channels electrons from nitrite to molecular oxygen. The molecular mechanism of oxidation nitrite is less described than oxidation ammonium. In new research (e.g. Woźnica A. et al., 2013) proposed new hypothetical model of NOB electron transport chain and NXR mechanism (Figure 2.). It should be noted that, in contrast to earlier models the NXR acts on the outside of the plasma membrane, directly contributing to postulated by Spieck and coworkers mechanism of proton gradient generation. Nevertheless, the molecular mechanism of nitrite oxidation is an open question.


 * + →  + 2H+ + 2e&minus; (2)

Characteristic of ammonia and nitrite oxidizing bacteria
==== Nitrifying bacteria that oxidize ammonia ====