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Nitrosomonas is a genus comprising Gram-negative, rod-shaped, and chemoautotrophic bacteria.

This organism oxidizes ammonia into nitrite as a metabolic process, known as nitritation (a step of nitrification). Nitrosomonas are useful in a polluted water and waste treatment technique known as bioremediation. They are important in the nitrogen cycle as they increase the bioavailability of nitrogen to plants whilst limiting carbon fixation. The genus is found in soil, freshwater, and on building surfaces, especially in areas that contains high levels of nitrogen compounds.

Nitrosomonas thrive in a pH range of 6.0–9.0, and a temperature range of 20–30 C. Most species are motile with a flagellum located in the polar region of the bacillus.

The organism has power-generating membranes, which form long, thin tubes inside the cell. These use electrons from the oxidation of ammonia to produce energy. It obtains the carbon it requires from the atmosphere via carbon fixation, which converts gaseous carbon dioxide into carbon bound in organic molecules.

Unlike plants, which fix carbon into sugars through energy gained through the process of photosynthesis, Nitrosomonas use energy gained through the oxidation of ammonia to fix gaseous carbon dioxide into organic molecules. Nitrosomonas must consume large amounts of ammonia before cell division can occur, and the process of cell division may take up to several days. This microbe is photophobic, and will generate a biofilm matrix, or form clumps with other microbes, to avoid light.

The species Nitrosomonas europaea has been identified as being able to degrade a variety of halogenated compounds including trichloroethylene, benzene, and vinyl chloride. Some Nitrosomonas species possess the enzyme urease, which catalyzes the conversion of the urea into ammonia and carbon dioxide. N. europaea, as well as populations of soil-dwelling ammonia-oxidizing bacteria (AOB), have been shown to assimilate the carbon dioxide released by the reaction to make biomass via the Calvin cycle, and harvest energy by oxidizing ammonia (the other product of urease) to nitrite. This feature may explain enhanced growth of AOB in the presence of urea in acidic environments.

Some sources regard Nitrobacteraceae to be the family of the genus Nicosomonas.

Genetics
Among the various species of Nitrosomonas that are known today, the complete genome of N. ureae strain Nm10, N. europaea, N.sp. Is79 has been sequenced.

All these species are characterized by the presence of the genes for the ammonia oxidation. The first enzyme involved in the ammonia oxidation is ammonia monooxygenase (AMO), which is encoded by the amoCAB operon. The AMO enzyme catalyzes the oxidation from (ammonia) to (hydroxylamine). The amoCAB operon contains three different gene: amoA, amoB and amoC. While N. europaea presents two copy of the genes, N. ''sp. Is79'' and N. ureae strain Nm10 have three copy of these gene.

The second enzyme involved in the ammonia oxidation is hydroxylamine oxidoreductase (HAO) encoded by the hao operon. This enzyme catalyzes the oxidation from to. The hao operon contains different genes such as the haoA that encodes for the functional cytochrome c subunit; the cycA that endodes for cytochrome c554 and the gene cycB that encodes for quinone reductase. These genes are present in different copies in various species; for instance, in ''Nitrosomonas sp. Is79'' there are only three copy, while in N. ureae there are four.

Nitrosomonas uses the Calvin-Benson cycle as a pathway for the Carbon fixation. for this reason all the species present an operon that encodes for the RuBisCO enzyme A peculiarity is found in N. sp Is79 in which the two copy of the operon encode for two different forms of the RuBisCO: the IA form and the IC form, where the first one has major affinity with the Carbon dioxide. Other species present different copies of this operon that encodes only for the IA form.

Important was the discovery of genes that encodes for enzymes involved in the denetrification. the first gene involved in this process is nirK that encodes for a Nitrite reductase with Copper. this enzyme catalyzes the reduction form (Nitrite) to  (Nitric oxide). While in N. europaea, N. eutropha and N. cryotolerans nirK is included in a multigenetic cluster ; in ''Nitrosomonas sp. Is79 and N. sp. AL212'' it is present as a single gene. An high expression of the nirK gene was found in N.ureae and this has been explained with the hypothesis that the NirK enzyme is also involved in the oxidation of in this species. The second genes involved in the denitrification are norCBQD that encodes for a nitric-oxide reductase that catalyzes the reduction from (Nitric oxide) to (NItrous oxide). These genes are present in ''N. sp. AL212, N.cryotolerans and N. communis strain Nm2''. In the Nitrosomonas europaea these genes are included in a cluster. These genes are absent in N. sp. Is79 and N. ureae.

Recently is found the norSY gene that encodes for a nitric-oxide reductase with copper in N. communis strain Nm2 and Nitrosomonas AL212.