Nitrosomonas

All species included in this genus have ellipsoidal or rod-shaped cells, which have extensive intracytoplasmic membranes displayed as flattened vesicles. Most species are motile, possessing a flagellum located in the polar region of the cell. Three basic morphological types of Nitrosomonas have been observed: short rods, rods, and cells with pointed ends. Nitrosomonas species have distinct characteristics regarding size and shape: • N. europaea cells appear as short rods with pointed ends, measuring 0.8–1.1 x 1.0–1.7 μm; motility has not been observed. • N. eutropha cells present as rod to pear-shaped cells with one or both ends pointed, measuring 1.0–1.3 x 1.6–2.3 μm. They show motility. • N. halophila cells have a coccoid shape, measuring 1.1–1.5 x 1.5–2.2 μm. Motility is possible due to a tuft of flagella. • N. communis cells are large rods with rounded ends, measuring 1.0–1.4 x 1.7–2.2 μm. Motility has not been observed in this species. • N. nitrosa, N. oligotropha, and N. ureae cells are spheres or rods with rounded ends. Motility has not been observed in these species. • N. marina presents slender rod cells with rounded ends, measuring 0.7–0.9 x 1.7–2.2 μm. • N. aestuarii and N. cryotolerans present as rod-shaped cells. == Genome ==

Genome sequencing of Nitrosomonas species is important for understanding the ecological role of these bacteria. Ammonia-oxidation genes The presence of genes for ammonia oxidation characterizes all these species. The first enzyme involved in the ammonia oxidation process is ammonia monooxygenase (AMO), which is encoded by the amoCAB operon. The AMO enzyme catalyzes the oxidation of NH3 (ammonia) to NH2OH (hydroxylamine). The amoCAB operon contains three different genes: amoA, amoB, and amoC. While N. europaea possesses two copies of these genes, N. sp. Is79 and N. ureae strain Nm10 have three copies. The second enzyme involved in the process of ammonia oxidation is hydroxylamine oxidoreductase (HAO), encoded by the hao operon. This enzyme catalyzes the oxidation of NH2OH to NO, a highly reactive radical intermediate that can be partitioned into the main products of the ammonia oxidation process: N2O, a potent greenhouse gas, and NO2-, a form of nitrogen more bioavailable for crops, but which conversely washes away from fields faster. The hao operon contains different genes such as haoA, which encodes for the functional cytochrome c subunit; cycA, which encodes for cytochrome c554; and cycB, which encodes for quinone reductase. Denitrification genes The discovery of genes that encode for enzymes involved in the denitrification process includes the first gene, nirK, which encodes for a nitrite reductase with copper. This enzyme catalyzes the reduction from NO2 (nitrite) to NO (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. A 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 NH2OH in this species. The second gene involved in denitrification is norCBQD, which encodes a nitric-oxide reductase that catalyzes the reduction from NO (nitric oxide) to N2O (nitrous oxide). These genes are present in N. sp. AL212, N. cryotolerans, and N. communis strain Nm2. In Nitrosomonas europaea, these genes are included in a cluster. These genes are absent in N. sp. Is79 and N. ureae. Carbon fixation genes Nitrosomonas species use the Calvin-Benson cycle as a pathway for carbon fixation. For this reason, all species possess an operon that encodes the RuBisCO enzyme. Transporter genes Since Nitrosomonas species are ammonia-oxidizing bacteria (AOB), ammonia carriers are essential. Bacteria adapted to high concentrations of ammonia can absorb it passively by simple diffusion. N. eutropha, which is adapted to high levels of ammonia, does not possess genes encoding an ammonia transporter. Bacteria adapted to low concentrations of ammonia possess a transporter (transmembrane protein) for this substrate. In Nitrosomonas, two different ammonia carriers have been identified, differing in structure and function. The first is the Amt protein (AmtB type), encoded by amt genes, found in Nitrosomonas sp. Is79. == Metabolism ==

Nitrosomonas is one of the genera included in AOB; species use ammonia as an energy source and carbon dioxide as the main source of carbon. The oxidation of ammonia is a rate-limiting step in nitrification and plays a fundamental role in the nitrogen cycle because it transforms ammonia, which is usually extremely volatile, into less volatile forms of nitrogen. The oxidation of ammonia to hydroxylamine is catalyzed by ammonia monooxygenase (AMO), a membrane-bound, multisubstrate enzyme. In this reaction, two electrons are required to reduce an oxygen atom to water: :NH3 + O2 + 2 H+ + 2 e– → NH2OH + H2O which occurs in the periplasm and is catalyzed by hydroxylamine oxidoreductase (HAO), a periplasm-associated enzyme. == Ecology ==

Habitat Nitrosomonas species are generally found in the highest numbers in habitats with an abundance of ammonia (examples include environments with plentiful protein decomposition and in wastewater treatment) and thrive in a pH range of 6.0–9.0 and a temperature range of . Some species can live and proliferate on the surface of monuments or on the walls of stone buildings, contributing to the erosion of those surfaces. but can also be found in fertilized soils. Leaching of soil In agriculture, nitrification by Nitrosomonas is problematic because the oxidized nitrite from ammonia can persist in the soil, leading to leaching and reduced availability for plants. Nitrification can be slowed by inhibitors that retard the oxidation of ammonia to nitrites by inhibiting the activity of Nitrosomonas and other ammonia-oxidizing bacteria, thereby minimizing or preventing the loss of nitrate. Application Nitrosomonas is used in activated sludge in aerobic wastewater treatment; nitrification treatment reduces nitrogen compounds in the water, avoiding environmental issues such as ammonia toxicity and groundwater contamination. Nitrogen, if present in high quantities, can cause algal development, leading to eutrophication and degradation of oceans and lakes. As a method of wastewater treatment, biological removal of nitrogen is cheaper and causes less environmental damage than physical-chemical treatments. Nitrosomonas also plays a role in biofilter systems, typically in association with other microbes, to consume compounds such as NH4+ or CO2 and recycle nutrients. These systems are used for various purposes, but mainly for odor elimination in waste treatment. == Other uses ==

Potential cosmetic benefits N. europaea is a non-pathogenic bacterium studied in connection with probiotic therapies. In this context, it may provide aesthetic benefits by reducing the appearance of wrinkles. The effectiveness of probiotic products has been studied to explore why N. eutropha, a highly mobile bacterium, has become extinct from the normal flora of human skin. Studies have investigated the potential benefits of repopulating human skin flora with N. eutropha. == See also ==