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Nitrification is the biological oxidation of ammonia or ammonium to nitrite followed by the oxidation of the nitrite to nitrate. The transformation of ammonia to nitrite is usually the rate limiting step of nitrification. Nitrification is an important step in the nitrogen cycle in soil. Nitrification is an aerobic process performed by small groups of autotrophic bacteria and archaea. This process was discovered by the Russian microbiologist, Sergei Winogradsky. ==Microbiology and ecology== The oxidation of ammonia into nitrite is performed by two groups of organisms, ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA〔Hatzenpichler R. (2012) Diversity, physiology and niche differentiation of ammonia-oxidizing archaea. Appl Environ Microbiol 78: 7501-7510〕).〔Treusch, A.H., Leininger, S., Kletzin, A., Schuster, S.C., Klenk, H.P., and Schleper, C. (2005) Novel genes for nitrite reductase and Amo-related proteins indicate a role of uncultivated mesophilic crenarchaeota in nitrogen cycling. Environ Microbiol 7: 1985–1995〕 AOB can be found among the β-proteobacteria and gammaproteobacteria.〔Purkhold, U., Pommerening-Roser, A., Juretschko, S.,Schmid, M.C., Koops, H.-P., and Wagner, M. (2000) Phylogeny of all recognized species of ammonia oxidizers based on comparative 16S rRNA and amoA sequence analysis: implications for molecular diversity surveys. Appl Environ Microbiol 66: 5368–5382〕 Currently, two AOA,'' Nitrosopumilus maritimus'' and ''Nitrososphaera viennensis'', have been isolated and described.〔Martens-Habbena, W., Berube, P. M., Urakawa, H., de la Torre, J. R., Stahl, D. A. 2009. Ammonia oxidation kinetics determine niche separation of nitrifying Archaea and Bacteria. Nature 461: 976-981.〕 In soils the most studied AOB belong to the genera ''Nitrosomonas'' and ''Nitrosococcus''. Although in soils ammonia oxidation occurs by both AOB and AOA, AOA dominate in both soils and marine environments,〔〔Wuchter, C., Abbas, B., Coolen, M.J.L., Herfort, L., van Bleijswijk, J., Timmers, P., et al. (2006) Archaeal nitrification in the ocean. Proc Natl Acad Sci USA 103: 12317–12322.〕〔Leininger, S., Urich, T., Schloter, M., Schwark, L., Qi, J., Nicol, G.W., Prosser, J.I., Schuster, S.C., Schleper, C. (2006) Archaea predominate among ammonia-oxidizing prokaryotes in soils. Nature 442:7014 pages 806-9.〕 suggesting that ''Thaumarchaeota'' may be greater contributors to ammonia oxidation in these environments.〔 The second step (oxidation of nitrite into nitrate) is done (mainly) by bacteria of the genus ''Nitrobacter'' and ''Nitrospira''. Both steps are producing energy to be coupled to ATP synthesis. Nitrifying organisms are chemoautotrophs, and use carbon dioxide as their carbon source for growth. Some AOB possess the enzyme, urease, which catalyzes the conversion of the urea molecule to two ammonia molecules and one carbon dioxide molecule. ''Nitrosomonas europaea'', as well as populations of soil-dwelling 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.〔Marsh, K. L., G. K. Sims, and R. L. Mulvaney. 2005. Availability of urea to autotrophic ammonia-oxidizing bacteria as related to the fate of 14C- and 15N-labeled urea added to soil. Biol. Fert. Soil. 42:137-145.〕 In most environments, organisms are present that will complete both steps of the process, yielding nitrate as the final product. However, it is possible to design systems in which nitrite is formed (the ''Sharon process''). Nitrification is important in agricultural systems, where fertilizer is often applied as ammonia. Conversion of this ammonia to nitrate increases nitrogen leaching because nitrate is more water-soluble than ammonia. Nitrification also plays an important role in the removal of nitrogen from municipal wastewater. The conventional removal is nitrification, followed by denitrification. The cost of this process resides mainly in aeration (bringing oxygen in the reactor) and the addition of an external carbon source (e.g., methanol) for the denitrification. Nitrification can also occur in drinking water. In distribution systems where chloramines are used as the secondary disinfectant, the presence of free ammonia can act as a substrate for ammonia-oxidizing microorganisms. The associated reactions can lead to the depletion of the disinfectant residual in the system.〔Zhang, Y, Love, N, & Edwards, M (2009), "Nitrification in Drinking Water Systems", ''Critical Reviews in Environmental Science and Technology'', 39(3):153-208, .〕 The addition of chlorite ion to chloramine-treated water has been shown to control nitrification.〔McGuire, M.J., Lieu, N.I. and Pearthree, M.S. (1999). “Using Chlorite Ion to Control Nitrification,” ''Journal American Water Works Association.'' 91:10 (October) 52-61.〕〔McGuire, M.J., Wu, X., Blute, N.K., Askenaizer, D., and Qin, G. (2009). “Prevention of nitrification using chlorite ion: Results of a demonstration project in Glendale, Calif.,” ''Journal American Water Works Association.'' 101:10 (October) 47-59.〕 Together with ammonification, nitrification forms a mineralization process that refers to the complete decomposition of organic material, with the release of available nitrogen compounds. This replenishes the nitrogen cycle. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nitrification」の詳細全文を読む スポンサード リンク
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