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Denitrification is a microbially facilitated process of nitrate reduction (performed by a large group of heterotrophic facultative anaerobic bacteria) that may ultimately produce molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products. This respiratory process reduces oxidized forms of nitrogen in response to the oxidation of an electron donor such as organic matter. The preferred nitrogen electron acceptors in order of most to least thermodynamically favorable include nitrate (NO3−), nitrite (NO2−), nitric oxide (NO), nitrous oxide (N2O) finally resulting in the production of dinitrogen (N2) completing the nitrogen cycle. Denitrifying microbes require a very low oxygen concentration of less than 10%, as well as organic C for energy. Since denitrification can lower leaching of NO3 to groundwater, it can be strategically used to treat sewage or animal residues of high nitrogen content. Denitrification allows for the production of N2O, which is a greenhouse gas that can have a considerable influence on global warming. The process is performed primarily by heterotrophic bacteria (such as ''Paracoccus denitrificans'' and various pseudomonads),〔Carlson, C. A., and J. L. Ingraham. 1983. Comparison of denitrification by ''Pseudomonas stutzeri'', ''Pseudomonas aeruginosa'', and ''Paracoccus denitrificans''. Appl. Environ. Microbiol. 45:1247-1253.〕 although autotrophic denitrifiers have also been identified (e.g., ''Thiobacillus denitrificans'').〔Baalsrud, K., and K. S. Baalsrud. 1954. Studies on ''Thiobacillus denitrificans''. Archives of Microbiology 20:34-62.〕 Denitrifiers are represented in all main phylogenetic groups.〔Zumft, W. 1997. Cell biology and molecular basis of denitrification. Microbiol. Mol. Biol. Rev. 61:533-616.〕 Generally several species of bacteria are involved in the complete reduction of nitrate to molecular nitrogen, and more than one enzymatic pathway has been identified in the reduction process.〔Atlas, R.M., Barthas, R. Microbial Ecology: Fundamentals and Applications. 3rd Ed. Benjamin-Cummings Publishing. ISBN 0-8053-0653-6〕 Direct reduction from nitrate to ammonium, a process known as dissimilatory nitrate reduction to ammonium or DNRA,〔An, S., and W. S. Gardner. 2002. Dissimilatory nitrate reduction to ammonium (DNRA) as a nitrogen link, versus denitrification as a sink in a shallow estuary (Laguna Madre/Baffin Bay, Texas). Marine Ecology Progress Series 237:41-50.〕 is also possible for organisms that have the nrf-gene.〔Spanning, R., M. Delgado, and D. Richardson. 2005. "It is possible to encounter DNRA when your source of carbon is a fermentable substrate, as glucose, so if you wanna avoid DNRA use a non fermentable substrate. The Nitrogen Cycle: Denitrification and its Relationship to N2 Fixation, p. 277-342."〕 This is less common than denitrification in most ecosystems as a means of nitrate reduction. Other genes known in microorganisms which denitrify include ''nir'' (nitrite reductase) and ''nos'' (nitrous oxide reductase) among others;〔Zumft, W. 1997. Cell biology and molecular basis of denitrification. Microbiol. Mol. Biol. Rev. 61:533-616.〕 organisms identified as having these genes include ''Alcaligenes faecalis'', ''Alcaligenes xylosoxidans'', many in the ''Pseudomonas'' genus, ''Bradyrhizobium japonicum'', and ''Blastobacter denitrificans''.〔Liu, X., S. M. Tiquia, G. Holguin, L. Wu, S. C. Nold, A. H. Devol, K. Luo, A. V. Palumbo, J. M. Tiedje, and J. Zhou. 2003. Molecular Diversity of Denitrifying Genes in Continental Margin Sediments within the Oxygen-Deficient Zone off the Pacific Coast of Mexico. Appl. Environ. Microbiol. 69:3549-3560〕 ==Nutrient limitation== All organisms require certain nutrients in their surroundings (available to them) for survival.〔Howarth, R. W. 1988. Nutrient Limitation of Net Primary Production in Marine Ecosystems. Annual Review of Ecology and Systematics 19:89-110.〕 Depending upon the ecosystem, nitrogen is most likely the limiting nutrient, although phosphorus is the other primary limiting nutrient and these two elements interact chemically.〔Vance, C. P. 2001. Symbiotic Nitrogen Fixation and Phosphorus Acquisition. Plant Nutrition in a World of Declining Renewable Resources. Plant Physiol. 127:390-397.〕 Some organisms appear to be able to denitrify and remove phosphorus.〔Kuba, T., M. C. M. Van Loosdrecht, F. A. Brandse, and J. J. Heijnen. 1997. Occurrence of denitrifying phosphorus removing bacteria in modified UCT-type wastewater treatment plants. Water Research 31:777-786.〕 The triple bond of N2 makes this a very stable compound; most organisms (i.e. plants) depend upon others to break this down to make it available for biochemical reactions.〔Seitzinger, S., J. A. Harrison, J. K. Bohlke, A. F. Bouwman, R. Lowrance, B. Peterson, C. Tobias, and G. V. Drecht. 2006. Denitrification Across Landscapes and Waterscapes: A Synthesis. Ecological Applications 16:2064-2090.〕 See Nitrification. Symbiotic relationships between ''Rhizobium'' species and legumes are well-documented.〔Daniel, R. M., I. M. Smith, J. A. D. Phillip, H. D. Ratcliffe, J. W. Drozd, and A. T. Bull. 1980. Anaerobic Growth and Denitrification by ''Rhizobium japonicum'' and Other Rhizobia. J Gen Microbiol 120:517-521.〕 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「Denitrification」の詳細全文を読む スポンサード リンク
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