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Concept
Comammox
Résumé
Comammox (COMplete AMMonia OXidation) is the name attributed to an organism that can convert ammonia into nitrite and then into nitrate through the process of nitrification. Nitrification has traditionally thought to be a two-step process, where ammonia-oxidizing bacteria and archaea oxidize ammonia to nitrite and then nitrite-oxidizing bacteria convert to nitrate. Complete conversion of ammonia into nitrate by a single microorganism was first predicted in 2006. In 2015 the presence of microorganisms that could carry out both conversion processes was discovered within the genus Nitrospira, and the nitrogen cycle was updated. Within the genus Nitrospira, the major ecosystems comammox are primarily found in natural aquifers and engineered ecosystems.
Complete nitrification step yield more energy (∆G°′ = −349 kJ mol−1 NH3) than either single oxidation alone (∆G°′ = −275 kJ mol−1 NH3 for ammonia oxidation to nitrite and ∆G°′ = −74 kJ mol−1 NO2− for nitrite oxidation to nitrate).
Complete nitrification of oxidizing ammonia to nitrate is energetically advantageous for Nitrospira. Due to the previous research done on Nitrospira, it was thought that all Nitrospira use nitrite as their energy source. Therefore, comammox Nitrospira were not discovered until 2015. All discovered nitrifiers belong to sublineage II of the genus Nitrospira. The genome of the nitrifying chemilithoautotrophic bacterium from the genus Nitrospira encodes for both ammonia and nitrite oxidation. The genes associated with the growth by ammonia oxidation to nitrate are ammonia monooxygenase and hydroxylamine dehydrogenases genes (e.g. amoA gene and hao cluster). This shows that complete nitrifying Nitrospira serve as cornerstones of the nitrogen-cycling microbial communities found in the environment. Nearly two years after the discovery of comammox organisms, Nitrospira inopinata was the first complete nitrifier to be isolated in pure culture. Kinetic and physiological analysis of Nitrospira inopinata demonstrated that this complete nitrifier has a high affinity for ammonia, slow growth rate, low maximum rate of ammonia oxidation, and high yield.
Source officielle
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