Nitrous oxide, ammonia-oxidizing bacteria

The ammonia-oxidizing bacteria oxidize ammonia to nitrous acid via hydroxyl-amine (NH2OH) (Lees, 1952 Hofman and Lees, 1953) ammonia is first oxidized to hydroxylamine by the catalysis of ammonia monooxygenase (AMO) (Dua et al., 1979 Hollocher et al., 1981). In this reaction, molecular oxygen is utilized. Then, hydroxylamine formed is oxidized to nitrous acid by the catalysis of hydroxylamine oxidoreductase (HAO). 

Hydroxylamine oxidoreductase was first purified by Hooper and Nason in 1965. They found that the enzyme catalyzed the reduction of horse ferricytochrome c with hydroxylamine but they found little nitrous acid formed as the result of the reaction nitrite formed was approximately 5% as much as cytochrome c reduced in 0.1 M glycine-NaOH buffer, pH 9.8. As the ammonia-oxidizing bacteria oxidize hydroxylamine to nitrite, they thought that one or more enzymes in addition to the oxidoreductase might participate in the oxidation of hydroxylamine to nitrite or additional factor(s) might be necessary for changing hydroxylamine to nitrite (Hooper et al., 1977). 

In the enzymatic oxidation of hydroxylamine catalyzed by hydroxylamine oxidoreductase, the electron acceptor for the oxidoreductase, cytochrome c-554, should be kept in the oxidized form as much as possible to accept electrons rapidly from hydroxylamine and NOH. For this purpose, sufficient air should be supplied for the bacteria to oxidize ammonia efficiently. If the air supply is not enough to oxidize hydroxylamine to nitrite, nitrous oxide (N20) occurs during the bacterial oxidation of ammonia (Poth, 1986 Anderson et al., 1993). Probably... 

A number of different enzymes can carry out the reduction of nitrite to either ammonium or nitric oxide and/or nitrous oxide. The latter types are involved with the denitrification process (Payne, 1973) and will not be considered here. Among the enzymes that catalyze the six-electron reduction of nitrite to ammonia, several different types are recognized. These are (I) assimilatory NiRs that function in biosynthetic nitrate assimilation of higher plants, algae, and fungi, (2) ammonia-forming dissimilatory NiRs involved in anaerobic nitrate respiration of diverse bacteria, and (3) assimilatory and dissimilatory sulfite reductases... 

These bacteria oxidize ammonia to nitrous acid via hydroxylamine (Fig. 1.7). A typical species of such bacteria is Nitrosomonas europaea. In addition to this, Nitrosomonas eutrophus, Nitrosococcus oceanus, Nitrosospira briensis, Nitrosolo-bus multiformis among others are known in this group. 

Emergence of acidic environments is associated with the activity of nitrifying bacteria (Nitrosomonas and Nitrobacter), which oxidize ammonia or ammonium entering in sufficient quantity the underground space with sewage effluents, to nitrous and nitric acids. 

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