Oxidation of inorganic nitrogen compounds as energy source.

This chapter covers one of the microbiological steps of the nitrogen cycle, nitrification, which is the biological oxidation of reduced forms of inorganic nitrogen to nitrite and nitrate. Nitrifying bacteria use the oxidation of inorganic nitrogen compounds as their major energy source. Reactions are catalyzed by two physiological groups of bacteria: ammonia-oxidizing bacteria, which gain energy from oxidation of ammonia to nitrite, and nitrite-oxidizing bacteria, which thrive by oxidizing nitrite to nitrate. Because of the toxic nature of nitrite, its rapid conversion to nitrate, assimilated by plants and microorganisms, is essential. Ammonia oxidizers are lithoautotrophic organisms using carbon dioxide as the main carbon source; ammonia monooxygenase oxidizes ammonia to hydroxylamine, which is convertedtonitritebythehydroxylamineoxidoreductase.When grown lithotrophically with nitrite, nitrite is oxidized to nitrate by the nitrite oxidoreductase and the oxygen atom in the nitrate molecule is derived from water. The enzyme also reduces nitrite to nitrate when Nitrobacter strains are grown heterotrophically in the presence of nitrate. Detailed schemes for electron flow and energy transduction as well as energy generation schemes are outlined and the role of nitrifying bacteria in the environment highlighted. The two groups of nitrifying bacteria are phylogenetically unrelated, as they are found in different classes of Proteobacteria and members of the nitrite oxidizers are even found in different phyla. This chapter also covers the physiology and phylogeny of recently detected anaerobic ammonium-oxidizing deep-branching members of the phylum Planctomycetes and of Nitrosomonas eutropha.