A strict separation of the reactions participating in aerobic, autotrophic nitrification and in anoxic, heterotrophic denitrification is not required, as concluded from N15-tracer experiments (Kuenen and Robertson, 1994). Autotrophic ammonia oxidizers seem to be able to produce NO, N2O, or N2 from nitrite if oxygen is limited, and ammonia as well as nitrite oxidizers can be isolated from anaerobic reactors (Kuenen and Robertson, 1994). Nitrosomonas europaea can use nitrite as an electron acceptor and pyruvate as an energy source under anoxic, denitrifying growth conditions (Ab-eliovich and Vonshak, 1992). In addition, several strains of Nitrobacter sp. were reported to denitrify during anoxic, heterotrophic growth (Bock et al., 1986).
Aerobic denitrification by Thiosphaera pantotropha was first described by Robertson and Kuenen (1984). T. pantotropha respires molecular oxygen and denitrifies nitrate simultaneously, provided that suitable electron acceptors are available. The conversion rate of acetate as electron donor with nitrate as electron acceptor was twice as high when the concentration of molecular oxygen was <30% of air saturation compared to 30%-80% air saturation (Robertson et al., 1988). Respiration, simultaneous nitrification, and denitrification were observed in the presence of oxygen and ammonia. During heterotrophic denitrification by T. pantotropha, ammonia is first oxidized to hydroxylamine by an ammonia monooxygenase, with ubiquinone serving as electron donor. Hydroxylamine is subsequently oxidized to nitrite by a hydroxylamine oxidoreductase. During coupled nitrification-denitrification, 3 of the 4 electrons from the oxidation of hydroxylamine are used for reduction of nitrite to nitrogen and are not available to the electron transport reaction catalyzed by cytochrome oxidase. Regeneration of ubiquinone is mediated by electrons that are generated during oxidation of an organic substrate (heterotrophic nitrification).
Conversion rates of ammonia by heterotrophic nitrifiers such as T. pantotropha (Kuenen and Robertson, 1994) are smaller than those of autotrophic nitrifiers (35.4 for T. pantotropha versus 130-1550 nmol NH3 min-1 mg-1 dry weight for Nitrosomonas sp.). If, however, the higher population density of heterotrophic nitrifiers, resulting from higher growth rates, is taken into consideration, the specific conversion rates are in a similar range. Heterotrophic nitrifiers can, in addition to their nit rifying capability, denitrify nitrite or nitrate to molecular nitrogen. In many wastewater treatment plants autotrophic nitrifiers may exist, producing nitrite from ammonia under moderately aerobic conditions; the ammonia is then converted to nitrate and/or reduced to nitrogen by heterotrophic nitrifiers in the presence of a suitable carbon source. In practice, the only disadvantage of heterotrophic nitrification is more surplus sludge generation for final disposal.
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