Many aerobic bacteria seem to be able to switch their oxidative metabolism to nitrate respiration. Similar to oxygen respiration, the nitrate respiration of heterotrophic bacteria requires a complex carbon source as an electron source for denitrification (e.g., Eqs. 24 and 25). Denitrification (Eq. 26) starts with the reduction of nitrate to nitrite by membrane-bound nitrate reductase A (a). Then a membrane-bound nitrite reductase (b) catalyzes NO formation. Finally, NO reductase (c) and N2O reductase (d) form N2. The theoretical stoichiometry of denitrification with methanol or acetate as a carbon source is shown in Eqs. 27 and 28. For practical application a surplus of carbon source must be supplied, since the wastewater is not free of oxygen, and part of the carbon source is respired until anoxic conditions are achieved.
To evaluate the stoichiometry of nitrate to organic compounds for denitrification with a complex carbon source, the oxidation-reduction state of the carbon substrates and the oxygen concentration in the wastewater should be known. In wastewater treatment plants more than 2.85 g COD is required for reduction of 1 g NO- -N (Bernet et al., 1996).
5 CH3-COOH + 8 HNO3 — 10 CO2 + 4 N2 + 14 H2O (28)
Instead of nitrate, many denitrifying bacteria can use NO-, NO, or N2O as terminal electron acceptors. Alternatively, they may release these intermediates during denitrification of nitrate under unfavorable conditions, as was observed in soil (Conrad, 1996). If, e.g., surplus nitrate is supplied and hydrogen donors are not sufficiently available, NO and N2O can be formed (Schon et al., 1994). Another condition for N2O formation is a pH below 7.3, at which nitrogen oxidoreductase is inhibited (Knowles, 1982).
Except for dissimilatory nitrate reduction, many aerobic and anaerobic bacteria are capable of assimilatory nitrate reduction to supply the cells with ammonia for growth (Eqs. 29 and 30). However, the enzymes for nitrate assimilation are expressed only at concentrations of ammonia <1 mM.
Here, nitrate reductase B (enzyme a in Eq. 29) reduces nitrate to nitrite, which is then reduced to ammonia by the nitrite reductase complex (enzyme b in Eq. 29). Whereas nitrate reduction by the oxygen-sensitive, membrane-bound enzyme nitrate reductase A conserves energy, no energy conservation is possible in the reaction catalyzed by the soluble enzyme nitrate reductase B, which is not repressed by oxygen. For details on the cell biology and the molecular basis of denitrification, please refer to Zumft (1997).
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