Some bacteria of the genera Arthrobacter, Flavobacterium, and Thiosphaera are able to catalyze heterotrophic nitrification of nitrogen-containing organic substances (Eq. 23):
Heterotrophic nitrifiers oxidize reduced nitrogen compounds, such as hydroxylamine and aliphatic and aromatic nitrogen-containing compounds, but in contrast to autotrophic nitrification, no energy is gained by nitrate formation. For this reason an organic substrate must be respired to satisfy the energy metabolism (Schlegel, 1992) .
Some heterotrophic nitrifiers can denitrify nitrate or nitrite under aerobic growth conditions. The nitrogen metabolism of Thiosphaera pantotropha and Paracoccus de-nitrificans are well documented examples (Stouthamer et al., 1997). These organisms express a membrane-bound nitrate reductase under anoxic growth conditions, which works only in the absence of molecular oxygen. Under aerobic growth conditions a periplasmic nitrate reductase is expressed, which catalyzes nitrate reduction at least to the state of nitrous oxide.
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