Aerobic oxidation of NH3 to NO2− and of NO2− to NO3− is not restricted to chemolithotrophs such as Nitrosomonas and Nitrobacter sp. Little is known of hetrotrophic nitrification but the denitrifier Paracoccus denitrificans provides an example of a heterotrophic nitrifier [19]. There is evidence that a similar ammonia mono-oxygenase may be present as in chemolithotrophs but that a distinct NH2OH-oxidase must be used. Heterotrophic nitrifiers, unlike autotrophic nitrifiers, are incapable of using nitrification to support growth. It has therefore been proposed that heterotrophic nitrification is not linked to the generation of a H+ motive force and that the electron acceptors for a ‘NH2OH oxidase’ in heterotrophic nitrifiers have more positive potentials than the cytochrome bc1 complex, for example cytochrome c550 and pseudoazurin. P. denitrificans has an AMO-like enzyme activity in membranes that can extract two electrons from the quinol pool and use these to convert NH3 to NH2OH using molecular O2 in a manner that is envisaged to be similar to that of autotrophic nitrifiers. The NH2OH will be released into the periplasm. An NH2OH-oxidase from P. denitrificans has been isolated from the periplasm and found to have different properties from the NH2OH oxidoreductase from N. europaea. The enzyme from P. denitrificans has a molecular mass of 20 kDa and contains non-heme, rather than heme, iron. A non-heme NH2OH-oxidase has also been partly purified from the Gram-positive heterotrophic nitrifier Arthrobacter globiformis, indicating that the non-heme-iron NH2OH-oxidase might be widespread among heterotrophic nitrifiers. Under anaerobic conditions, N2O is the product of the P. denitrificans NH2OH-oxidase. This is probably the product of a 2e− oxidation of NH2OH to yield nitroxyl radicals that can then dimerize to form N2O. By contrast, under aerobic conditions NO2− can be formed, presumably as a result of an oxidation of nitroxyl to NO2− by O2. In N. europaea, NO2− is the product of NH2OH oxidoreductase even in the absence of O2. As discussed earlier, four electrons are passed to cytochrome c554 as NH2OH is oxidised to NO2−. Two of these electrons are passed into the quinol pool and then used by ammonia mono-oxygenase. The other two electrons are used in the electron transport chain, either to generate a H+ motive force, via the cytochrome bc1 complex to an oxidase, or to generate NADH via reverse electron transfer to NADH. Therefore more than two electrons from the NH2OH oxidoreductase must reduce cytochrome c554, so that growth can be coupled with autotrophic nitrification. If the product of NH2OH-oxidase in heterophic nitrification is nitroxyl and the reaction to NO2− involves reaction with O2, then only two electrons are produced per NH2OH. This would not allow for autotrophic growth. Instead it is proposed in P. denitrificans that the electrons are transferred to the cytochrome c550/pseudoazurin pool. These two electrons can then be passed from cyt c500/peudoazurin to the non-H+-motive NO2−, N2O or NO reductases. This then allows for further reduction of the NO2− generated from NH2OH via aerobic denitrification reactions that are not coupled to energy conservation. Thus the overall result is to withdraw two electrons from the quinol pool and dissipate these without energy conservation. This is an example of redox balancing and reflects the proposed role for heterotrophic nitrification in energy spillage, rather than energy conservation. Heterotrophic nitrifiers are believed to be widely distributed in soils. However, most of these organisms cannot as yet be cultured when performing heterotrophic nitrification. Although the enzymology, molecular biology and ecology of heterotrophic nitrification remains poorly understood, it is hoped that the emerging genome sequences of some heterotrophic nitrifiers will begin to help to redress this gap in our knowledge on nitrification.