Nitrification is a bacterial process in which ammonia (NH3) is oxidized to nitrate (NO3"). Nitrification is carried out by two groups of bacteria (Bock et al. 1986). NH3 oxidizing bacteria transform NH3 to nitrite (NO2") while NO2" oxidizing bacteria subsequently oxidize the N02~ to NO3". In both cases, the oxidation of the N compound provides the reductant required by the bacteria for energy and biosynthesis, i.e. the bacteria are chemolithotrophs. The bacteria that oxidize NH3 are obligate chemolithotrophs (Bock et al. 1986), i.e. they are entirely dependent upon NH3 as a source of electrons in spite of the fact that NH3 is not a particularly good energy source thermodynamically (Hooper 1989). These bacteria are also obligate autotrophs - they obtain most or all of their carbon for growth from CO2 (Killham 1986).

In croplands fertilized with NH3 or urea-based compounds, oxidation of NH3 by these bacteria leads to the loss of available N through leaching of the oxidized forms of nitrogen. While NH3, which is present primarily as ammonium (NH4+) in soils, remains bound to typical soils (i.e. having a net negative charge), NO3" is more mobile in the soil and is readily leached into ground and surface waters. Furthermore, when the NO3" subsequently serves as a substrate for denitrification (the process where NO3" and other forms of oxidized N are reduced to N2), then the NH3 fertilizer is lost to the atmosphere as N2. Trace amounts of NO and N2O, both greenhouse gases, are also released in the processes of nitrification and denitrification (McLaren et al. 1991). The industrial production of ammonia requires large inputs of energy in the form of natural gas and H2. The loss of N through the processes initiated by nitrification corresponds to a loss of energy used in the production of NH3. In spite of the negative role of nitrification in croplands, nitrification is beneficial to the treatment of NH4+ in sewage (by initiating the conversion of NH4+ to N2) and may have potential in the bioremediation of polluted soils and waters (through the indiscriminate action of the monooxygenase that initiates nitrification) (Rasche et al. 1991; Vitousek et al. 1997). NH3 oxidizing bacteria in general and N. europaea in particular are the focus of this review.

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