Nitrification chemoautotrophic process

There are two types of dissimilatory denitrification that are considered here. The first is denitrification that takes place during the chemoautotrophic process of nitrification. Nitrification is the sequential oxidation of to N02, which is... 

Recently research has revealed that nitrification is not exclusively associated with chemoautotrophic bacteria of the P and y proteobacteria, but occurs in many Crenarcheaota as well (Francis et al, 2005 Konneke et al, 2005 Wuchter et al, 2006) who may in fact may dominate this process in seawater (Wuchter et al, 2006) (see Chapter 5 by Ward, this volume). The extent that archael nitrification occurs in coral reef habitats remains to be determined. Ammonium oxidation has also been shown to occur anoxicaUy in some marine sediments at the expense of N02 (Thamdrup and Dalsgaard, 2002) (see Chapter 6, Devol, this volume). 

Nitrification is the process of oxidation of ammonia to nitrite and nitrate by chemoautotrophs of the Gram-negative genera Nitrosomonas and Nitrobacter that employ nitrification as their sole source of energy. Nitrosomonas mediates oxidation of ammonia to nitrite according to the overall reaction ... 

Nitrification is the biological oxidation of ammonium (NH4) to nitrate (NOJ), with nitrite (NOJ) as an intermediate under aerobic conditions (NHj NOJ NOJ). Under oxygen-limited conditions nitrifiers can use NOJ as a terminal electron acceptor to avoid accumulation of the toxic NOJ, whereby N2O and NO are produced. During that process, because separate bacteria (Meiklejohn 2006) oxidize NHj into NOJ and NOJ into NOJ, the process can lead to the temporary accumulation of NOJ in soil and water. These bacteria are generally chemoautotrophic, requiring only CO2, H2O and O2. The nitrifying bacteria Nitro-somas, which converts NHj to NOJ, is also able to reduce NOJ. This nitrite can decompose abiotically, yielding NO or NO2, substantially favoured in acidic soils... 

Recall from Section 2.4.2 that nitrification is a multistep process in which ammonium (NUt ) is oxidized to nitrate using O2. Nitrification is carried out by various nitrifiers, chemoautotrophic bacteria that occur in oxic environments where ammonia is present (Ha)mes, 1986). Nitrifiers use chemical energy released by ammonium oxidation to reduce CO2 to organic matter, much as green plants use energy captured from light to reduce CO2. The intermediate species of nitrification are shown in Eq. (4.55). Note that N2O can be produced by side reactions from either of two intermediates by chemical rearrangement of the short-lived nitroxyl molecule (HNO) or by the reduction of nitrite (N02 ) within the cells of the nitrifiers. 

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