Bacteria ammonia-oxidizing, denitrification

Bodelier, P. L. E., Lihochant, J. A., Blom, C. W. P. M., and Laanbroek, H. J. (1996). Dynamics of nitrification and denitrification in root-oxygenated sediments and adaptation of ammonia-oxidizing bacteria to low-oxygen or anoxic habitats. Appl. and Environ. Microbiol. 62, 4100-4107. 

Ammonia is oxidized in nature to nitrate via several intermediates in the process of nitrification. Nitrate may be reduced to nitrite by either a dissimilatory or an assimilatory process. Nitrite may be assimilated into the cell via reduction to ammonia, or it may be reduced by microorganisms to N20 and N2 in denitrification. A major part of the total nitrogen in this pathway is lost to the atmosphere. However, in turn, atmospheric dinitrogen is converted to ammonia by various bacteria in nitrogen fixation. 

There are four different types of nitrite reductases the copper-containing protein Copper Enzymes in Denitrification and cytochrome cd perform a one-electron rednetion of nitrite to nitric oxide, and are involved in denitrification " the siroheme-containing protein and the cytochrome c ititrite reductase (cNiR) both perform the complete, six-electron reduction, of nitrite to ammonia. The cNiR is present in the y, 5 or e-subclasses of proteobacteria, and is encoded by the nrf operon (nitrite reduction with /ormate), which has different gene composition in the different classes of bacteria, having in common only the gene for the catalytic subunit, ntfA. 

A number of different enzymes can carry out the reduction of nitrite to either ammonium or nitric oxide and/or nitrous oxide. The latter types are involved with the denitrification process (Payne, 1973) and will not be considered here. Among the enzymes that catalyze the six-electron reduction of nitrite to ammonia, several different types are recognized. These are (I) assimilatory NiRs that function in biosynthetic nitrate assimilation of higher plants, algae, and fungi, (2) ammonia-forming dissimilatory NiRs involved in anaerobic nitrate respiration of diverse bacteria, and (3) assimilatory and dissimilatory sulfite reductases... 

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