Nitrifying Bacteria

Two groups of highly specialized organisms—the ammonia and the nitrite oxidizers—are called nitrifiers. The organisms are characterized by the capacity for lithotrophic growth in which energy is generated by the oxidation of ammonia to nitrite (ammonia oxidizers) or nitrite to nitrate (nitrite oxidizers). Nitrifiers are autotrophic bacteria, fixing CO2 as the main carbon source via the Calvin cycle. In addition to aerobic lithotrophic growth, the organisms possess a high metabolic diversity. The nitrogen of the biosphere exists in the oxidations states of NH 3 . Lithotrophic nitrification is considered to be the main process by which the transformation of more reduced nitrogen compounds to nitrate occurs. The nitrogen cycle is characterized by mobilization, immobilization, and transformation of the various nitrogen‐containing compounds. Nitrifying bacteria are of ecological importance because these organisms convert ammonia, which is often absorbed to soil particles, to nitrate, which is mobile in soil water. Although Nitrosomonas and Nitrobacter are the two most commonly isolated nitrifiers, they are not necessarily the most abundant ones in natural habitats. Their dominance in enrichment cultures may be explained by the fact that conditions in the commonly used enrichment culture methods promote the growth of these organisms.