Nitrifying Bacterium

Nitrifying Bacterium 

an aerobic, gram-negative, flagellate microorganism that converts ammonia and ammonium salts to nitrates. Nitrifying bacteria are found in soil and water. These bacteria were isolated and described in 1890 by the Russian microbiologist S. N. Vinogradskii. (This work enabled him to subsequently discover chemosynthesis.)

The conversion of NH₃ to nitrates—nitrification—is achieved in two stages. The bacteria of the genus Nitrosomonas first oxidize NH₃ to nitrite: NH⁺₄ + 1½O₂ → NO^-₂ + 2H⁺ + H₂O. In the second stage, the bacteria of the genus Nitrobacter oxidize nitrite to nitrate: NO^-₂ + ½ O₂ → NO^-₃. Thus, the two groups of nitrifying bacteria are metabiotically interrelated: bacteria that oxidize NH3 provide a substrate for bacteria that oxidize nitrite.

Like other chemoautotrophs, nitrifying bacteria can use the energy of oxidation to assimilate carbon from carbonates or from atmospheric CO₂; the carbon is then used to synthesize microstructural elements within the cell. Nitrifying bacteria cannot assimilate carbon from the organic compounds that are present in soil, freshwaters, or artificial nutrient media.

A. A. IMSHENETSKII