GENERAL MICROBIOLOGY - T.P. Pirog - 2004

16. UTILIZATION OF INORGANIC HYDROGEN DONORS: AEROBIC CHEMOLITHOTROPHIC BACTERIA

16.1. OXIDATION OF AMMONIA AND NITRITE (NITRIFICATION)

The conversion of ammonia (ammonium) to nitrate—nitrification—is carried out by nitrifying Bacteria. However, no bacteria are known to convert ammonia directly to nitrate. Its oxidation always involves two distinct groups of bacteria: one group oxidizes ammonia to form nitrite, while the other oxidizes nitrite to nitrate.

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Nitrifying bacteria are Gram-negative bacteria belonging to the family Nitrobacteriaceae. Until recently, they were considered obligate chemolithoautotrophs because they do not utilize organic substrates added to growth media. However, there are currently many doubts regarding the obligate nature of nitrifiers. For instance, it has been shown that Nitrobacter winogradskyi is capable of utilizing acetate for the synthesis of certain cellular components (Proteins, Poly-β-hydroxybutyrate).

The Role of nitrification processes in soil. In well-aerated soils, ammonium ions released during the mineralization of nitrogen-containing compounds are rapidly oxidized. The transition from the cation (NH₄) to the anion (NO₃) is accompanied by soil acidification and increased solubility of minerals (potassium, calcium, magnesium, and phosphoric acid salts). Therefore, nitrifiers were long viewed as a crucial factor in enhancing soil fertility. Today, however, this perspective has shifted. It has been found that ammonium ions are retained in the soil much longer and more effectively than nitrate, which is prone to leaching. Consequently, There is a growing trend to limit nitrification in agricultural soils. Researchers are actively searching for substances capable of specifically inhibiting the growth of nitrifying bacteria to act as «stabilizers» of soil nitrogen.

It should be noted that the growth and METABOLISM of autotrophic nitrifiers proceed normally only within a pH range of 7–8. The pH range supporting complete nitrification from ammonia to nitrate is very narrow, as both ammonia (at high pH values) and nitric acid (at low pH values) exert toxic effects on Nitrobacter.

Nitrifying bacteria are indirectly involved in the degradation of various structures built with lime and cement. This occurs because nitrifiers oxidize ammonia present in the atmosphere and animal excreta into nitric acid.