Activity of Chemolithotrophic Nitrifying Bacteria under Stress in Natural Soils

Nitrification is an important process in the biogeochemical cycle of nitrogen, linking its reduced and oxidized parts. Since the conversion of ammonium to nitrate has a great impact on the environment, such as weathering of soils, production of greenhouse gases, and eutrophication of surface and ground waters (Van Breemen and Van Dijk, 1988), it is important to know the characteristics of the responsible organisms. Although many organotrophic microorganisms are able to produce oxidized nitrogenous compounds such as nitrite and nitrate (Focht and Verstraete, 1977; Killham, 1987; Kuenen and Robertson, 1987), chemolithotrophic nitrifying bacteria are considered to be the most important group producing these compounds from ammonia. A contribution to nitrate production by organotrophic microorganisms has only been observed in some acid coniferous forest soils (Schimel et al., 1984; Killham, 1986, 1990). According to Bergey’s Manual of Systematic Bacteriology (Watson et al., 1989), the family of nitrifying bacteria is a diverse group of rods, vibrios, cocci, and spirilla, all having the ability to utilize ammonia or nitrite as a major source of energy and carbon dioxide as the chief source of carbon. With the exception of Nitrobacter species, all others are obligate chemolithotrophs, but some can grow mixotrophically on a mixture of CO2 and small organic compounds. All strains are aerobic, but some may proliferate at low oxygen concentrations. Some Nitrobacter species might even grow at the expense of nitrate reduction in the absence of oxygen.