Tolerance of nitrifying bacteria to copper and nickel

Evidence is mounting that soil microorganisms can become increasingly tolerant to metals on exposure. However, in situ investigations regarding the effects of metals, particularly Cu and Ni, on specific soil functions/communities are still limited in number. Here, we investigated whether preexposure of nitrifying bacteria to Cu or Ni can induce increased tolerance to these metals. We also investigated whether changes in the tolerance of populations exposed to Cu under field conditions (long term) or in a laboratory-spiked soil (short term) occur. The method used was specifically designed to avoid possible confounding factors because of aging of metals in soil. Sterilized soils were enriched with different concentrations of Ni or Cu and were inoculated with the same soil that was either uncontaminated or had been contaminated previously with metals. Nitrification was measured after 28 d. In the laboratory-spiked soil, the exposed nitrifier community showed an increased tolerance to Ni but not to Cu. However, we found an increased tolerance to Cu in the case of a nitrifying community exposed to Cu for nearly 80 years under field conditions. These results indicate that the capacity of nitrifying bacteria to adapt to at least some metals is a widespread phenomenon. However, acquisition of tolerance to Cu may be more difficult, or require more time, compared with tolerance to Ni.