Response of soil bacterial communities pre-exposed to different metals andreinoculated in an unpolluted soil

Changes in tolerance levels of soil bacterial communities pre-exposed to di fferent metal concentrations and then reestablished in an unpolluted soil w ere examined during a one year incubation period using the thymidine incorp oration technique. The study was performed with a sterilized agricultural s oil, which was reinoculated with bacteria extracted from soils previously c ontaminated with high doses of Zn, Cu or Cd, and from an unpolluted soil. B acteria pre-exposed to metal addition initially exhibited a greater toleran ce than those non-preexposed. The microbial communities responded to the ab sence of metals in the reinoculated soil with a rapid decrease in community tolerance. losing most of the acquired tolerance (70-90%) within the first week. After that no changes in community tolerance were detected. Thus, a long-lasting effect of the original community tolerance of the inoculum was detected even 12 months after the metal stress was removed. At this time t here was still a dose-response effect left since higher tolerance levels we re usually found in soils with higher tolerant inoculum. Changes in toleran ce levels over time showed similar trends, irrespective of which metal the bacterial communities were initially tolerant. (C) 2001 Elsevier Science Lt d. All rights reserved.