Pollution-induced community tolerance of soil microbes in response to a zinc gradient

The long-term accumulation of Zn in soils provides the microbial community time to adapt to this heavy metal. To assess the effects of long-term exposure to Zn on the metabolic diversity and tolerance to Zn of soil microbial community, the pollution-induced community tolerance (PICT) method, based on the Biolog microtitre plate system, was used. It especially is useful to study gradients of pollutants for detecting PICT. Such a Zn gradient was found by selecting soils at increasing distances from galvanized electricity pylons at two sites where Zn accumulation had occurred over a period of decades. Soil metabolic profiles showed a humpbacked response to increasing soil Zn concentrations, indicating that accumulation of Zn up to 2,000 mg/kg did not decrease the metabolic biodiversity in the culturable fraction of the microbial community. This fraction of the microbial community showed significant PICT, because the effective concentration that reduces the metabolic biodiversity by 50% (EC50), values for Zn added to the Biolog wells increased by up to two orders of magnitude with increasing soil-Zn concentration along the transects. Significant PICT was detectable at soil Zn concentrations above approximately 300 mg/kg. The EC50 values correlated more closely with soil total Zn than with soil pore water Zn. The results suggest that, although long-term exposure to Zn imposes stress on soil microbes, resulting in an increased tolerance, detectable PICT does not necessarily lead to a decrease in metabolic diversity.