Trace metal exposure of soil bacteria depends on their position in the soil matrix

Micropores and biofilms of soils may protect bacteria against chemical stress, predation, and competition phenomena, explaining the great diversity and robustness of soil microbial communities and functions. We used sequential dispersion/density gradient centrifugation to separate free and loosely attached cells (FLA) from strongly attached cells (SA). The two fractions of the soils communities were investigated along a Zn and Cd pollution gradient, and the pollution-induced trace metal community tolerance (PICT) for SA and FLA was analyzed. FLA had developed a strong PICT in response to the 80 years of Zn and Cd pollution, whereas SA was virtually unaffected. It appears that the position of SA in biofilms and micropores has effectively protected them against toxic metal concentrations. The estimated free ion activity showed that the Cd activity was too low to reach toxic levels (PICTCd was probably caused by Zn). In contrast, the estimated Zn ion activity was close to a critical level, and could have caused the observed PICTZn in FLA, at least if temporal/spatial fluctuations of soil pH are taken into account. Such fluctuations could also explain the protection of SA as a result of diffusion constraints; which would be of little help under constant conditions because chemical equilibrium would be reached throughout.