Acclimation of subsurface microbial communities to mercury.

We studied the acclimation to mercury of bacterial communities of different depths from contaminated and noncontaminated floodplain soils. The level of mercury tolerance of the bacterial communities from the contaminated site was higher than those of the reference site. Furthermore, the level of mercury tolerance and functional versatility of bacterial communities in contaminated soils initially were higher for surface soil, compared with the deeper soils. However, following new mercury exposure, no differences between bacterial communities were observed, which indicates a high adaptive potential of the subsurface communities, possibly due to differences in the availability of mercury. IncP-1 trfA genes were detected in extracted community DNA from all soil depths of the contaminated site, and this finding was correlated to the isolation of four different mercury-resistance plasmids, all belonging to the IncP-1beta group. The abundance of merA and IncP-1 plasmid carrying populations increased, after new mercury exposure, which could be the result of selection as well as horizontal gene exchange. The data in this study suggest a role for IncP-1 plasmids in the acclimation to mercury of surface as well as subsurface soil microbial communities.