Acclimation of aquatic microbial communities to Hg(II) and CH3Hg+ in polluted freshwater ponds

The relationship of mercury resistance to the concentration and chemical speciation of mercurial compounds was evaluated for microbial communities of mercury-polluted and control waters. Methodologies based on the direct viable counting (DVC) method were adapted to enumerate mercury-resistant communities. Elevated tolerance to Hg(II) was observed for the microbial community of one mercury-polluted pond as compared to the community of control waters. These results suggest an in situ acclimation to Hg(II). The results of the methylmercury resistance-DVC assay suggested that minimal acclimation to CH₃Hg⁺ occurred since similar concentrations of CH₃HgCl inhibited growth of 50% of organisms in both the control and polluted communities. Analyses of different mercury species in pond waters suggested that total mercury, but not CH₃Hg⁺ concentrations, approached toxic levels in the polluted ponds. Thus, microbial acclimation was specific to the chemical species of mercury present in the water at concentrations high enough to cause toxic effects to nonacclimated bacterial communities.