Response of a freshwater bacterial community to mercury contamination (HgCl2 and CH3HgCl) in a controlled system

An experimental study was made on the effects of inorganic mercury (HgCl₂) and methylmercury (CH₃HgCl) on freshwater aerobic heterotrophic bacterial population. The experimental system was based on two-compartment biotopes: natural sediment, from the Garonne River, and dechlorinated tap water. The response of bacterial communities to mercury additions (to the water column) was monitored by determining total Hg in water and enumerating the total number of bacteria and the evolution of the mercury resistant community. Isolation was carried out by plate count method. Enumeration of mercury resistant strains was made with a general medium (Iron-Tryp-tone agar) amended with 10 g Hg/ml of HgCl₂. The response to 2 g Hg/L (HgCl₂) was fast approximately 50% of the maximum percentage of mercury resistant bacteria being reached after one hour (21.7% after 17 h exposure). Spikes of CH₃HgCl (2 g Hg/L) in the water column caused an initial inhibition of growth of Hg-resistant and sensitive bacteria followed by a complete recovery of the background microbial population after 84 h. Seven mercury resistant bacterial strains were isolated from the experimental systems and each of them was checked for HgCl₂ and CH₃HgCl transformation. All were able to volatilize HgCl₂ by producing elemental mercury, but none was able to degrade methylmercury. Additions of different concentrations of HgCl₂ (0.02 g Hg/L to 2 g Hg/L) to the water column caused a proportional increase in the percentage of mercury-resistant bacteria. Low concentrations (<0.6 g Hg/L) of CH₃HgCl also induced the Hg-resistant community, whereas 2 g Hg/L of CH₃HgCl inhibited the growth of both Hg-sensitive and Hg-resistant heterotrophic bacteria.