Arch. Environ. Contam. Toxicol. 22, 439-444 (1992) E Archives of nvironmental contamination and Iloxicology © 1992Springer-Verlag New York Inc. Response of a Freshwater Bacterial Community to Mercury Contamination (HgCl2 and CH3HgCI) in a Controlled System Franco Baldi *l, Alain Boudou**, and Francis Ribeyre** *University of Siena, Department of Environmental Biology, Via P.A. Mattioli, 4. 1-53100 Siena, Italy and **University of Bordeaux I, Laboratory of Fundamental Ecology and Ecotnxicology, URA CNRS 1356, 33405 Talence Cedex, France Abstract. An experimental study was made on the effects of inorganic mercury (HgCI2) and methylmercury (CH3HgC1) on freshwater aerobic heterotrophic bacterial population. The ex- perimental 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 Ixg Hg/ml of HgCI2. The response to 2 Ixg Hg/L (HgC12) 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 CH3HgC1 (2 [xg Hg/L) in the water column caused an initial inhibition of growth of Hg-resistant and sensitive bacteria fol- lowed 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 HgC1 z and CH3HgC1 transformation. All were able to volatilize HgC12 by producing elemental mercury, but none was able to degrade methylmercury. Additions of differ- ent concentrations of HgC12 (0.02 jxg Hg/L to 2 Ixg Hg/L) to the water column caused a proportional increase in the percentage of mercury-resistant bacteria. Low concentrations (<0.6 Ixg Hg/L) of CH3HgC1 also induced the Hg-resistant community, whereas 2 jxg Hg/L of CH3HgC1 inhibited the growth of both Hg-sensitive and Hg-resistant heterotrophic bacteria. Mercury resistance in heterotrophic aerobic bacteria has been known for 30 years (Moore 1960). The mer operon has been sequenced (Bennet et al. 1978; Ogawa et al. 1984) and the enzymes mercury reductase and organomercurial lyase, which degrade inorganic and organic mercury to Hg° and their respec- tive hydrocarbons from organomercurials, have been purified ~To whom correspondence should be addressed. (Begley et al. 1986; Fox and Walsh 1982). The enzyme mech- anism was recently reviewed by Silver and Misra (1988). Several reports (Nelson and Colwell 1975; Porter et al. 1982; Baldi et al. 1989) exist on mercury resistant bacteria in the environment, but the response of mercury resistant communi- ties to inorganic mercury pollution is not well investigated and even less, the response to organomercurials (Nakamura et al. 1990). In terrestrial (Baldi et al. 1991) and aquatic environ- ments (Olson et al. 1979; Barkay and Olson 1986; Barkay 1987), the percentage of mercury resistant bacteria is high and often correlated to inorganic mercury distribution in polluted areas. However, contradictory results appear in natural condi- tions. Barkay (1987) found that the enumeration of mercury resistant bacteria in aquatic environments by agar plates amended with 10 p~g Hg/ml HgCI2 gave a concentration two orders of magnitude higher than the DNA gene probe method prepared for gram-negative bacterial communities (Barkey et al. 1985). In a terrestrial environment around a geothermal source (Baldi et al. 1991), the percentage of mercury resistant bacteria isolated from mosses was not always correlated with the total concentration of mercury found in bryophyte leaves, which are regarded as conventional bioindicators for air-borne metal pollution (Rasmussen 1977). Investigation of mercury resistant bacteria communities in a simplified freshwater system could therefore be useful for studying their response to additions of mercury compounds and subsequently for understanding their role in polluted environ- ments. The aim of this study was to study the kinetics and develop- ment of aerobic heterotrophic bacteria populations, both resis- tant and sensitive to mercury compounds, in an experimental system. The system was based on two-compartment biotopes (water column and sediment) and different biological models: rooted macrophytes (Elodea densa, Ludwigia natans), burrow- ing mayfly nymphs (Hexagenia rigida). They were used to study: 1) the actions and interactions of the abiotic factors: temperature, pH, and photoperiod: 2) the sediment and water column as initial contamination sources of inorganic and or- ganic mercury species; 3) mercury bioaccumulation and trans- fer from freshwater environment to biota (Maury-Brachet et al. 1988, 1990; Ribeyre and Boudou 1989, 1990; Saouter et al. 1990). Determination of the chemical forms of Hg accumulat-