The Role of Bacteria in Controlling Methylmercury

by either decreased methylmercury production or increased methylmercury degradation in ecosystems containing high levels of total mercury. Methylmercury degradation is mediated by bacteria in sediments, in natural waters and in soils. This activity is a part of the bacterial mercury detoxification mechanism, resulting in the degradation of methylmercury to gaseous elemental mercury, which can escape the local environment. The enzymes mediating these activities are only produced when mercury is present in the environment of the active bacteria. This is achieved by regulation of gene expression (the mer genes) that encode the functions required for mercury resistance. These genes are induced primarily by inorganic mercury, and thus, the enzymes responsible for methylmercury degradation are synthesized only when the level of mercury is high enough to activate their expression. Therefore, we hypothesized that methylmercury degradation by mercury resistant bacteria may explain the observed inverse relationship between total mercury and the proportion of (Continued on page 7) M ercury is a toxic heavy metal that has contaminated a wide variety of aquatic habitats across New Jersey through atmospheric deposition and industrial contamination. Mercury enters the environment mostly as inorganic Hg(II) where it is converted to methylmercury by bacteria in anaerobic sediments. It is this methylated form of mercury which is of particular concern since, unlike other forms of mercury, methylmercury is readily bioaccumulated and biomagnified in the aquatic food chain, posing health risks to humans who consume fish and shellfish. Because mercury toxicity is a function of its chemical speciation in the environment the reactions constituting the biogeochemical cycle of mercury (Fig. 1) are important for determining mercury toxicity. Methylmercury toxicity can lead to a variety of adverse health effects, including neurological and reproductive defects. Minimizing exposure to methylmercury in young children and women of childbearing age is especially important, since the developing brain is particularly susceptible to the toxic effects of methylmercury. Mercury concentrations in tissues of freshwater fish are elevated throughout NJ but there is no direct relationship between the degree of mercury contamination and methylmercury production and accumulation in aquatic biota. In fact, we have observed an inverse relationship between the concentrations of total mercury and the proportion of methylmercury in the total mercury pool in samples collected in contaminated streams and lakes in the Meadowlands and Pine Barrens lakes. This inverse relationship might explain the inconsistencies of fish data. Water samples contained 100 – 6800 ng/L total mercury in the Meadowlands and only 0.4 – 6 ng/L total mercury in the Pine Barrens, while methylmercury concentrations in both systems were 0.05 – 0.8 ng/L, a range common in natural waters. (Fig. 2). This apparent paradox is not unique to NJ and it may be explained By Jeffra K Schaefer, Ph.D., John Reinfelder, Ph.D., and Tamar Barkay, Ph.D., Dept. of Biochemistry and Microbiology and Environmental Sciences, Rutgers, The State University