The bioaccumulation of mercury in aquatic organisms is controlled by a large number of biotic factors such as the position of the organism in the food web and various biological parameters. It is also controlled by abiotic factors such as pH, dissolved organic carbon and other seasonal and limnological conditions. The possible interactions between chemical elements and Hg accumulation are poorly known and the existing information is often obtained in restricted laboratory conditions with unrealistically high concentrations of Hg and elements and under inadequate exposure time. The main objective of our research is to understand how selenium affects mercury accumulation in natural conditions in the food chain of freshwater lakes.
Our first results showed a clear antagonistic effect between Se and Hg in the tissues of two freshwater fish species, yellow perch and walleye. It was confirmed by the low levels of Hg in tissues containing high concentrations of Se and vice-versa (Figure 1). The concentrations of mercury and selenium as well as information on the speciation of the latter were also measured in the sampled lakes. In Year 2 of the project, our attention was focused on other aquatic species representing lower levels of the food web in lakes characterized by various levels of dissolved Hg and Se. Samples of zooplankton, mayflies and amphipods were collected in ten lakes located at increasing distances from Sudbury smelters. Water samples were also collected at two different periods of the year. Concentrations of total Hg in tissues can be directly correlated with distance from Sudbury smelters and inversely correlated with Se concentrations in lake water, once again suggesting the action of an antagonistic mechanism by Se on Hg assimilation. The correlations between total Hg and total Se in organisms from the lower levels of the food web were not as clear as those in fish tissues. However when total concentrations of Se are replaced by a ratio Se/Hg, correlations improved remarkably for all species, including those with fish species in our previous work. This suggests that Se is not an independent variable and that the assimilation of Hg is somehow affected by the presence of Se. When the concentration of Se is significantly higher than that of Hg, Se plays a dominant role in Hg assimilation processes.
It is now clear that Se play an important role in the bioassimilation of mercury in aquatic systems and this information is now being considered in several other COMERN projects.
Figure 1. Concentration of Hg vs concentration Figure 2. Concentration of Hg in of Se in muscle tissues of walleye. tissues of amphipods vs that of Se. Each point represents samples from one lake. in lakewater.
We have also been investigating on the role of Se on Hg assimilation in bacteria and cells. This project looks more specifically at the distribution of Se and Hg in various compartments (solution, cell membrane, precipitate, etc) of bacteria and considers some important biochemical parameters. The results show that bacteria can eliminate most of Se from a stressed culture by reducing it to an insoluble form of Se(0) and eliminate Hg by generating a volatile form of the metal, likely Hg(0) through the onset of some mercury reductase enzymes. When the two elements are present in the culture, it appears clearly that Se is playing a detoxifying role by limiting the availability of Hg to the bacteria.
Other projects developed here are related to the behavior of Se in sediments and to the influence of the chemical species of Se on the geochemistry of Hg in lake sediments. This aspect will be more developed during Year 3 and following years to investigate on the role of Se in Hg methylation and bioaccumulation. Sediment and pore water samples have been collected in two Sudbury lakes and are now being processed and analyzed for Hg and Se. These two projects are more specifically related to one of our objectives which are to understand the effect of high Se contents in sediments on the methylation of Hg.
A significant amount of time was also spent on the speciation of selenium in water, more particularly on the identification and characterization of Se(VI) and the extraction of Se(0) from sediments in collaboration. A significant attention has also been given to the determination of methylmercury and we are now capable of obtaining good reproducible results after solving several technical problems.
Since the beginning of Year 3, we focus our sampling efforts on one lake located in the middle of the City of Greater Sudbury. Our objective is to determine the seasonal effects in the bioaccumulation process of Hg and Se for zooplankton in four different basins of Ramsey Lake. Ramsey Lake is one of the main drinking water reservoirs for the city of Sudbury and it is the subject of an extensive monitoring program and health risk assessment project supported by Inco Ltd and Falconbridge Ltd.
Our study on the geochemical behavior of Hg in lake sediments is well underway and we expect to continue with fieldwork and laboratory simulations to test the influence of Se on the Hg cycle.