Biogeochemical Cycling of Mercury in the Environment A. Russell Flegal, Microbiology and Environmental Toxicology The Flegal lab investigates biogeochemical cycling of trace metals in the environment. One of the lab's current research project involves examination of the transport, fate, and cycling of mercury, a potent neurotoxin to both aquatic life and humans. The results of this research and other similar studies will play an important role in assessing programs designed to mitigate the impact of such trace elements on human health and the environment.

Research carried out by Russ Flegal's lab aims to better understand key aspects of the biogeochemical cycling of trace metals in the environment. Some of these metals, such as zinc and copper, are unique in that they can be both essential and toxic to biological systems, depending upon their concentrations. Others, such as lead and mercury, are not believed to have any positive function in biological systems, and are believed to be toxic at nearly all concentrations. By investigating the transport, fate, and biogeochemical cycling of these metals in the environment we can hope to better understand the pathways by which humans and other biological systems are exposed to these toxicants, the extent to which human perturbations are responsible for this exposure, and means by which their toxicity can be mitigated in the environment.

In the past Dr. Flegal's work focused largely on lead, and dealt both with human perturbations in its global and regional cycling, as well as the toxic effects of lead in biological systems. And while recent research has included using lead isotopes to discern the origin of lead found in chocolate and vinegar and to elucidate the cycling of lead in the Hudson River estuary, the lab has expanded its expertise and now deals with numerous other trace metals. These include mercury, silver, chromium, zinc, cupper, iron, cadmium, and nickel. Recent research has involved analyses of the concentration, speciation, and isotopic compositions of trace metals in water, sediment, aerosols, and organisms in study areas such as the San Francisco Bay estuary, the Pacific Ocean, and numerous surface and groundwater systems throughout California.

Mercury, a current focus of the Flegal lab, is a potent neurotoxin which presents a health threat to both aquatic life and humans, and is found at elevated levels in the environment due to anthropogenic activities. The chemical and biological cycling of mercury in the natural environment is extremely complex and incompletely understood. Mercury's toxicity is a function of its speciation, or chemical form, which also plays a major role in the rates of its methylation, bioavailability, and biomagnification in aquatic systems. The form of mercury of most concern is monomethyl mercury, as a result of its toxicity and ability to biomagnify up food chains, resulting in concentrations in fish more than 100,000 times greater than the associated waters. Concentrations of methyl mercury in some fish have been found to exceed health standards, and health advisories for those most susceptible to mercury toxicity, namely pregnant women and young children, have been posted at popular fishing areas and supermarket meat counters.

Mercury in aquatic systems can originate from atmospheric deposition, point source industrial and medical discharges, as well as runoff from mercury and gold mines. The last of these represent a legacy of ongoing mercury contamination to California's waters. The two largest mercury mines in North America, the historic New Almaden and New Idria mining districts, are located in California and drain into San Francisco Bay. Much of the mercury from these mines was used in the prolific gold mining carried out in California during the 19th centruy, with mercury contaminated sediments still found throughout the Sierra Nevada today. Industrial point sources, medical waste, and waste water effluent still pose significant environmental inputs of mercury in some areas. As such, mercury represents not only a long standing toxic contaminant of concern in California, but will also continue to pose serious concerns for both human and aquatic life well into the future.

The long term success of current programs and attempts to remediate and mitigate the risks of mercury remains to be validated, and is severely limited by the current lack of knowledge regarding many aspects of the cycling of mercury in the environment. As a result, the Flegal lab is currently carrying out research which will help to elucidate poorly understood aspects of this cycling, and help during the formulation of future strategies at environmental mercury cleanups.

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