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Environmental implications of selenium's effects on mercury retirement.

Growing awareness of the presence of mercury (Hg) in the environment and in food sources has led to mounting public concern and increasing policy consideration by legislative and regulatory agencies. Because of its longevity in the ecosystem and production and release by industrial processes, Hg contamination is essentially global. Elemental mercury ([Hg.sup.0]) is a uniquely volatile heavy element that is released into the atmosphere from numerous natural and human sources. Once airborne, [Hg.sup.0]joins the atmospheric pool and is globally redistributed until it oxidizes in the upper atmosphere and precipitates. Variable portions will be photoreduced by solar radiation and reemitted, while the rest remains in the oxidized form ([Hg.sup.+2]) to be temporarily retained where it falls. Certain quantities of this retained [Hg.sup.+2] form stable complexes with materials in the environment that reduce its biological availability while the rest is accumulated by living organisms. When Hg accumulation rates exceed the capacity of natural mechanisms to release it into the atmosphere or geologically retire it from active cycling, Hg concentrations can build up in aquatic biota.

Anaerobic bacteria employ a biochemical pathway that adds a methyl group to Hg they have incorporated. This process creates methylmercury (C[H.sub.3]Hg), the predominant form of rig that bioaccumulates in organisms of the food web. Creatures accumulate C[H.sub.3]Hg and [Hg.sup.+2] from their food sources, biomagnifying Hg concentrations from prey to predator. This begins at the bottom of the food web. Mercury present in bacteria and plankton is consumed by microinvertebrates. These invertebrates are consumed by larger creatures that are consumed by fish that are in turn consumed by even larger fish. As a result of these amplification steps, fish at the top of the food web can harbor tissue Hg concentrations > [10.sup.6]-fold higher than that of the water in which they swim.

Measuring the amount of rig present in the environment or food sources provides an incomplete and potentially inaccurate indication of Hg associated risks if the presence and effects of selenium (Se) are not also considered. Chemically similar to sulfur, Se is a nutritionally essential element required to support synthesis of a number of proteins that are expressed in tissues of all creatures. The selenide formed during each cycle of protein synthesis has an extremely high affinity for Hg ([k.sub.d] [10.sup.-45]). As a result, HgSe complexes readily form, especially in reducing environments. Selenium addition to aquatic ecosystems has been found to enhance Hg retirement and reduce Hg bioaccumulation, potentially as a result of HgSe formation. Since Se suspended in the water column will be in an oxidized state that is unable to bind Hg, the mechanism of HgSe formation is apt to be biologically mediated, potentially occurring through direct interaction with intracellular selenide. The HgSe interaction is apparent throughout the Hg cycle, influencing its transport, biogeochemical exposure, bioavailability, toxicological consequences, and remediation.

While the amount of rig present in the environment and food sources is important to know, it is equally important to determine whether sufficient Se is available to sponsor Hg retirement through formation of HgSe and still support synthesis of Se-dependent proteins. The molar relationship between Hg and Se governs not only Hg accumulation in aquatic food webs, it also determines sensitivity or robustness of Hg-exposed organisms. Sensitivity to Hg toxicity in exposed creatures appears to depend more upon the balance of rig and Se present in the system than upon the amount of Hg involved.

Phytoremediation, the use of plants to remove mercury from contaminated soil or water, may involve the formation of HgSe complexes within the plants. Certain plants are known to accumulate large amounts of rig and Se, but the interactions of the two elements in these plants remains inadequately understood. Known and potential Hg hyperaccumulator plant species are under study to determine whether the presence of Se influences Hg accumulation. If HgSe complexes are formed in the tissues of these plants, efficient, permanent, and inexpensive retirement of rig may be possible.

Studies are under way to determine the effects of different concentrations of Se in the diets of insects on the accumulation and retention of dietary Hg. Since the invertebrate digestive system is not acidified, insoluble HgSe complexes may not be biologically available. An artificial food web is being constructed that will feed these insects to other insects and fish. These studies and related projects being conducted in mollusks and mammals are designed to examine the influence of Se availability on Hg bioaccumulation. These data will allow us to assess the potential magnitude of these effects in the environment.

C.R. Jordan, * L.J. Raymond, and N.V.C. Ralston

University of North Dakota, Energy & Environmental Research Center, Grand Forks, ND
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Title Annotation:Professional Communications
Author:Jordan, C.R.; Raymond, L.J.; Ralston, N.V.C.
Publication:Proceedings of the North Dakota Academy of Science
Date:Apr 1, 2005
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