Mercury Nick Boerner Biol 464

Chemical and Physical Properties Atomic number: 80 Chemical series: transition metal Appearance: silvery Phase: liquid Density: g/cm3 Atomic weight: g/mol Melting point: º F Boiling point : º F

History In china, India, and Tibet, mercury use was thought to prolong life The ancient Greeks used mercury in ointments and the ancient Egyptians and Romans used it in cosmetics More recent uses include preserving wood, silvering mirrors, anti-fouling paints, herbicides, handheld maze games and road leveling devices in cars and in the making of felt hats Mercury compounds have been used in antiseptics, laxatives, antidepressants, and antisyphilitics. They were also allegedly used by allied spies to sabotage German airplanes Today mercury is used primarily for the manufacture of industrial chemicals or for electrical and electronic applications

Releases into the environment Mercury is a extremely rare element in the Earth’s crust, having an overall abundance by mass of.08 ppm Natural sources such as volcanoes are responsible for approximately half of atmospheric mercury emissions Anthropogenic emissions of mercury include the combustion of fossil fuels, mining and reprocessing of gold, copper, and lead, and the disposal of batteries, fluorescent lamps and thermometers. Elemental mercury in the gaseous state reoxidizes relatively slowly to the mercuric state Hg (2)….this allows its residence time in the atmosphere to be on the order of a year which permits a global distribution The return of mercury from the atmosphere to the Earths surface occurs via wet precipitation of dissolved Hg (2) which distributes mercury into the soil,rivers, lakes and oceans. Residence time for mercury in soils is 1,000years,ocean water: 2,000 years and ocean sediments: >1 million years

Toxicity to Aquatic life All mercury compounds interfere with thiol metabolism causing inhibition or inactivation of proteins containing thiol ligands and ultimately leading to mitotic disturbances pH, alkalinty, redox and other variables lead to a variety of chemical species that have varying toxicities Methyl mercury is the most hazardous mercury species due to its high stability, its lipid solubility, and its possession of ionic properties that lead to a high ability to penetrate membranes in living organisms Sulfate reducing bacteria are responsible for the bulk of mercury methylation in natural waters Methyl mercury bioaccumulates in the food web and remains in the fatty tissue of animals As a result, the average proportion of methyl mercury over total Hg increases from about 10% in the water column to 15% in phytoplankton, 30% in zooplankton, and 95% in fish

Mode of entry Mercury is first transported across the lipid membrane that surrounds unicellular organisms (compounds with Kow of 3.0 or higher are able to cross the membrane readily) The microbial uptake of mercury is thus a key step both in its methylation and its bioaccumulation The accumulation of methyl mercury in higher organisms results mainly from the ingestion MeHg-containing food rather than direct uptake of mercury from the water Inorganic mercury is absorbed through gills, intestine, or skin and is distributed through the blood and accumulated in liver and kidney Organic mecurials such as methyl mercury accumulate in the muscles and brain more than other tissues

Acute and Chronic Toxicity In general, toxicity is higher at elevated temperatures, at reduced salinities in marine organisms, and in the presence of other metals such as zinc and lead Signs of acute mercury poisoning in fish include flaring of gill covers, increased frequency of respiratory movements, loss of equilibrium, and sluggishness Signs of chronic mercury poisoning in fish include brain lesions, cataracts, diminished response to change in light intensity, inability to capture food, and abnormal motor coordination Toxic concentrations of mercury salts range from less than.1ug/l to 200 ug/l for species of marine and freshwater organisms Rainbow trout LC-50 (96 h)= 24.0 ug/l Copepod Acartia tonsa LC-50 (96h)= 13.0 ug/l Mammal lethal residues 6.0 mg/kg in the brain, in the liver, 17.0 in the whole body and 37.7 in the kidney

Biotransformation High metabolic rate of the liver allows the excretion of inorganic forms of mercury in fish Organomercurials accumulate particularly in the brain where more than 98% of the mercury is in the form of methyl Hg Biotransformations and mechanisms of breakdown appear to occur quite slowly but these mechanisms are undetermined UV radiation can photoreduce free methyl mercury in the water column Methyl mercury is decomposed by bacteria in two phases. First hydrolytic cleave the C- Hg bond releasing the methyl group. Second, a reductase enzyme converts the ionic Hg to the elemental form, which is then free to difuse from the aquatic environment into the vapor phase Some drugs have been used to counteract the mercury poisoning in humans –2,3-dimercaptopropanol, polthiol resins, selenium salts, vitamin E and sulfhydryl agents –These compounds compete with Hg for protein binding sites

Bibliography Clarkson T.W. Mercury: Major issues in environmental health. Environmental Health Perspective. 1993, 100:31-38 Lann, Hans. Mercury accumulation in food chains. Nordic Society Oikos. 1971, 22, Morel, Francois The chemical cycle and bioaccumulation of mercury. Annual review of ecology and systematics Vol 29: Trudel, M and Rasmussen, J. Modeling the Elimination of Mercury by Fish. Environmental Science Technology. 1997, 31, Vallee, Bert. Ulmer, David. Biochemical effects of mercury, lead, and cadmium. Annual Review Biochemistry :91-128