Mercury

1. Mercury Nick Boerner Biol 464

2. Chemical and Physical Properties • Atomic number: 80 • Chemical series: transition metal • Appearance: silvery • Phase: liquid • Density: 13.534 g/cm3 • Atomic weight: 200.59 g/mol • Melting point: -37.89º F • Boiling point : 674.11º F

3. 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

4. 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

5. 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

6. 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

7. Acute and Chronic Toxicity • Ingeneral, 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, 10.0-55.6 in the liver, 17.0 in the whole body and 37.7 in the kidney

8. 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

9. 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, 403-406 • Morel, Francois The chemical cycle and bioaccumulation of mercury. Annual review of ecology and systematics. 1998. Vol 29: 543-566 • Trudel, M and Rasmussen, J. Modeling the Elimination of Mercury by Fish. Environmental Science Technology. 1997, 31, 1716-1722 • Vallee, Bert. Ulmer, David. Biochemical effects of mercury, lead, and cadmium. Annual Review Biochemistry. 1972. 41:91-128