School of Biological Sciences
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School of Biological Sciences |
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Environmental Pollution and Toxicology |
Mercury in the Marine EnvironmentA lecture by Dr Rick Leah
(Overheads from the Lecture are available here as a Word File 
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Metals occur in varying amounts in sea water and since the sea is the final repository of soluble elements from the terrestrial part of the planet all are present. Some (so-called Heavy Metals), such as iron, copper, cobalt and zinc are essential in small quantities for the healthy growth of marine organisms. Others, such as mercury, lead and cadmium have no known biological role. All of these metals are toxic if present in excess but the most important marine contaminants are generally considered to be those non-essential heavy metals.
Of all of the heavy metals, most attention by environmental managers is currently paid to Mercury because it still poses the greatest threat to human health from pollution of the marine environment. There are continuing efforts to tighten environmental restrictions on this element as new research reveals that it may be causing neurological damage to unborn and young children.
In marine sediments bacteria may convert the less toxic, inorganic form of the metal to the more toxic, organic form of methyl mercury. This chemical form is relatively mobile in the environment and tends to accumulate in fish. Consumption of marine (or other) food containing methyl mercury over an extended period may lead to the onset of Minimata disease. This disease, first observed in Minimata Bay in Japan where the local fish-eating community received large doses of mercury derived from a local chemical factory manufacturing acetaldehyde and vinyl chloride, has neurological symptoms which include, mental disturbance, ataxia and impairment of gait, speech and hearing. In the original incident a considerable number of fatalities (43) were caused with many more people (>700) being irreversibly affected. Mercury is an important industrial material used, for example, in the chlor-alkali industry and in the manufacture of small batteries.
Mercury is a natural contaminant of the marine system and since methyl mercury moves up the food chain easily (in some circumstances actually biomagnifying) some large predatory fish such as tuna and swordfish have naturally high levels which may approach or even exceed acceptable limits for human consumption in large, older specimens. There are normally no apparent adverse health effects in the fish themselves, the risk is to the next level of consumer in this case humans. (However, wildlife such as birds can be adversely affected directly - see our new website dedicated to Mercury for this and other detailed information.
Mercury in tuna has recently been back in the news because of the risk to
pregnant women and the unborn foetus (Guardian
Article on the risk to pregnant women)
US Campaigning Website
on the risks of eating tuna
February 2003 : even American Supermarkets are becoming involved because of legal threats over the implications of selling contaminated fish to consumers