What is methylmercury and why is it such a concern in aquatic environments?

Mercury is found in the environment as a result of natural and human activities. The amount of mercury that cycles in the environment has increased since the industrial age. The main source of mercury is air emissions from power generation and other industrial and waste disposal activities. During its movement among the atmosphere, land, and water, mercury undergoes a series of complex chemical transformations.

One of the products of these transformations is an organic form called methylmercury. Methylmercury is easily absorbed into the living tissue of aquatic organisms and is not easily eliminated. Therefore, it accumulates in predators. The degree to which mercury is transformed into methylmercury and transferred up the food chain through bioaccumulation depends on many site-specific factors (such as water chemistry and the complexity of the food web) through processes that are not completely understood.

Most attention toward mercury pollution has focused on methylmercury because it has been found to be a potent neurotoxin. Research has also shown that this form of mercury has the capacity to bioaccumulate, in the aquatic food chain, to concentrations that are a million-fold that of the background aquatic system. Methylmercury is highly toxic to mammals, including people, and causes a number of adverse effects.

Health studies and information showing neurotoxicity, particularly in developing organisms, are most abundant. The brain is the most sensitive organ for which suitable data are available to quantify a dose-response relationship. A recent study by the National Academy of Science concluded that the population at highest risk is the children of women who consume large amounts of fish and seafood during pregnancy, and that the risk to that population is likely to be sufficient to result in an increase in the number of children who have to struggle to keep up in school and who might require remedial classes or special education.

According to a 2005 EPA study, women living in US coastal communities - and presumably eating more fish than inland residents do - had higher average blood levels of methylmercury. Women living on the Atlantic coast had the highest average levels, followed by women on the Pacific and then women on the Gulf coasts. Many had methylmercury levels that the EPA considers unsafe for adults. Which fish are harmful? There is limited information about methylmercury in fish because there is no national or statewide system in place to monitor amounts. Most states, Native American tribes, and U.S. territories issue advisories that warn people when they are aware of methylmercury contamination. The advisories indicate what types, size, and amounts of fish are of concern. Pollution can result in high mercury levels in fish. Otherwise, methylmercury levels for many fish are relatively low, ranging from less than 0.01 part per million (ppm) to 0.5 ppm. A few fish are so high in methylmercury that they should be totally avoided by pregnant or nursing women, young children, and other at-risk populations.

In March 2004, the Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) issued a joint Consumer Advisory warning about methylmercury in fish. The advisory continues a previous warning against four particular species of fish and for the first time includes a specific warning about the consumption of tuna. The advisory recommends that women who might become pregnant, who are pregnant or nursing, and young children:

• Avoid eating shark, swordfish, king mackerel, and tilefish (also known as golden bass or golden snapper);
• Limit consumption of all other types of fish to 12 ounces per week;
• Limit their consumption of canned albacore ("white") tuna or fresh tuna to no more than 6 ounces per week;
• Limit the fish eaten by young children to even smaller portions per week (no specific advice is given);
• Check local advisories about the safety of fish caught in local lakes, rivers, and coastal areas. If no advice is available, eat no more than 6 ounces per week of locally caught fish, and do not consume any other fish during that week;

Methylmercury is the form of mercury that is most available and most toxic to biota including zooplankton, insects, fish and humans. In lakes and streams, bacteria and chemical processes transform inorganic mercury to the more toxic methylmercury. This form of mercury is easily taken up by biota and it accumulates in their tissues. Unlike many other fish contaminants, such as dioxin and DDT, mercury does not concentrate in the fat, but the muscle tissue. Thus, there is no simple way to remove mercury-contaminated portions from fish that are to be eaten. The processes involved in biomethylation of mercury in water are not yet fully understood, but the extent of this transformation appears to be related to pH, alkalinity, presence of dissolved organic materials containing sulfur and availability of sulfur bacteria in aquatic environments.