Mercury in the environment

Mercury is a highly toxic element that is found both naturally and as an introduced contaminant in the environment. Although its potential for toxicity in highly contaminated areas such as Minamata Bay, Japan, in the 1950s and 1960s, is well documented (e.g. see here), research has shown that mercury can be a threat to the health of people and wildlife in many environments that are not obviously polluted. The risk is determined by the likelihood of exposure, the form of mercury present (some forms are more toxic than others), and the geochemical and ecological factors that influence how mercury moves and changes form in the environment.

The toxic effects of mercury depend on its chemical form and the route of exposure. Methylmercury [CH₃Hg] is the most toxic form. It affects the immune system, alters genetic and enzyme systems, and damages the nervous system, including coordination and the senses of touch, taste, and sight. Exposure to methylmercury is usually by ingestion, and is absorbed more readily and excreted more slowly than other forms of mercury. Elemental mercury, Hg(0), the form released from broken thermometers, causes tremors, gingivitis, and excitability when vapors are inhaled over a long period of time. Although it is less toxic than methylmercury, elemental mercury may be found in higher concentrations in environments such as gold mine sites, where it has been used to extract gold. If elemental mercury is ingested, it is absorbed relatively slowly and may pass through the digestive system without causing damage.

Mercury in the environment can have natural sources, such as volcanic activity and the weathering of mercury-containing bedrock (e.g. biotite-rich granite). Mercury is also found naturally in oil, coal and gas reserves. Combustion of fossil fuels is therefore a major source of mercury entering the environment. As emissions from the combustion of fossil fuels are often to the atmosphere through tall chimney stacks etc, mercury can be transported over long distances before it is deposited in what may appear to be pristine sinks.

Methylmercury is formed through biomethylation. Methylation is the process through which methane is produced in the environment, and is associated with waterlogged, anaerobic conditions. Biomethylation is mediated by microbes that inhabit waterlogged, anaerobic conditions. An example of the microbes involved are sufate (SO₄)-fixing (or reducing) bacteria. They obtain their energy from breaking-off sulfur (S) atoms from sulfate compounds and then combining the S with another atom, such as hydrogen ~ the latter to form hydrogen sulfide (H₂S), which is a toxic gas that also gives rotten eggs their smell.

Biomethylation in the environment (see above) results in transforming certain heavy metals, such as arsenic and mercury, into much more mobile and biologically lethal forms. This is done by adding methyl (a compound formed of one carbon atom and three hydrogen atoms, CH₃) to the mercury atoms, to form CH₃Hg. We have already come across this problem in GE3246 – methylation is the process that increases the toxicity of arsenic in groundwater in countries such as Bangladesh, especially where that groundwater is contaminated by organic matter (e.g. sewerage). The conversion of inorganic mercury to methylmercury is important because its toxicity is greater and because organisms require considerably longer to eliminate methylmercury than non-methylated forms.

The concentration of dissolved organic carbon (DOC) and pH have a strong effect on methylation in the environment. Studies have shown that for the same species of fish taken from the same region, increasing the acidity of the water (decreasing pH) and/or the DOC content generally results in higher mercury levels in fish, an indicator of greater net methylation. Higher acidity and DOC levels enhance the mobility of mercury in the environment, thus making it more likely to enter the food chain.

The geography of biomethylation

Although mercury is a globally dispersed contaminant, it is not a problem everywhere. Problems of mercury toxicity are linked to biomethylation, and biomethylation as a distinct focality (or geography!). Aside from grossly polluted environments, mercury is normally a problem only where the rate of natural formation of methylmercury from inorganic mercury is greater than the reverse reaction. Environments that are known to favor the production of methylmercury include certain types of wetlands, dilute low-pH lakes, muddy, low energy coastlines etc. In other words, locations that are also often favoured by fish/shrimp farmers.

How does mercury enter the food chain?

People are exposed to methylmercury almost entirely by eating contaminated fish and wildlife that are at the top of aquatic foodchains. The exact mechanisms by which mercury enters the food chain remain largely unknown and may vary among ecosystems. As already mentioned, certain microbes play an important role in the production of methylmercury. These methylmercury-producing bacteria may be consumed by the next higher level in the food chain, thereby directly passing on the methylmercury they contain, or the bacteria may excrete the methylmercury to the water where it can quickly adsorb to plankton, which are also consumed by the next level in the food chain. Because animals accumulate methylmercury faster than they eliminate it (mercury ingested can take up to two months to be excreted), animals consume higher concentrations of mercury at each successive level of the food chain. Small environmental concentrations of methylmercury can thus readily accumulate to potentially harmful concentrations in fish, fish-eating wildlife and people. Even at very low atmospheric deposition rates in locations remote from point sources, mercury biomagnification can result in toxic effects in consumers at the top of these aquatic food chains.