Biogeochemistry

The Biogeochemical Cycle

Natural transformations and environmental pathways of mercury are very complex and are greatly affected by local conditions. To assess the environmental fate and the impacts of anthropogenic mercury emissions, researchers must examine a range of biogeochemical interactions affecting mercury in its different physical states and chemical forms. Understanding the relationships between local conditions and mercury levels in various environmental media and living organisms is critical to predicting changes in concentration and form.

There are two main types of reactions in the mercury cycle that convert mercury through its various forms: oxidation-reduction and methylation-demethylation. In oxidation-reduction reactions, mercury is either oxidized to a higher valence state (e.g. from relatively inert Hg0 to the more reactive Hg2+) through the loss of electrons, or mercury is reduced, the reverse of being oxidized, to a lower valence state.

Mercury Oxidation

The oxidation of Hg0 in the atmosphere is an important mechanism involved in the deposition of mercury on land and water. Elemental mercury (Hg0) can volatilize relatively easily and be emitted to the atmosphere, where it may be transported on wind currents for a year or more and be re-deposited in the environment for further cycling. In contrast, Hg2+ has an atmospheric residence time of less than two weeks due to its solubility in water, low volatility and reactive properties. Hence, when (Hg0) is converted to Hg2+, it can be rapidly taken up in rain water, snow, or adsorbed onto small particles, and be subsequently deposited in the environment through "wet" or "dry" deposition.

In the Arctic, the conversion of Hg0 to Hg2+ in the atmosphere occurs very rapidly in a phenomenon known as "mercury depletion" at the end of dark polar winters. This phenomenon was discovered by a renowned scientist, Dr. William Shroeder, of Environment Canada's Meteorological Services in Downsview, Ontario. When the sun rises in the spring, atmospheric Hg0 is converted photochemically to Hg2+ in the presence of reactive chemicals released from sea salt (for example, bromine and chlorine ions) and mercury levels in the atmosphere are "depleted" as the Hg2+ is then deposited on snow and ice surfaces. As a consequence, a pulse of reactive mercury enters the Arctic environment when the short lived growing season is beginning. It remains a research question what fraction of this reactive mercury is converted to toxic methylmercury and taken up by animals and plants.

Mercury Methylation

In the environment, mercury is transformed into methylmercury when the oxidized, or mercuric species (Hg2+), gains a methyl group (CH3). The methylation of Hg2+ is primarily a natural, biological process resulting in the production of highly toxic and bioaccumulative methylmercury compounds (MeHg+) that build up in living tissue and increase in concentration up the food chain, from microorganisms like plankton, to small fish, then to fish eating species like otters and loons, and humans.

Understanding the variables influencing the formation of methylmercury is critically important due to its highly toxic, bioaccumulative and persistent nature. A variety of microorganisms, particularly methanogenic (methane producing) and sulfate–dependant bacteria are thought to be involved in the conversion of Hg2+ to MeHg under anaerobic (oxygen poor) conditions found, for example, in wetlands and river sediments, as well as in certain soils. Methylation occurs primarily in aquatic, low pH (acidic) environments with high concentrations of organic matter.

Rates of biomethylation are a function of environmental variables affecting mercuric ion availability as well as the population sizes of methylating microbes. Alkalinity, or pH, plays a strong role in regulating the process because it is affected by, and in turn effects, the adsorption of various forms of mercury on soil, clay and organic matter particles, thus influencing mercuric ion availability. Acid rain may increase biomethylation as more MeHg is formed under acidic conditions. Mercury can be bound by sulfide ions and made unavailable for methylation; however, sulfate may stimulate growth of certain methylating microbes. Organic matter can stimulate microbial populations, reduce oxygen levels, and therefore increase biomethylation. Biomethylation increases in warmer temperatures when biological productivity is high, and decreases during the winter.

Land use changes affecting some of these variables can result in increased rates of mercury methylation. For example, the construction of hydro-electric dams can mobilize mercury stored in the submerged forest floor and vegetation. The presence of organic matter (in the form of newly submerged vegetation) in combination with anaerobic conditions can stimulate microbial growth and lead to elevated methylmercury levels.

In general, the form of mercury in the environment varies with the season, with changes in organic matter, nutrient and oxygen levels and hydrological interactions within an ecosystem. In addition, the quantity and forms of mercury are, to a large extent, a function of emission sources and transportation processes. All of these variables in turn affect the global mercury budget.