Mercury ($\text{Hg}$) is a naturally occurring heavy metal found throughout the Earth’s crust. This element has become a global environmental concern because of its ability to move easily across air, land, and water, changing its chemical form as it travels. While natural geological processes constantly release mercury, human activity has significantly increased its circulation, leading to widespread contamination. Understanding the different forms mercury takes and the transformations it undergoes is fundamental to grasping its influence on nature and the food web.
The Forms of Mercury
Mercury exists in three primary forms, each with distinct chemical characteristics and differing levels of risk. The most basic form is elemental mercury, $\text{Hg}^0$, a shiny, silver-white metal that is a liquid at room temperature. This form is highly volatile and easily evaporates, becoming an invisible, odorless vapor that can travel long distances in the atmosphere.
When elemental mercury reacts with elements like sulfur or chlorine, it forms inorganic mercury compounds, often called mercury salts. These compounds, such as mercuric chloride, are generally white powders that are water-soluble and can be transported in soil and water. While inorganic mercury is more reactive than its elemental counterpart, it is less readily absorbed by living organisms.
The most concerning form is methylmercury ($\text{MeHg}$), an organic compound where mercury is bonded to a carbon atom. Methylmercury is highly toxic and is the form that living organisms absorb most easily.
Natural Sources and Releases
Mercury originates from the Earth’s geological makeup, found in mineral ores like cinnabar. The continuous, background release of mercury occurs through the natural weathering of mercury-rich rocks, which slowly mobilizes the element into soil and water. This geological process ensures a constant, though low-level, input of inorganic mercury into the cycle.
Volcanic activity is another significant natural source, releasing large amounts of elemental mercury vapor directly into the atmosphere during eruptions and through passive degassing. Forest fires also contribute to the natural cycle by releasing mercury that has been previously deposited and absorbed by vegetation and soil. While human activities, such as burning coal, are responsible for the majority of current atmospheric mercury pollution, these natural processes formed the original, historical mercury cycle.
The Environmental Mercury Cycle
Once released, mercury moves through a complex global process that involves the atmosphere, land, and water. Elemental mercury vapor, due to its volatility, can circulate in the atmosphere for up to a year, allowing it to be transported across continents before it eventually returns to the Earth’s surface. This return occurs through atmospheric deposition, where mercury falls as either dry particles or is washed out of the air by rain and snow, known as wet deposition.
After deposition, inorganic mercury lands on soil and water bodies, where a chemical transformation takes place, primarily in aquatic sediments. The process of methylation is performed by specific microorganisms, mainly anaerobic bacteria that thrive in oxygen-poor environments like wetlands and lake bottoms. These bacteria, particularly sulfate-reducing and iron-reducing types, convert inorganic mercury ($\text{Hg}^{2+}$) into the highly toxic methylmercury ($\text{MeHg}$) as a byproduct of their metabolic processes.
The conversion to methylmercury makes mercury biologically available. Methylmercury can then be released from the sediment into the water column or remain attached to the bacteria. This transformation is influenced by various local environmental factors, including the amount of organic matter present and the acidity of the water. The mercury cycle is dynamic; methylmercury can also be converted back to less toxic inorganic forms through demethylation.
Bioaccumulation and Food Chain Effects
The methylmercury produced in aquatic environments is the form that enters the food chain, beginning with primary producers like phytoplankton. Organisms absorb methylmercury efficiently, and because they eliminate it very slowly, the compound begins to build up in their tissues over time. This initial process, where the concentration of a substance increases within a single organism, is known as bioaccumulation.
As methylmercury moves up the food chain, its concentration increases dramatically at each successive feeding level. This phenomenon is called biomagnification, and it occurs because predators consume large quantities of prey that already contain accumulated mercury. For example, zooplankton eat phytoplankton, small fish eat zooplankton, and larger predatory fish consume the small fish.
The most significant concentrations of methylmercury are found in organisms at the top of the aquatic food web, such as swordfish, shark, and large tuna. These top predators can harbor mercury levels that are millions of times higher than the surrounding water. This biomagnification poses a direct risk to wildlife, including fish-eating mammals and birds, and is the primary route of exposure for humans who consume contaminated seafood.