Mercury is a naturally occurring element found in the Earth’s crust, air, water, and soil globally. It exists in various forms, including elemental, inorganic compounds, and organic compounds like methylmercury. Its ability to transform and accumulate in aquatic environments is important for understanding its presence in fish and its relevance to human consumption, as it can pose health risks.
Sources of Environmental Mercury
Mercury enters the environment from both natural processes and human activities. Natural sources include volcanic eruptions, forest fires, rock weathering, and ocean emissions. For example, volcanic activity can release significant amounts of mercury into the atmosphere. These processes contribute to baseline mercury levels in ecosystems.
Human-caused, or anthropogenic, sources are the primary contributors to current environmental mercury levels. Burning fossil fuels, particularly coal for power, is a major source of atmospheric mercury. Other industrial activities, such as gold mining, cement manufacturing, and waste incineration, also release mercury. Once emitted, mercury can travel thousands of miles through the atmosphere before depositing onto land and into water bodies via rainfall or dry deposition.
Bacterial Conversion to Methylmercury
Inorganic mercury released into the environment does not readily accumulate in fish. A significant transformation occurs in aquatic sediments and soils, where specific microorganisms convert inorganic mercury into methylmercury. This process, known as methylation, is primarily carried out by anaerobic bacteria, which thrive in oxygen-free environments, such as those found in lake and ocean sediments.
Sulfate-reducing bacteria, iron-reducing bacteria, and methanogens are among the bacteria responsible for this conversion. Methylmercury is an organic form of mercury that is more readily absorbed and retained by living organisms than its inorganic counterpart. This transformation allows mercury to enter the aquatic food web.
Uptake and Concentration in Aquatic Life
Methylmercury from sediments and water enters aquatic organisms through two main processes: bioaccumulation and biomagnification. Bioaccumulation describes the uptake of methylmercury by an individual organism from its environment, including water and food, at a rate faster than it can excrete it. For example, small aquatic insects or plankton absorb methylmercury directly from their surroundings, leading to a buildup in their tissues over time.
Biomagnification occurs as methylmercury concentrations increase at successively higher levels of the food chain. When smaller organisms containing methylmercury are consumed by larger predators, the mercury transfers and concentrates in the predator’s tissues. For example, small fish may eat contaminated plankton, and then larger fish consume those small fish. This explains why long-lived, predatory fish, such as shark, swordfish, king mackerel, and certain types of tuna, typically have the highest mercury levels. These top predators accumulate mercury from all the prey they consume throughout their lives.