Mercury is a globally distributed pollutant, a heavy metal that has infiltrated every corner of the world’s oceans. Its presence is not confined to coastal areas near industrial centers; it is found everywhere from the surface waters to the deepest trenches of the Pacific, even in remote marine sediments where human contact is minimal. The core problem is that mercury travels vast distances before settling in the marine environment, accumulating there over time. This widespread distribution means the entire global ocean acts as a massive reservoir for this metal, impacting marine life across all latitudes and depths.
Natural and Geological Sources
The presence of mercury in the ocean is not entirely a new phenomenon, as the element is naturally part of the Earth’s crust. Geological processes release mercury into the environment without any human intervention, providing a background level that has always existed. This baseline input occurs through the slow, continuous weathering of mercury-rich rocks, such as cinnabar, which releases the metal into soils and waterways. Volcanic eruptions and geothermal activity also represent significant natural sources of oceanic mercury. Sub-sea hydrothermal vents, particularly those found at mid-ocean ridges, are the only direct natural source of mercury into the ocean water column, demonstrating that mercury has always cycled through the marine system.
Human Contributions
Human activity is the primary driver behind the elevated levels of mercury currently measured in the ocean, having significantly increased the global mercury pool since the Industrial Revolution. One of the most substantial sources is the combustion of fossil fuels, particularly coal, which naturally contains trace amounts of mercury. Burning this coal releases the mercury into the atmosphere as a gas, accounting for a significant percentage of global anthropogenic emissions.
Artisanal and small-scale gold mining (ASGM) is another massive source of contamination, responsible for the largest single share of global anthropogenic mercury emissions. Miners use elemental mercury to amalgamate with gold particles, and the subsequent burning of this amalgam to recover the gold releases large quantities of mercury vapor directly into the air and discharges liquid mercury into local waterways. Industrial processes further contribute, with facilities like chlor-alkali plants historically using mercury cell technology. Although regulations are phasing out this technology, the legacy of these industrial discharges and the ongoing emissions continue to pour mercury into the environment.
The Journey to the Sea
The ocean receives mercury from its sources through two primary transport mechanisms: atmospheric deposition and hydrological runoff. Elemental mercury, released as a gas from sources like coal plants and gold mining, has a long atmospheric lifetime, allowing it to be carried by global air currents for thousands of miles. This long-range atmospheric transport is why mercury pollution is found even in the most remote ocean basins and polar regions. While airborne mercury can deposit directly into the ocean as dry particles or wet deposition, a substantial amount first settles on land. Hydrological transport then moves this deposited mercury from terrestrial landscapes into the sea via river runoff, flushing contaminated soils into estuaries and coastal waters, a process that is particularly pronounced during storm events and snowmelt.
Chemical Change: Why Mercury Becomes Dangerous
The inorganic mercury that enters the ocean is not the most toxic form; the danger emerges through methylation, a transformation process that converts inorganic mercury into methylmercury. The conversion is facilitated by specific types of anaerobic microbes, mainly sulfate-reducing bacteria and iron-reducing bacteria, that thrive in oxygen-poor environments like ocean sediments and coastal wetlands. These microorganisms possess a gene, $hgcA$, that enables them to perform this methylation process. Methylmercury is highly fat-soluble, or lipophilic, allowing it to easily cross biological membranes, including the blood-brain barrier and the placental barrier in mammals. Once inside an organism, this potent neurotoxin can cause severe neurological and developmental damage.
Accumulation in the Food Chain
The fat-soluble nature of methylmercury drives its movement and concentration within the marine environment, beginning with bioaccumulation. Individual organisms, starting with phytoplankton and small invertebrates, absorb methylmercury from the water and sediment faster than they can excrete it, building up concentration in their body tissues over their lifespan. The problem intensifies through biomagnification as the methylmercury moves up the trophic ladder. This results in a million-fold increase in concentration from the water to the top of the food chain, where apex predators like tuna, swordfish, and sharks exhibit the highest levels. Consumption of these top-level marine fish is the primary route of methylmercury exposure for people, leading to fish consumption advisories worldwide.