Inside bacterial cells, a methyl group (CH3) replaces the sulfide to produce monomehtyl mecury (CH3Hg or MMHg), which diffuses into seawater and is taken up by phytoplankton.
Sulfide combines with ionized mercury (Hg2+) in seawater to form mercuric sulfide (HgS), which can diffuse into bacterial cells.
The Mercury Cycle
Food Chain & Biomagnification
Dangerous levels of monomethyl mercury (MMHg) accumulate through biomagnification, the natural process that builds some substances to toxic levels as it travels up the food chain. Biomagnification starts with phytoplankton, which contain 10,000 times more MMHg than the seawater around them. Phytoplankton are eaten by zooplankton, which are eaten by small fish, which are eaten by larger fish and marine mammals. Instead of being expelled, MMHg accumulates in tissues resulting in an increase in concentration to 10 million times more MMHg than seawater – a level toxic to people.
Eating large amounts of seafood, even over a long period, increases the risk of mercury poisoning. Children and fetuses are especially vulnerable. For that reason, the U.S. Environmental Protection Agency and Food and Drug Administration maintain up-to-date recommendations and guidelines for consumption of fish and shellfish by adults, children, and pregnant or nursing women.
The direct source of toxic monomethyl mercury (MMHg) is unknown. MMHg levels are highest near the top of a low-oxygen zone between 100 and 400 meters below the surface. Scientists are investigating whether bacteria in this zone produce MMHg, just as bacteria in oxygen-poor sediments do. They are also exploring whether reactions in seawater, rather than in cells, convert some ionic mercury (Hg2+) to MMHg. Such reactions may involve dimethylmercury (DMHg), which is present in seawater at very low levels, but is not a health concern because it does not bioaccumulate.
In the atmosphere, sunlight converts elemental mercury (Hg) to ionized mercury (Hg2+), which falls to Earth in rain. About 80 percent of the Hg2+ that enters the ocean is converted back to Hg and evaporates back to the atmosphere. Some of the Hg2+ that remains in the ocean sticks to particles of organic matter and falls to the seafloor. Some enters the food chain as animals consume the organic matter and each other.
So far, the only known sources of toxic monomethyl mercury (MMHg) are bacteria in oxygen-poor sediments. A side-effect of that process enables elemental mercury (Hg) to enter bacterial cells and be converted into MMHg.
With little or no oxygen available, some bacteria use sulfate (S042-) for respiration, expelling sulfide (S2-) as a waste product.
Bacteria: A source of toxic MMHg
Ionized mercury (Hg2+) in seawater and sediments cannot enter bacterial cells.
The major sources of mercury in the atmosphere are human activities. In the U.S. alone, burning of high sulfur coal and other fossil fuels releases about 160 tons of mercury into the atmosphere every year. Volcanic eruptions are a natural source of mercury in the atmosphere. Rain carries much of the atmospheric mercury to the land and oceans. Over millions of years, plate tectonics and volcanic activity in Earth’s crust bring mercury from the depths to the surface.
Mercury (Hg) cycles through the Earth’s atmosphere, ocean and rock. In the ocean, mercury is converted to monomethyl mercury (MMHg), a neurotoxin that moves up the food chain and becomes highly concentrated in tuna, swordfish, and other seafood eaten by humans.