Mercury in the natural world
Q
What is a food chain?
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A
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A food chain refers to the relationship between eating and being eaten (prey versus predator) that exists among creatures in the natural world.
In the ecosystem, the relationship between eating and being eaten is linked in the manner of a pyramid that runs from bacteria or plants to carnivorous animals.
Many pollutants, such as methylmercury, released into the environment show higher concentration in organisms whose position in the food chain is higher.
Q
Where does mercury in the atmosphere come from?
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A
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It is as a result of volcanic eruptions or combustion of coal or petroleum that mercury comes out into the atmosphere in gaseous forms.
Annual mercury releases into the atmosphere are estimated to amount to approximately 80 to 600 tons from volcanic eruptions, approximately 2,000 to 2,950 tons from oceanic evaporation, and approximately 2,000 tons from combustion of fossil fuels (mainly coal). In addition, mercury is emitted from the land surface and from plants (about 1,700 to 2,800 tons), and the mercury in the atmosphere eventually comes back to the land surface or the ocean surface in rain or fog.
Q
How does natural methylmercury contained in fish and shellfish come into being?
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Certain types of bacteria (microbes) change ambient inorganic mercury to methylmercury.
Certain types of bacteria that live in the aquatic environment (Desulfovibrio desulfuricans LS is a well-known example) take in inorganic mercury and change it to methylmercury.
Methylmercury so generated gets accumulated in fish and shellfish through the food chain of aquatic creatures in stages, from plankton to small fish, then small fish to carnivorous fish, and so forth.
Q
What form of mercury is easily absorbed from the digestive tract (small intestine)?
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Among the forms of mercury present in nature, methylmercury is the most easily absorbed.
Among the forms of mercury present in nature, a majority of methylmercury gets absorbed from the digestive tract (small intestine). In contrast, only a few percent of inorganic mercury salts get absorbed that way.
Methylmercury, bound with a type of amino acid called cysteine, has a property of getting easily absorbed from the digestive tract and then penetrating into various parts of the body. Therefore, it can also make its way inside the brain and damage nerve cells, or cross the barrier of the placenta that connects a fetus with its mother, eventually entering the body of the fetus as well.
Metallic mercury, which hardly gets absorbed from the digestive tract, is easily vaporized and nearly 80% of inhaled metallic mercury vapor gets absorbed from the lungs. Depending on the volume absorbed, this may cause poisoning. There are examples of cases overseas that relate to exposure to vaporized mercury used in the process of gold mining.
Q
Can the amount of methylmercury contained in fish be reduced by cooking?
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A
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The amount of methylmercury contained in fish cannot be reduced by any method of cooking.
Since methylmercury in the body of fish is tightly bound with protein, it never disappears by adding heat at a temperature used for cooking, unless the fish is burnt until it is charred black.
It also virtually never dissolves into the cooking oil used for frying food, or into the stewing broth.