Download Biological conversion and uptake of mercury

Biological conversion and uptake of mercury
One of the most critical components of the mercury cycle is the conversion of inorganic
forms of mercury to organic methylmercury. This process, known as “methylation,” is
caused by small bacteria that live in lakes, rivers, wetlands and marshes. Understanding
how and where methylmercury is formed is very important because methylmercury is far
more toxic than the inorganic forms of mercury. For instance, mercury deposited from
the atmosphere can be transformed in methylmercury within the wetlands surrounding a
lake and transported to the aquatic environment; but methylmercury can also be produced
directly in the sediments of the lakes from mercury that has come from the atmosphere or
that was transported by streams and runoffs from the watershed
Inorganic mercury is toxic to humans and wildlife when exposed to high levels for a short
period of time, while methylmercury can accumulate in humans and wildlife over time,
eventually causing harm, even when exposed to small amounts of this compound.
Methylmercury has the three properties that make substances particularly harmful to
humans; it persists, it bioaccumulates and it is toxic to most life forms.
Bioaccumulation describes the process whereby methylmercury can enter the aquatic
food chain directly from the water or through the consumption of small organisms,
insects or fish. Bioaccumulation will occur when the mercury intake rate will exceed the
metabolic capability of excretion. For example, aquatic microorganisms such as plankton
absorb the methylmercury created by bacterium and, over time, mercury levels increase
in their systems. Mercury also “biomagnifies” meaning that mercury levels become more
and more concentrated higher up in the food-chain, as a result of eating contaminated
food. For example, large predator fishes, such as pikes or walleyes, that eat a smaller
fish species, that have themselves ingested microorganisms containing methylmercury,
will have much higher concentrations of mercury in their bodies than the microorganisms
and even the smaller fishes. Birds, animals or humans that eat large predator fish are then
at risk of mercury poisoning. Mercury is thought to have a greater power of
biomagnification than almost any other substance known. A large predator fish may have
100,000 to one million times more mercury in its body than the water in which it swims.