Mercury is a naturally occurring element, but some 2,000 tons of it enter the global environment each year from human-generated sources. Deposited onto land or into water, mercury is picked up by some types of microorganisms, which convert a small portion of it to methylmercury, a highly toxic form that builds up in fish and the animals---and people---that eat them.
The primary way people in the United States are exposed to methylmercury is by eating fish and shellfish. Health effects include damage to the central nervous system, heart and immune system. The developing brains of young and unborn children are especially vulnerable.
In San Francisco Bay, Gehrke, Blum and coworkers suspected small fish such as silverside and topsmelt were acquiring mercury from sediments on the bay floor and then passing it along to larger fish and other fish-eating animals, but it also was possible that mercury from the atmosphere or localized industrial sources was ending up in the fish.
To resolve the question, the research team compared chemical "fingerprints" of mercury in sediments and in fish, much as a detective compares a suspect's fingerprints to those found at a crime scene. The fingerprints result from a natural phenomenon called isotopic fractionation, in which different isotopes of mercury react to form new compounds at slightly different rates. In one type of isotopic fractionation, mass-dependent fractionation (MDF), the different rates depend on the masses of the isotopes. In mass-independent fractionation (MIF), the behavior of the isotopes depends not on their absolute masses but on whether their masses are odd or even.
The team sampled sediment at 20 sites in the bay and fish at 26 sites. "We used young fish, less than four months old, that have a very small home habitat," said Gehrke. "Because they're restricted t
|Contact: Nancy Ross-Flanigan|
University of Michigan