New Fingerprinting Method Tracks Mercury in Environment

ANN ARBOR, Mich. - With mercury polluting our air, soil and water and becoming concentrated in fish and wildlife as it is passed up the food chain, understanding how the potent nerve toxin travels through the environment is crucial. A new method developed at the University of Michigan uses natural "fingerprints" to track mercury and the chemical transformations it undergoes.

Mercury is a naturally occurring element, but some 150 tons of it enter the environment each year from human-generated sources in the United States, such as incinerators, chlorine-producing plants and coal-fired power plants. Mercury is deposited onto land or into water, where microorganisms convert some of it to methylmercury, a highly toxic form that builds up in fish and the animals that eat them. In wildlife, exposure to methylmercury can interfere with reproduction, growth, development and behavior and may even cause death.

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Effects on humans include damage to the central nervous system, heart and immune system. The developing brains of young and unborn children are especially vulnerable.

Because of such profound and irreversible effects on health and the environment, "it's very important to understand how and where mercury transforms into its most toxic forms and how it moves around in the environment, leading to human and animal exposure," said research fellow Bridget Bergquist, who is first author on the paper.

"I have often dreamed of how useful it would be if we could mark individual atoms of mercury with an indelible fingerprint of key chemical reactions and use this fingerprint to follow them around in the environment," said co-author Joel D. Blum, who has been working on the problem for more than a decade. "This is precisely what we have been able to achieve with the experiments that we're reporting. Our work opens the door to an entirely new method for tracing mercury pollution and for investigating mercury behavior in the environment and in the food chains of humans and other animals."

Bergquist and Blum based their new tracking method on a natural phenomenon called isotopic fractionation, in which different isotopes (forms) of mercury react to form new compounds at slightly different rates, something like bicycle racers in the Tour de France. Some riders perform better in the mountainous stages of the race and are separated from the pack due to their strength; others distinguish themselves on the flat stages of race due to their superior speed. With mercury isotopes, it's mass, not athletic ability, that dictates their behavior—in one type of isotopic fractionation, at least. In this mass-dependent fractionation (MDF), different mercury isotopes participate differently in chemical reactions, based on their masses.

"While mass-dependent fractionation is a well-known phenomenon in lighter elements and forms the basis for how we determine such things as past climates on the Earth and dietary food chains of animals, mercury was thought to be too heavy for the signal to show up," said Blum, who is the John D. MacArthur Professor of Geological Sciences. But in this work, Bergquist and Blum show that mass-dependent fractionation can be used to track mercury. Because the process is observed naturally in fish as they grow, the mercury the fish excrete must have a different isotopic composition than the mercury they take in, so MDF may reveal how much mercury fish consume, how much they excrete and how it changes during the fishes' lifetimes.

In the current work, the researchers exploited both MDF and another type of isotopic fractionation called mass-independent fractionation (MIF), in which isotopes segregate based not on absolute mass but on whether their masses are odd or even. Bergquist and Blum discovered that this type of fractionation occurs only in reactions involving sunlight, such as those that take place in surface waters and result in methylmercury being detoxified and released to the atmosphere. Mass-independent fractionation of mercury and other heavy elements had been predicted but never carefully documented in nature.

By combining two methods that provide distinct isotope signatures, Bergquist and Blum came up with a tracking tool that is more powerful than either one alone.

"We found that fish from a wide range of lakes and from the ocean all have large degrees of both mass-independent and mass-dependent isotope fractionation," Bergquist said. "So now we're able to use the mass-independent isotope signatures to estimate the proportion of toxic methylmercury at each location that was detoxified and released to the atmosphere by photochemical reactions, and we're also able to use the mass-dependent isotope signatures to study the accumulation of mercury in fish as they age and grow larger.

Together the two signatures provide a label that allows us to understand the sources of methylmercury to fish and to differentiate fish from different localities."Using the method in this way illustrates its potential for much wider application, Blum said. "One example is a complementary study that we reported at a recent scientific meeting." In that study research fellow Abir Biswas, working with Bergquist and Blum, found that mercury in coals from various coal-producing regions around the world vary in their mass-dependent and mass-independent isotopic composition. "This suggests that we may be able to use the mercury isotope studies to distinguish different sources of mercury to the atmosphere, which has far-reaching practical applications," Blum said. "In short, this entirely new approach to studying mercury sources, mobility and toxicity in the environment paves the way for a wide range of studies that should enhance our understanding of this important toxin in the environment."The researchers received funding from the Division of Earth Sciences of the National Science Foundation and the Turner Postdoctoral Fellowship from the U-M Department of Geological Sciences.