Scientists scuba dive for core samples to learn the fate of mercury.

This story first appeared in the SERC quarterly Newsletter winter 2005/2006

Dr. Cindy Gilmour; Studying the Fate of Mercury in the Environment  

New U.S. food guidelines suggest we all "choose fish more often for lunch or dinner," while the American Heart Association touts the benefits of eating fish for its heart-healthy Omega3 fatty acids. At the same time, the EPA cautions some people against eating more than a few servings of certain fish and advises consumers to check for local advisories before eating any fish not commercially caught. Even if no advisory exists, "consumers are advised to restrict consumption [of non-commercial fish] to one meal per week."

Choosing to eat fish is no longer a simple affair. At issue, the presence of high levels of dangerous toxins found in fish the world over.

In 2004, the United States alone issued 3,221 advisories on fish consumption, 2,436 of them for mercury. While there are other toxins appearing in fish, mercury is at the top of the list, and the clear threat to human health that it poses prompted the Environmental Protection Agency to issue the first Clean Air Mercury Rule in March of last year. Aimed at lowering industrial mercury emissions by about 20 percent in five years and 70 percent by 2018, the rule has been controversial. Some say it should be more rigorous while others dispute the impact of any such regulation no matter how aggressive.

Once a common household item found in everything from thermometers to cosmetics and paint, mercury pollution is abundant in our landscape. While the coal-fired power industry is responsible for the majority of new mercury emissions from the United States and at the center of much of the regulatory debate, it represents only a small fraction of the total mercury in our environment because 150 years of contamination is already stored in the soils. Some say that reducing current emissions will have little or no impact on the threat to human health while those seeking more rigorous regulation argue that it's the new supply of mercury to the environment that poses the biggest threat.

While the debate rages on in Washington, microbial ecologist Cynthia Gilmour is steadily working on answering questions that might help settle the matter. Gilmour is trying to understand how mercury reacts in the environment and how it accumulates in food webs--information that may ultimately predict the most effective methods for reducing the threat to human health.

Once released into the atmosphere through industrial emissions, mercury is carried to the ground by rain and accumulates in sediments and soils. This inorganic form of mercury does not accumulate in food webs, but under certain conditions, natural bacteria can convert it into an organic form called methylmercury.

That's where the trouble starts. In aquatic environments, small organisms such as filter-feeding oysters eat the bacteria and thus consume the methylmercury. Then larger organisms such as fish eat the filter-feeders. The more they eat, the more methylmercury they accumulate in their bodies. As even larger fish eat the little ones, the process continues and methylmercury concentrations "biomagnify" up the food web until, by the time a nice juicy filet arrives on your dinner plate, it can contain up to a million times more mercury than the water from which it came.

Because bacterial mercury methylation occurs mainly in wetlands and aquatic sediments, aquatic organisms including wading birds and commercially important fish, are particularly susceptible. As awareness of mercury levels in fish has climbed in recent years, nearly every region in North America has been faced with issuing fish consumption advisories.

Fixing the problem will rely on understanding where the mercury in fish is coming from and exactly how it's getting there. Gilmour and her colleagues are exploring the processes by which microbes convert mercury pollution into methylmercury. One idea she is exploring is that as mercury "ages" in soils, it becomes more tightly bound within particles and is less readily taken up and methylated by bacteria.

She and her colleagues are testing this hypothesis as part of the Mercury Experiment to Assess Atmospheric Loading In Canada and the United States—a.k.a. METAALICUS. In one field study at the remote Experimental Lakes Area in northwest Ontario, they are depositing mercury into a small lake and its watershed at levels equivalent to the current mercury deposition in the highly-impacted Eastern United States, and tracking what happens to it.

According to Gilmour, there are two unique aspects of the study. "This is the first study where the deposition of mercury to a whole, intact ecosystem has been experimentally manipulated," she said. The second unique feature of the study is that they are able to trace the behavior of newly deposited mercury separately from older mercury contamination in the ecosystem. Gilmour and her colleagues are using slightly different variations of mercury called isotopes. Most elements, like mercury, are made up of multiple isotopes, or variants with the same number of protons but differing numbers of neutrons in the atom's nucleus. This difference accounts for a very slight difference in mass or atomic weight. Mercury contains seven different stable isotopes—seven forms which do not degrade over time. (Radioactive isotopes are unstable and decay over time.) Generally, isotopes of the same element have the same chemical characteristics and react in the same way, but that slight difference in atomic weight allows Gilmour to keep track of the mercury she adds in her field experiments.

When the team adds a mercury stable isotope somewhere in the experimental watershed, they can track it in follow-up samples by measuring the ratio of that isotope compared to others. When that ratio rises above the expected "background" levels, they know it represents mercury they have added to the system.

To further refine their understanding of the way mercury behaves in the ecosystem, they are also able to differentiate between the mercury they add in different areas of their study site. "We are using three different stable mercury isotopes, one each on the lake surface, an adjoining wetland, and the upland watershed, which allows us to follow the behavior of mercury deposted to each part of the ecosystem individually," Gilmour said.

Understanding how ecosystems respond to changing levels of mercury will help predict how effective mercury emissions regulations will be, and how rapidly methylmercury contamination of fish may decline in response to emissions reductions. "If the older mercury contamination in sediments and soils is readily methylated and bioaccumulated, then the response time to emissions reductions is going to be very long," Gilmour said. "However, if most methylmercury is formed from recent mercury pollution, then we may see a rapid response to emissions reductions."

For more information, or to reach Dr. Gilmour, please contact SERC science writer Kristen Minogue.