Environmental ▼News study of two New Jersey lakes and the bacteria that live in them could help unravel the paradox of why fish swimming in waters with very different levels of mercury contamination can often have similar methylmercury concentrations. Microbiologist Jeffra Schaefer and colleagues in Tamar Barkay’s Rutgers University lab in research published in this issue of ES&T (pp 4304–4311) present evidence that bacteria that efficiently break down methylmercury thrive in the waters of contaminated sites but are largely absent at more pristine sites. By reducing the proportion of a water body’s total mercury that stays methylated, these bacteria can significantly reduce the toxic metal compound that gets into the food web and hence into fish. Mercury contamination is so widespread in the United States that some 40 states have issued advisories against frequent consumption of freshwater fish. What to do about mercury is high on the political agenda, as the Bush Administration’s cap-and-trade proposals have come under widespread criticism (Environ. Sci. Technol. 2004, 38, 126A– 127A). But the mercury cycle—from atmospheric deposition of inorganic mercury through methylation by bacteria and biomagnification up the food web—has proved so complex that many unanswered questions still remain, including the relationship between varying mercury levels in waters and methylmercury contamination in fish. “This is one of the first papers to spell out this paradox,” says microbiologist Mark Marvin-DiPasquale, with the U.S. Geological Survey (USGS) in Menlo Park, Calif. “And they’ve done a great piece of work using molecular biology to show that water column bacteria serve as
TAMAR BARKAY
Bacteria could be key to methylmercury paradox
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Researchers from Rutgers University study the fate of methylmercury in a lake in the Pine Barrens. Findings here were compared with measurements in a more urban setting.
a natural mercury defense mechanism that can beat back pollution,” he adds. But the bacteria explanation only goes so far, DiPasquale warns. The Rutgers scientists compiled data showing that, in general, the proportion of total mercury in the water column that is methylated is lower in contaminated waters than in more pristine sites. However, the concentration of methylated mercury in contaminated waters is still higher than in pristine locales, he cautions, so that the analysis doesn’t necessarily explain contamination levels in fish. However, some mercury scientists argue that the paradox is not real. “The statement that methylmercury in fish in contaminated ecosystems is no higher than in ‘pristine’ ecosystems is just plain wrong,” said one prominent mercury scientist who asked not to be named. “There are lots of examples of elevated methylmercury levels in
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fish in contaminated systems—and of rising and falling mercury levels in fish in response to contamination—and it is irresponsible to state otherwise.” Bacteria resistant to methylmercury contain the mer operon, a cluster of genes responsible for synthesizing enzymes that degrade methylmercury into elemental mercury and methane in a process called reductive demethylation. The Rutgers scientists used this information to create genetic biomarkers to assess the level of mer activity in bacteria. Armed with their test, the researchers sampled lakes over a three-year period in the industrialized Meadowlands region of northern New Jersey and in the more pristine Pine Barrens in the South. Total mercury concentrations ranged from 4220 nanograms per liter (ng/L) in the Meadowlands to 5.4 ng/L in the Pine Barrens. However, methylmercury concentra© 2004 American Chemical Society
to harbor bacteria that reductively demethylate mercury; oxidative demethylation appeared to prevail in the Everglades. This finding changes a prevailing assumption about mercury fluxes in freshwater ecosystems, says USGS scientist David Krabbenhoft. When mercury scientists try to understand the causes of methylmercury levels in fish at a particular site, “We tend to assume that demethylation rates are constant across systems,” he says. “We expect that it’s changes in methylation rates that account for the variability.” The findings have no significance for bioremediation, cautions Barkay. You can’t dump mercury into a system and expect the amount of methylmercury to go down, she says, pointing out that the bacteria in contaminated systems only reduce the fraction of mercury that remains methylated. The contaminated lakes still have higher levels of methylmercury, she says. —REBECCA RENNER
Genomics data are no panacea Genomics, proteomics, and other systems biology data offer a promising solution to the U.S. EPA’s enormous task of evaluating an ever-increasing number of chemicals in the environment. But the agency must overcome significant challenges before it can fully apply such an approach to risk assessment, according to a draft EPA white paper released this spring. The paper outlines the “overarching challenges” in the areas of research, technical development, and staffing that EPA needs to address as these data become increasingly common. The white paper is the latest step in EPA’s effort to develop a genomics policy. It builds on EPA’s 2002 Interim Policy on Genomics that states that the agency can use genomic data in the decision-making process,
but that the data alone are insufficient as a basis for decisions. EPA scientists wrote the white paper at the request of the agency’s Science Policy Council (SPC), which asked them to examine the broader implications that genomics could have for EPA. The paper takes a careful position, say observers, and recommends practical steps that EPA could take to strengthen its capability to use genomics information. It raises technical issues such as how to standardize and specify quality controls for genomic tests when the field is changing so quickly and how to statistically evaluate systems biology data. The paper also recommends that EPA hire staff with expertise in systems biology and train existing staff.
News Briefs Kids in Europe One in three child deaths across Europe can be attributed to environmental problems, according to a report published by the World Health Organization in June. The report examined the effects of indoor and outdoor pollution, water sanitation, injury, and lead exposure on children and adolescents up to 19 years old. Of note, outdoor air pollution kills 13,000 children under age 4 annually, with an additional 10,000 deaths from pollution created while heating homes with solid fuel. Poor water sanitation kills 13,000 kids under 14. Much of the problem resides in the former Soviet bloc countries. The authors conclude that intervention measures to reduce environmental factors will result in substantial gains in human health. (Lancet 2004, 363 [9426], 2032–2039).
Water research agenda The science necessary to practically resolve the growing number of serious water resource issues in the United States is not available, according to a report by the U.S. National Research Council (NRC) released in June. Over the past 30 years, federal investment in water science, much of which has been deferred to states, has remained stagnant, the NRC panel finds. As a result, most studies have focused on short-term problems, “with a limited outlook for cross-cutting issues, longer-term problems, and more basic research that often portends future solutions.” Because of the growing competition and emerging challenges, the NRC panel recommends that an additional $70 million in federal funding go annually to water research. For more information on Confronting the Nation’s Water Problems: The Role of Research, go to www.nap.edu.
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tions varied only from 0.03 to 1.6 ng/L for lakes in both regions, with only slightly lower concentrations in Pine Barrens waters. Fish from these waters had similar levels of methylmercury. The Rutgers scientists found a higher proportion of mercury-resistant bacteria from the Meadowlands site, which expressed mer genes and, in lab tests, reductively demethylated methylmercury. The Pine Barrens bacteria demethylated mercury much slowly, producing carbon dioxide and some methane, which suggests that these bacteria are engaged in oxidative demethylation. The observation that lakes have different bacterial communities that demethylate mercury is consistent with an earlier study by MarinDiPasquale and co-workers that compared some of the most contaminated sites in the world with the relatively pristine Florida everglades (Environ. Sci. Technol. 2000, 34, 4906–4908). The sediments at their most contaminated sites appeared
