Research led by Dartmouth scientists found that aquatic organisms fed nutritious, high-quality food end up with much lower concentrations of toxic methylmercury in their tissues. The result suggests ways in which methylmercury—a neurotoxin that can accumulate to hazardous levels—can be slowed in its passage up the food chain to fish.
Methylmercury is a neurotoxin found in all water bodies. While normally present in the water only in trace amounts (at the ng/L level or below), methylmercury presents a serious health hazard to humans due to bioaccumulation and biomagnification, a process in which a toxin occurs in higher and higher concentrations in animal tissue as one moves up the food chain. Daphnia and other zooplankton are a major source of methylmercury for lake fish. The highest levels of methylmercury are found at the highest trophic level (e.g. piscivorous fish), where mercury levels can exceed the "black list limit" of 0.5 mg/kg.
Photo: An enlarged view of Daphnia pulex, which has an actual length of 2-3 mm. (Photo courtesy Paul Hebert, University of Guelph.)
New research results:
In laboratory experiments, Karimi and colleagues from Dartmouth, Lakeland College, and Stony Brook University, studied the translucent water flea Daphnia pulex, a species of zooplankton that is one of the chief food sources for freshwater fish. The team measured, over five days, the growth of two groups of juvenile Daphnia, which in their mature state are about 2-3 millimeters in length. Both groups were fed the same amount of algae contaminated with trace amounts of methylmercury; however, one group's algae was of greater nutritional value.
The animals that received the nutritious, phosphorous-rich algae grew 3.5 times faster than the other group, the research found. Although the faster growing zooplankton ingested roughly the same amount of methylmercury as the other group, they ended up with one-third the concentration of toxin in their tissues because, as they grew faster, the toxin was diluted.
"This research provides evidence that by eating high-quality food, organisms may reduce their bodily concentration of a contaminant," said lead author Roxanne Karimi
, a graduate student in the Dartmouth Department of Biological Sciences
. "These findings allow us to predict the conditions under which freshwater fish are likely to carry relatively high mercury levels."
This same effect could occur in other organisms for other contaminants, such as PCBs and DDT, which also biomagnify in a food chain, Karimi said. "These contaminants pose health risks because they tend to remain in the body and so accumulate to high concentrations. When organisms have the optimal combination of nutrients available to them, they are able to gain more weight relative to the amount of toxin they get from their food. This is what results in the process of diluting the toxin by rapid growth."
The research is reported in a paper titled "Stoichiometric controls of mercury dilution by growth," to be published in the April 23, 2007 online "Early Edition" of Proceedings of the National Academy of Sciences
This study is one of a number of ongoing research projects at Dartmouth that look at methylmercury and other toxic heavy metals in aquatic food webs. Many of these projects are being undertaken by Dartmouth's Center for Environmental Health Sciences
and one of its programs, the Dartmouth Toxic Metals Research Program
, which is funded by the the Superfund Basic Research Program
"One of the most distinctive aspects of Dartmouth's center and the SBRP in general is the drawing together of scientists from multiple disciplines to solve problems of significant human impact," said Karimi's advisor and co-author Carol Folt
, Dartmouth dean of faculty, professor of biological sciences, and associate director of the Dartmouth toxic metals program. "Dartmouth's program has made great strides in addressing public and environmental impacts of both arsenic and mercury with this approach. Our particular focus—understanding the environmental factors that drive mercury to reach some of the highest levels in fish from the most pristine systems—is of special value for public policy. Mercury in fish is a worldwide issue of concern, meriting region-wide approaches for effective and timely mitigation and global cooperation."
The other authors of the paper are Celia Y. Chen
, research associate professor, Department of Biological Sciences, Dartmouth; Paul C. Pickhardt, Department of Biology, Lakeland College
; and Nicholas Fisher, Marine Sciences Research Center, Stony Brook University
. In related activities, these researchers have demonstrated several key ecological mechanisms
that explain why greater concentrations of methylmercury are often found in fish from more pristine systems
and recently sponsored a workshop
attended by scientists and policy makers throughout the region to share data, identify data gaps and discuss mercury mitigation. Chen also participated recently in the region-wide assessment of mercury impacts
that identified hotspots of mercury contamination requiring national attention.
Source: Dartmouth College (modified to fit the EVISA format)Original study
Roxanne Karimi, Celia Y. Chen, Paul C. Pickhardt, Nicholas S. Fisher, Carol L. Folt, Stoichiometric controls of mercury dilution by growth
, Proc. Natl. Acad. Sci. U.S.A., 104 (2007) 7477-7482. doi: 0.1073/pnas.0611261104Related studies
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