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Research led by Dartmouth scientists has found that animals fed nutritious,
high-quality food end up with much lower concentrations of toxic methylmercury
in their tissues. The results suggest ways in which methylmercury—a neurotoxin
that can accumulate to hazardous levels—can be slowed in its passage up the
food chain to fish.
From left: Celia Chen '78, Roxanne Karimi, and Carol Folt are co-authors on a
study that examines how diet affects methylmercury concentrations in freshwater
fish. (Photo by Joseph Mehling '69)
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"This research provides evidence that by eating high-quality food,
organisms may reduce their bodily concentration of a contaminant," says
lead author Roxanne Karimi, a graduate student in the Department of Biological
Sciences.
The research is reported in a paper titled "Stoichiometric Controls of
Mercury Dilution by Growth," was published in the April 23, 2007, online
early edition of Proceedings of the National
Academy of Sciences.
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 to 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.
Methylmercury is a neurotoxin found in all water bodies. While normally
present in the water only in trace amounts, methylmercury presents a serious
health hazard to humans due to biomagnification, a process in which a toxin
occurs in higher and higher concentrations in animal tissue as it moves up the
food chain. Daphnia and other zooplankton are a major source of methylmercury
for lake fish. When water fleas and other zooplankton grow rapidly by feeding
on high-quality food, the rate at which methylmercury is accumulated and
transferred through the food chain may decrease.
This same effect could occur in other organisms for other contaminants, such
as PCBs and DDT, which also biomagnify in a food chain, Karimi says.
"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."
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 Superfund Basic Research Program
(SBRP).
"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," says Karimi's advisor and
co-author Carol Folt,
dean of the faculty of arts and sciences, professor of biological sciences, and
associate director of Dartmouth's 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."
Other authors of the paper are Celia Y. Chen '78,
research associate professor of biological sciences; Paul C. Pickhardt,
Department of Biology, Lakeland College;
and Nicholas Fisher, Marine Sciences Research Center, Stony Brook University. In related
activities, the 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.
By REBECCA BAILEY
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