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The research is reported in a paper titled "Stoichiometric
controls of mercury dilution by growth," to 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 2-3 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 one 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, the
research suggests.
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."
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 (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," 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." |
