Mercury Content Reduced in Daphnia Fed High Quality Algae

Celia Y. Chen, Ph.D. and Carol L. Folt, Ph.D.
Dartmouth College
P42ES007373

New research by NIEHS-funded scientists at Dartmouth College finds that 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 in fish and ultimately to humans.

"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 and supported by the NIEHS Superfund Basic Research Program. "These findings allow us to predict the conditions under which freshwater fish are likely to carry relatively high mercury levels."

Laboratory experiments conducted Karimi and her colleagues from Dartmouth, Lakeland College, and Stony Brook University, studied the water flea Daphnia pulex, a species of plankton 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 Daphnia that received the nutritious, phosphorous-rich algae grew 3.5 times faster than the other group. 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 the toxin in their tissues because the toxin was diluted.

Methylmercury is a neurotoxin found in most large bodies of water. While normally present in trace amounts, methylmercury presents a serious health hazard to humans due to biomagnification, a process in which the toxin occurs in higher and higher concentrations in animal tissues progressing up the food chain. Daphnia and other plankton are a major source of methylmercury for lake fish. The research suggests that when water fleas and other organisms grow rapidly by feeding on high quality food, the rate at which methylmercury is accumulated and transferred through the food chain may decrease.

Citation: Karimi R, Chen CY, Pickhardt PC, Fisher NS, Folt CL. Stoichiometric controls of mercury dilution by growth. Proc Natl Acad Sci U S A. 2007 May 1;104(18):7477-82.