Good Eating

The water flea Daphnia pulex Copyright Holder: P.D.N. Hebert, University of Guelph

Monday, April 16th, 2007--

The concentration of the toxin methylmercury in freshwater fish may be linked to how fast zooplankton grow.

The zooplankton Daphnia, a common water flea, is a staple food source for many freshwater fish. Daphnia eat algae. But, not all the algae they eat is equal. According to a study published in the Proceedings of the National Academy of Sciences, the ratio of the nutrients in the algae determines how much methylmercury Daphnia pulex harbors.

And because Daphnia are low on the food chain, the amount of methylmercury they absorb is likely a key factor in how much of the toxin accumulates in fish, birds and other higher-level organisms.

Roxanne Karimi, a researcher at Dartmouth University in New Hampshire and her colleagues Celia Y. Chen, Paul C. Pickhardt, Nicholas S. Fisher and Carol L. Folt, found that Daphnia pulex grow differently depending on their diet. “Daphnia tend to have high demand for phosphorus,” says Karimi. “They use it to synthesize proteins; they use it to assimilate carbon or respire less carbon.”

Daphnia grown in the lab that ate "high-quality" food, algae with a higher proportion of phosphorus to other nutrients, grew up to five times faster than those which fed on algae with less phosphorus. The scientists found that faster-growing Daphnia also had lower methylmercury concentrations in their bodies: faster growth means that the mercury is dispersed over a larger area.

Mercury enters the aquatic food chain when tiny microbes living in the sediments of lakes transform inorganic mercury (the kind that comes out of smokestacks) into organic mercury, or methylmercury. Methylmercury’s molecular structure allows it to easily pass into algal cells, while inorganic mercury “will tend to stay outside of the cell on the cell wall,” Karimi explains.

Once methylmercury is in the algae, it is absorbed by the Daphnia. From the flea, the methylmercury travels largely unimpeded up the food chain. “That’s why methylmercury poses a significant health risk, because it gets into the tissue and once it gets in there, it is very difficult for any organism to get rid of it,” Karimi explains.

This study shows for the first time that presence of methylmercury in an organism is dependent, to some degree, on the quality of food the organism eats. But Karimi says dumping phosphorus into your favorite fishing hole is probably not the answer. Phosphorus may not work the same way in every organism: “Other organisms might have a high demand for another nutrient,” says Karimi. Plus, too much phosphorus can cause algae blooms which can kill fish. Karimi says: “I don’t think it translates into some obvious remedial measure.” Instead, Karimi suggests, the application value may be in helping predict, based on the nutrients in the algae, which lakes will have fish with higher methylmercury concentrations.

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--Flora Lichtman