Fish can absorb toxins from the environment and accumulate them within their flesh to dangerous levels in a process known as bioaccumulation. In particular, there have been numerous advisories warning people--particularly pregnant women and children--to restrict their intake of particular species to limit their exposure to mercury, which has been linked to neurological damage and an increased risk of heart attacks. Now a report published today in the journal Science indicates that the type of mercury present in swordfish and tuna might not be as harmful as previously thought.

Graham N. George, now at the University of Saskatchewan in Canada, and his colleagues bought samples of fresh fish from a market in California. Using X-ray absorption spectroscopy (XAS), the scientists investigated which atoms surround mercury found in muscle tissue and determined that the metal was most often bound to a carbon atom on one side and sulfur on the other. In this form, known as methylmercury cysteine, mercury is less likely to cross cell membranes than it is when present in the form of methylmercury chloride, which is typically used to model its potential toxic effects. Indeed, according to the report, day-old zebrafish larvae tolerated 20 times more methylmercury cysteine than methylmercury chloride. "There may be reason for cautious optimism," George says. "The mercury in fish may not be as toxic as many people think but there is a lot we need to find out before we can make this conclusion."

The scientists next plan to study what form of mercury accumulates in mammals that consume fish containing mercury. "Once we understand how mercury is bound in mammalian tissue," George notes, "we'll be ideally poised to design a drug that could perhaps remove it."

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