Researchers at Duke University, Durham, N.C., believe they have discovered the reason that saltwater fish like tuna, mackerel and shark that have ingested mercury tend to end up posing a more serious health threat to humans who eat them.

Even though freshwater concentrations of mercury are far greater than those found in seawater, saltwater fish have mysteriously yet definitively been linked to greater risk. The answer, according to the researchers, is in the seawater itself.

According to a study by Heileen Hsu-Kim, assistant professor of civil and environmental engineering at Duke's Pratt School of Engineering, the potentially harmful version of mercury, known as methylmercury, latches onto dissolved organic matter in freshwater, while it tends to latch onto chloride, i.e. the salt itself, in seawater.  

"The most common ways nature turns methylmercury into a less toxic form is through sunlight," said Hsu-Kim. "When it is attached to dissolved organic matter, like decayed plants or animal matter, sunlight more readily breaks down the methylmercury. However, in seawater, the methlymercury remains tightly bonded to the chloride, where sunlight does not degrade it as easily. In this form, methylmercury can then be ingested by marine animals."

Methylmercury is a potent neurotoxin that can lead to kidney dysfunctions, neurological disorders and even death. In particular, fetuses exposed to methylmercury can suffer from these same disorders as well as impaired learning abilities. Because fish and shellfish have a natural tendency to store methylmercury in their organs, they are the leading source of mercury ingestion for humans.

"The exposure rate of mercury in the U.S. is quite high," said Hsu-Kim. "A recent epidemiological survey found that up 8 percent of women had mercury levels higher than national guidelines. Since humans are on the top of the food chain, any mercury in our food accumulates in our body."

The results of Hsu-Kim's experiments, which have been published early online in the journal Nature Geoscience, suggest that scientists and policymakers should focus their efforts on the effects of mercury in the oceans, rather than freshwater.

Her research was supported by the National Institute of Environmental Health Science.

In the past, most of the scientific studies of effects of mercury in the environment have focused on freshwater, because the technology had not advanced to the point where scientists could accurately measure the smaller concentrations of mercury found in seawater. Though the concentrations may be smaller in seawater, mercury accumulates more readily in the tissues of organisms that consume it.

"Because sunlight does not break it down in seawater, the lifetime of methlymercury is much longer in the marine environment," said Hsu-Kim. "However, the Food and Drug Administration and the Environmental Protection Agency do not distinguish between freshwater and seawater."

The key to the sun's ability to break down methylmercury is a class of chemicals known as reactive oxygen species. These forms of oxygen are the biochemical equivalent of the bull in the china shop because of the way they break chemical bonds. One way these reactive oxygens are formed is by sunlight acting on oxygen molecules in the water.

"These reactive forms of oxygen are much more efficient in breaking the bonds within the methylmercury molecule," said Hsu-Kim. "And if the methylmercury is bonded to organic matter instead of chloride, then the break down reaction is much faster."

Tong Zhang, a Ph.D. candidate in Hsu-Kim's laboratory, was first author on the paper.

SOURCE: Press Release from Duke University