Why presence of mercury in seawater is more dangerous

The presence of mercury in seawater can be a threat to the health of people consuming saltwater fish like tuna, mackerel and shark. Duke university researcher have revealed why.

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

Hsu-Kim said: "The most common ways nature turns methylmercury into a less toxic form is through sunlight. 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 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.

Hsu-Kim said: "The exposure rate of mercury in the U.S. is quite high. 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 have appeared in the journal Nature Geoscience.