Antioxidant Chemicals Could Alter Mercury’s Environmental Fate

Heileen Hsu-KimAntioxidant chemicals, including one produced by aquatic life during times of stress, may have a hand in the fate of mercury in watersheds, potentially influencing the toxic metal's entry into the food chain, according to a report by a researcher at Duke University's Pratt School of Engineering.

The researcher reports in the April 1 Environmental Science & Technology that mercury and other trace metals react with a common antioxidant defense molecule to form stable complexes that can persist for days. Organisms from bacteria to mammals produce and sometimes release antioxidants, which neutralize harmful free radicals and other toxic chemicals. Similar compounds, prevalently found in some aquatic environments, would be expected to react in the same manner.

The stability of such chemicals linked with mercury might increase their persistence in surface waters and the distance they could travel, said Heileen Hsu-Kim, an assistant professor of civil and environmental engineering. The interaction might therefore influence the rate at which mercury trickles down into sediments, where it can be converted into the form that builds up in fish and makes its way to dinner tables.

"The formation of these really stable mercury-thiol compounds may interfere with environmental processes that cause mercury to convert to the form that bio-accumulates," Hsu-Kim said. Thiols are defined by the presence of a sulfur-containing chemical group with antioxidant effects.

Mercury released into the air from coal-burning power plants or other sources can get deposited on surface waters, Hsu-Kim explained. But the mercury has to make its way to oxygen-deprived sediments before it can be converted by anaerobic bacteria, those that grow in the absence of oxygen, into the methylmercury that concentrates in fish.

"If mercury is released into the air in locations upwind of the anaerobic environments where methylmercury is produced, you could end up with hotspots," Hsu-Kim said. "That condition depends on the whole transport process of mercury after it deposits into watersheds and surface waters.

"Mercury's interaction with thiols is one component of that whole chain of events that could lead to the development of methylmercury hotspots."

Thiols normally break down easily in the presence of oxygen, a characteristic that gives them their antioxidant capacity. In the new study, Hsu-Kim tested whether trace metals, including mercury, could interfere with that process.

She mixed one such molecule, called glutathione, with individual trace metals and with chemicals, including copper and hydrogen peroxide, that would normally spur the antioxidant's break down. The glutathione formed complexes with both mercury and silver that remained stable for at least two days, she found. That interaction, in effect, prevented the glutathione from reacting with the other chemicals. A similar, though weaker, interaction took place between the glutathione and other trace metals, including cadmium, zinc and lead, Hsu-Kim reported.

In addition to their potential implications for the fate of mercury and other trace metals in the environment, the results also support the notion that the toxic effects of some metals stem from their ability to disable protective antioxidants, Hsu-Kim added.

"It tells us what some metals would do if organisms are exposed," she said. "The metals basically bind up the antioxidant site, blocking glutathione's ability to neutralize harmful free radicals that can damage DNA or other cell structures."

The research was supported by the Pratt School of Engineering.