Natural dissolved organic matter plays dual role in cycling of mercury

Background:
Mercury is distributed around the globe mainly through the burning of coal, industrial uses and through natural processes such as volcano eruptions. Various forms of mercury are widely found in sediments and water.

Microbes in aquatic environments transform inorganic mercury through methylation to methylmercury, a more toxic form of mercury that enters the food chain and accumulates in fish. Such biomethylation predominanantly occurs under anaerobic conditions. Interestingly, such biomethylation is not a one-way road, and other types of bacteria can transform methylmercury to less toxic forms. The environmental factors that determine the mercury availability to methylating bacteria and its transformation under these conditions remain poorly understood.


The new study:

Findings published by Oak Ridge National Laboratory's Liyuan Liang and Baohua Gu help explain previously reported seemingly contradictory findings. (ORNL photo by Jason Richards)

Researchers at the Department of Energy's Oak Ridge National Laboratory have made a discovery that ultimately could help explain the complex cycling of mercury. They performed their experiments by simulating conditions found in nature.

"Until now, reactions between elemental mercury and dissolved organic matter have rarely been studied in anoxic environments," said Baohua Gu of the the lab's Environmental Sciences Division.

In a paper published January 25 in the Proceedings of the National Academy of Sciences, a team led by Gu reports that compounds from the decay of organic matter in aquatic settings affect mercury cycling. Low concentrations of these compounds can chemically reduce mercury, but as those concentrations increase, that reaction is greatly inhibited.

"This study demonstrates that in anoxic sediments and water, organic matter is not only capable of reducing mercury, but also binding to mercury," said co-author Liyuan Liang. "This binding could make mercury less available to microorganisms for making methylmercury."

The authors also noted that their paper offers a mechanism that helps explain the seemingly contradictory reports on the interaction of organic matter and mercury in nature.

Gu and Liang hope this newly gained knowledge will play a role in helping to understand how mercury cycles in aquatic and sediment environments and help in informed decision-making for mercury-impacted sites around the nation.

"Our long-term goal is to understand the mechanisms controlling the production of methylmercury in the environment, " Liang said. "This understanding could lead to ways to reduce levels of mercury in fish as this is a global problem of enormous significance."

Source: adapted from Oak Ridge National Laboratory


The original study:

 Baohua Gu, Yongrong Bian, Carrie L. Miller, Wenming Dong, Xin Jiang, and Liyuan Liang, Mercury reduction and complexation by natural organic matter in anoxic environments, Proc. Natl. Acad. Sci. USA, 108/4 (2011) 1479-1483. DOI: 10.1073/pnas.1008747108



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