EcoPedia

Sorry Charlie, the Mercury Methylation Cycle

Michael Forlini | Jun 5, 2009

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The Food and Drug Adminsitration (FDA) recommends that women who are pregnant, or may become pregnant, and nursing mothers avoid fish that may contain unsafe levels of methylmercury. Such fish includes swordfish, king mackerel, shark, or tilefish.

A recent study released last month by the United States Geological Survey (USGS) has shown the rise of methylmercury in ocean life. This study focused on human activities in releasing mercury in the north Pacific and challenged the validity of a long-held theory that it was geologic in origin and associated with deep-sea spreading. This theory assumed that methylmercury originated 30,000 feet below the sea surface. A team of researchers at the USGS sampled Pacific Ocean water from 16 sites between Honolulu and Kodiak, Alaska. In addition, atmospheric mercury emissions were studied as a potential source with the use of an ocean circulation model.

Samples taken from the Pacific Ocean showed that 2006 mercury levels were approximately 30 percent greater than mid-1990s levels. The initial cause of this dramatic increase was thought to be mercury emissions associated with the combustion of fossil fuels. However, air modeling computations could not prove this theory, given that the air emissions did not travel a far enough distance to impact the middle Pacific. The question of how mercury gets into tuna and other fish in the ocean has challenged the scientific community for decades.

This new study provides a significant step forward in understanding oceanic methylmercury sources. Data and modeling results from the study support the notion of a mercury methylation cycle, in which much of the methylmercury in the open ocean is the result of biologically mediated transformation of mercury into methylmercury. In the mercury methylation cycle, most of the mercury originates from atmospheric fallout to the ocean surface and the subsequent transport of the mercury to greater ocean depths (200 to 700 meters) where the methylmercury production process occurs. At these depths, naturally occurring bacteria decompose organic matter, which is largely comprised of settling algae (commonly referred to as ocean rain) that are produced in the sunlit waters near the surface (the photic zone). However, the decomposition of organic matter also results in unintentional conversion of mercury to methylmercury, which is then passed up the food web through the bioaccumulation process, eventually to top predator fish like tuna.

The model used in this study predicts that Pacific Ocean mercury levels will rise another 50 percent by the year 2050 if mercury emission rates continue to rise as projected. Such increases could have implications for resulting methylmercury levels in Pacific Ocean fish. The overall findings from this study suggest that measurable changes in mercury levels of very large systems (such as the Pacific Ocean) are plausible over reasonably short time frames and it is equally plausible that reductions in ocean mercury levels would follow if mercury emissions were decreased.