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Fish respond when the mercury rises

Approximately 40 years ago, researchers in Sweden and Canada made the startling discovery that fish from relatively pristine northern lakes contained unusually high levels of mercury (Hg) in their muscle tissue. Up until that point, mercury-contaminated fisheries were known only from locations receiving end-of-pipe discharges of mercury-laden wastes. In this case, however, lakes were remote from any such point-sources but still had fish mercury levels high enough to cause human-health problems, principally neurological damage, if consumed in sufficient quantities (1). It would be another decade before the development of clean sampling and analytical techniques would allow researchers to link the exceedingly low levels of mercury present in lake water to their biomagnification in the aquatic food chain and back to their ultimate source in industrial atmospheric emissions (2, 3). Since that time, mercury scientists have determined that thousands of lakes worldwide (the vast majority of those tested) are similarly contaminated; they have unraveled the microbial processes by which elemental mercury is converted to its bioaccumulative form, methylmercury (4); and they have determined through the analysis of natural geologic archives (cores of lake sediments and peat bogs) that human activities are responsible for approximately two-thirds of the mercury now present in the global atmosphere (5). In this issue of PNAS, Harris et al. (6) add a key piece to this giant biogeochemical puzzle: the experimental determination that mercury levels in fish respond rapidly and directly to changes in atmospheric mercury deposition.