BioEd Online

Mercury levels in the Pacific Ocean are rising, a new study suggests.¹ The increase may mean that more methylmercury, a human neurotoxin formed when mercury is methylated by microbes, accumulates in marine fish such as tuna.

The research comes as researchers and policymakers, who have tended to focus on atmospheric concentrations of the element, are looking for a fuller picture of the mercury cycle. US guidelines on methylmercury in fish are currently under review.

It remains unclear exactly how atmospheric mercury — whether dumped directly into oceans or carried there through rivers or coastal deposits — is methylated and eventually taken up by fish, which are a major source of human exposure to methylmercury. But the new data, collected by Elsie Sunderland of Harvard University and colleagues, also provide a possible mechanism for mercury methylation within the ocean.

Ocean update

The researchers collected samples from the eastern North Pacific, an area also monitored by research cruises in 1987 and 2002.² They estimate that methylated mercury accounts for as much as 29% of all mercury in subsurface ocean waters, with lower concentrations occurring in deeper water masses. The group's modelling indicates that atmospheric deposition of mercury could lead to a doubling of the total ocean mercury concentrations recorded in the mid-1990s by 2050.

Sunderland's team also found a relationship between levels of methylated mercury and organic carbon. Particles of organic carbon from phytoplankton or other sources may provide surfaces on which microbes could methylate mercury in the ocean, the researchers suggest. That methylated mercury could then be released back into the water.

"We don't have a causal mechanism yet, but it does seem to be linked to the biological pump in the ocean," says Sunderland. Previous findings in the southern and equatorial Pacific, she adds, observed similar high methylmercury concentrations where biological activity was highest. That connection has implications for climate change and the mercury cycle: warmer, more productive oceans, with more phytoplankton and more fish, might increase the amount of methylated mercury that ends up on human plates.

The researchers also hypothesize that waters in the western Pacific could be picking up mercury deposited from increasing atmospheric emissions in Asia, and then moving to the northeast Pacific. The ocean may only now be responding to higher mercury loads from past atmospheric deposition, Sunderland says.

Daniel Cossa of the French Research Institute for Exploitation of the Sea (IFREMER) in La Seyne sur mer and his colleagues have gathered another set of mercury data, this time from the Mediterranean, to be published in the May issue of Limnology and Oceanography.³ Both papers indicate that not all methylated mercury comes direct from coastal or river sources, and confirm that methylation occurs at moderate depth in oceans, says Cossa's co-author Nicola Pirrone, director of Italy's National Research Council Institute for Atmospheric Pollution in Rende.

Natural controversy

"The ocean is a big missing part" of the mercury cycle, says Pirrone, who also headed the United Nations Environment Programme's scientific assessment last year for future mercury policy making.

Robie Macdonald, an Arctic mercury specialist at Canada's department of oceans and fisheries, says that although mercury in the atmosphere has increased by about 400% in the past 100–150 years, concentrations seem to have risen by only about 30% in the oceans.⁴ "We've been so busy looking at the atmosphere, not really looking at the oceans," he says. "Both papers are really important in terms of changing community attention to what mercury does and its risks."

Any control measures on methylmercury, however, must take into account how much comes, unavoidably, from natural sources and how much is from anthropogenic sources such as the combustion of fossil fuels, points out Pirrone.

And controversy continues on that score. A lack of data on changes in methylmercury levels in fish, and on natural or anthropogenic origins of the compound, led to a California court decision in March 2009 that allowed tuna-canning companies to avoid labelling methylmercury levels in their fish products. The US Food and Drug Administration is currently evaluating its guidelines on the risks of consuming methylmercury in fish.