Sulfur regulates methylation of mercury in aquatic environments

If you've ever read a post on this blog, you know that methylmercury is the inorganic species of mercury most toxic and bioavailable to wildlife. But we haven't discussed regulation of methylmercury in the environment - an important process partly regulated by global sulfur dioxide emissions and deposition. Of course to most chemists sulfur  regulation of methylmercury is nothing new, but to fisheries biologists it's actually pretty interesting.

Source: http://www.timemastermd.com/?tag=natural

A group of researchers lead by Dr. Drevnick of the University of Miami were recently puzzled when they found that mercury concentrations in northern pike (Esox lucius) from Isle Royale in Lake Superior had declined significantly over the past decade. They were puzzled because atmospheric deposition - the predominate route of mercury loading  in Isle Royal lakes - had remained steady or slightly increased over the last several decades.  In fact, sediment core samples indicate an increasing trend of Hg deposition around Isle Royale for the past hundred years.  But despite increasing Hg deposition, Drevnick's study found that pike from 6 interior lakes having recent fish consumption advisory warnings for Hg contamination had all declined below the mercury advisory thresholds.



This of course is good news since fish consumption is the major exposure route for mercury in humans, but the researchers wanted to know why they observed this anomoly. They proposed three hypotheses: 1) there was a reduction in atmospheric deposition of inorganic mercury over the past few decades 2) something had changed in the pike's ecology resulting in a reduced exposure to mercury, or 3) something else in the environment had changed causing a reduction in the amount of available methylmercury.

The authors tested each hypothesis using a combination of live and museum-preserved fish, sediment profiles, water chemistry, and historic data to reconstruct temporal trends of mercury deposition in Isle Royale and mercury concentrations in pike, as well as other environmental indicators like stable isotopes and historic sulfur deposition rates at Isle Royale.

Source: http://www.neiwpcc.org/mercury/environment.asp

To explain the declining mercury concentrations in pike, the first hypothesis was easily rejected because historic data combined with the researchers sediment cores indicated no decline in sediment inorganic mercury concentrations. The second hypothesis was also rejected because there was no significant change in pike ecology between museum-preserved and live specimens based on carbon and nitrogen isotope analysis. Further, growth rates remained constant over time, all leading to the conclusion that pike growth and feeding habits have remained consistent over the past hundred years.

The authors then turned to other environmental factors that may have caused a reduction in the bioavailability of methylmercury.  They found that climate, landscape, and water quality had not significantly changed over recent decades, but that atmospheric sulfur deposition had appreciably declined since 1976, six years after the U.S. government adopted the U.S. Clean Air Act of 1970. Here, they were convinced that they found the cause. But just how does sulfur deposition explain the bioavailability of methylmercury?

It turns out the methylation of mercury happens in sediments at the bottom of aquatic ecosystems. Often under oxygen starved conditions, anaerobic bacteria thrive, essentially using sulfur instead of oxygen to "breath". As these microbes respire, they turn inorganic mercury into methylmercury. So more sulfur in aquatic sediments means more sulfate-reducing bacteria- the microbes responsible for methylmercury.  Thus even though atmospheric deposition of mercury has not declined, controls on sulfur emissions from the 1970 Clean Air Act have indirectly reduced the amount of bioavailable methylmercury in aquatic environments.

This is good news because sulfur emissions are regulated chiefly to prevent acid rain and these programs will likely not abate anytime soon. However, there has been limited discussion to increase sulfur dioxide emissions because it reflects sunlight - in effect combating global climate change. But this is a dangerous proposal and restrictions on greenhouse gases would be a safer, smarter solution to global climate change. And if we're seriously considering reductions in green house gas emissions, it might also behoove us  to implement strict regulations on mercury emissions. This would act cooperatively with existing reductions in sulfur emissions to  improve the health of wildlife and ultimately ourselves. Just an idea.

Reference:

Drevnick, P. E., Canfield, D. E.,  Gorski, P. R.,  Shinneman, A. L. C.,  Engstrom, D. R.,  Muir, D. C. G., Smith, G. R., Garrison, P. J., Cleckner, L. B., Hurley, J. P., Noble, R. B.,  Otter, R. R., Oris, J. T. 2007. Deposition and cycling of sulfur controls mercury accumulation in Isle Royale fish Environ. Sci. Technol. 41 7266 - 7272