Andrew Graham and Seth Van Helten sampling groundwater near a capped pipe and stump

Science Research Stories: Environmental Chemist Andrew Graham

The Surprising Way that the Neurotoxin Methylmercury Is Degraded

Student preparing solution
Charlotte Kanzler ’17 preparing solutions in an anaerobic bag

Like many people, chemistry associate professor Andrew Graham eats fish. Unlike many people, he thinks a lot about mercury when he does. That is because much of his research focuses on mercury, and the main way people are exposed to mercury is by eating fish.

From Coal and Gold to Disease and Death

The largest human sources of atmospheric mercury are coal combustion and artisanal gold mining. The mercury then moves from the atmosphere into aquatic systems, where bacteria and archaea (a group of single-celled organisms without nuclei) methylate it — they add a methyl group to it consisting of 1 carbon atom and 3 hydrogen atoms. After methylation, the mercury becomes methylmercury.

Methylmercury is neurotoxic to humans and other organisms. At its worst, high mercury consumption causes Minamata disease, which is characterized by paralysis, tremors, and death. Making matters worse, methylmercury is bioaccumulative. “Organisms that take up the methylmercury have a hard time getting rid of [it],” Graham explains. It is also biomagnified. Methylmercury concentrations increase by a factor of about 10 each step up the food chain. For this reason, Graham says, “I don’t eat any swordfish or shark,” 2 organisms high on the food chain that have high methylmercury concentrations as a result. Though he laughs and adds, “I don’t know if I would [even] if I didn’t know what I know about mercury.”

Deep Research with a Purpose

Graham standing in water
Andrew Graham

Graham wants his research to improve mercury modeling, which informs preventative actions such as emissions policies, as well as fish consumption advisories about how much of what type of fish are safe to eat in a given place. More specifically, he wants to understand what causes mercury methylation and demethylation (removing a methyl group), the main drivers of changes in methylmercury concentrations.

In 2018, he copublished a paper in Environmental Science: Processes & Impacts on “Methylmercury speciation and dimethylmercury production in sulfidic solutions.” Part of this paper focused on one of four pathways for methylmercury demethylation: demethylation by organisms in waters without oxygen.

There have been few studies on this pathway, and none of those studies included controls without organisms but with the same water chemistry as the experiments with organisms. As such, they could not demonstrate that the organisms were directly doing the demethylation themselves. Rather, the products of the organisms’ metabolism could be doing the demethylation.

Graham’s study did include those controls. His team measured demethylation by the amount of mercury lost over time. Three students at the College, Xiaoxuan Yang ’17, Emma Leverich Trainer ’16, and Charlotte R. Kanzler ’17, did the control experiments.

Grinnell College has advanced science facilities that allow faculty and students to do advanced research such as this. The instruments Graham uses in his lab are “not something you’ll find most places and not something undergraduates would get exposure to most places,” says Graham. That exposure is especially valuable for students pursuing chemistry research post-graduation.

Questioning Assumptions, Discovering Alternatives

Two students working with equipment in an anaerobic bag
Deborah Msekala ’17 and Xiaoxuan Yang ’17 working in an anaerobic bag

The control experiments did not go as expected. “The really surprising thing was that when we did those control experiments, we saw loss of methylmercury in our control experiments that was not all that different than our biological experiments,” says Graham. That meant the products of the organisms’ metabolism were doing the demethylation, not the organisms themselves. In a society where we assume humans and other organisms drive most change, it was surprising to discover that neither directly drives such an important reaction.

“We go in expecting something, and when we get results that don’t match with our expectation, our initial response can often be ‘Well, something went wrong with the experiment.’ We don’t immediately go to questioning the major underlying assumption,” Graham says. As a result, “The initial challenge was actually convincing ourselves that we hadn’t messed up somehow.” In the end, they did convince themselves they had not messed up, and they had the evidence to prove it.

These findings were a big step in understanding the causes of mercury demethylation and thus how the harms of mercury can be reduced. It is not yet known how significant this demethylation without oxygen is in natural systems. However, with Graham’s team actively working in the lab and field to find that out, it may not be unknown for much longer.


Vishva Nalamalapu ’20 is the content specialist fellow in the Office of Communications and Marketing at Grinnell College. She loves writing about scientific research in a way that is accessible and interesting to readers with or without science backgrounds. Her series on scientific research projects focuses on doing just that. If you are a Grinnell College professor or student interested in having your scientific research project featured or think someone else’s project would be a good fit, please contact her.

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