Guilt by Association – Tracking methylmercury with genes

By Elizabeth White

Fish consumption advisories are designed to protect people from unsafe levels of mercury and other contaminants, but what if many of them are based on incorrect assumptions about how methylmercury forms and accumulates in aquatic environments? Sea Grant-funded researcher Trina McMahon, professor of bacteriology at UW-Madison, is exploring that possibility, but first, she’ll have to track down where the methylmercury is coming from. 

Inorganic mercury is transformed into methylmercury, the most toxic and easily bioaccumulated form of mercury, by the actions of microbes naturally found in aquatic environments. Accurate predictions of the amount of methylmercury in fish are based on how much methylmercury the fish is likely to have consumed.

Unfortunately, it’s impossible to tell how much methylmercury is being produced at a particular location in real time—bacteria convert mercury into methylmercury and back again, chemical reactions can de-methylate it, and physical processes can settle out particles.

McMahon describes trying to measure methylmercury: “What people do is measure the concentration of methylmercury over space and time. If they see higher levels in a particular area or at a particular time, they infer it’s being produced there because if it wasn’t being produced it would have been de-methylated by something. A lot of it is inferred.”

A group at Oak Ridge National Laboratory in Tennessee enabled researchers to bypass some of that inference by identifying a gene required to convert mercury to methylmercury. If the gene is present in a water sample, the bacteria that convert mercury to methylmercury are there too.

McMahon and her team, including graduate students Elizabeth McDaniel and Ben Peterson, used a new detection method to search for that gene. Their research revealed that mercury-methylating bacteria are present at locations in the water column where oxygen is depleted, suggesting that mercury can be methylated in the water column, not exclusively in anaerobic environments with lots of organic matter (like swamps and wetlands) as had previously been believed.

McMahon is currently working with water samples from the Great Lakes and Madison’s Lake Mendota to quantify the abundance of mercury-methylating bacteria and determine where they are in the water.

She said, “You see that there is methylmercury there; we’re hoping to see that it correlates with the genes being there. But we really have to show the organisms are methylating it at that moment, and that’s hard to do. We just have guilt by association.”

The concentration of methylmercury in the water (instead of the sediments) has serious implications for the bioaccumulation of mercury in the food chain. Zooplankton, which form the first link of the food chain, spend most of their time at the bottom of the aerobic zone. If methylmercury is forming at the top of the anaerobic zone, zooplankton may be much closer to methylmercury production sites than previously thought. The more methylmercury zooplankton consume, the more fish consume.

It’s possible there are other genes involved in methylation than the one found at Oak Ridge, and graduate student McDaniel is comparing micro-organisms that can methylate to see if there’s something about their metabolisms that gives them the ability to methylate mercury.

She said, “There’s this big question: How are they methylating mercury, and why?”

Answering that question might resolve more than curiosity. Once McDaniel locates the genes that are involved, she could delete them to see how that affects the methylation process. The implications of that remain to be seen.

McMahon said, “It’s hard to imagine at this moment how something you learn from the genomes could remediate, clean up, methylmercury in a system. But you never know. Basic science has a way.”