A big fat source of mercury (Image: Dr. Priya Ganguli)

Heavy metal is moving in mysterious ways. Two studies this week show that pristine areas, far from industrial pollution, are building up surprisingly high levels of mercury from unexpected sources – moulting elephant seals and bacteria in Arctic estuaries.

The mercury released by factories is dangerous, but it becomes more harmful when bacteria at the bottom of the food chain convert it into an organic form: methylmercury, which is a neurotoxin. Over time, as the bacteria get eaten by organisms higher up the food chain, methylmercury levels can build up in top predators.


But mapping how this substance flows in the wild – and how it enters our food – is a work in progress. “The overwhelming majority of methylmercury exposure is from the consumption of fish and shellfish,” says Dave Krabbenhoft of the US Geological Survey in Wisconsin. “Yet our understanding of where the methylmercury was getting produced and its pathways to species we consume isn’t very clear.”

It appears that some top predators, such as elephant seals, don’t just hold on to the methylmercury that builds up in their systems, they also dump it back into otherwise remote, protected ecosystems.

Sealing in the mercury

California’s Año Nuevo State Reserve has higher mercury levels than any other site on the US West Coast. Now, researchers have pinpointed its source: the moulting seals and sea lions that crowd the reserve’s waterfront.

“Elephant seals undergo a catastrophic moult,” says Jennifer Cossaboon of San Diego State University in California. “It comes off in big sheets of fur and the top few layers of skin.”

For comparison, from examining the fur, Cossaboon calculates that elephant seals in the US and Mexico release about the same amount of methylmercury as the nearby San Francisco bay does. Around 40 per cent of California, the most populous US state, drains into San Francisco Bay, including areas of heavy industry.

It isn’t yet known how well methylmercury in hair can work its way back into the food chain, but if it happens, the same process probably occurs in other mammal and seabird species in the area.

“I just can’t get over how high the concentrations of methylmercury in the water are,” says Amina Schartup of Harvard University. “It’s incredible. I’ve never seen anything like that.”

Schartup’s team found another mercury surprise in Canada’s Lake Melville, a river-fed fjord that meets the ocean. Methylmercury levels in the system were much higher than they expected. “The question is why?” Schartup says. “It’s not really a contaminated environment.”

The big story

They believe the excess methylmercury is being made by bacteria floating in the lake, where layers of warm water are stacked down into colder depths. Bacteria settle into mats of mucus that rest between layers, and they process the mercury washed into the lake from nearby soil. That kind of methylmercury production has been seen in the ocean before, but not in estuaries.

A hydroelectric dam is planned for the river, and the resulting flooding could wash more mercury downstream to the fish that local Inuit communities depend on. Damming the river could double the methylmercury in the lake, Schartup warns – and a similar thing could happen with hydroelectric developments currently planned throughout the Arctic.

“For me that’s the big story,” says Krabbenhoft. Although soil mercury can leach out into the environment and be converted into methylmercury, this process in the water column is much quicker. That would mean changes to the amount of mercury released by industry could be quickly felt in food webs, with unpredictable results.

Krabbenhoft stresses that these sources of methylmercury are likely to change with the climate. For example, global warming will result in more temperature stratification in oceans and estuaries, giving bacteria more layers of water to settle between, which will lead to the production of more methylmercury.

“We’re opening the methylmercury era,” Schartup says. “We’re going to uncover more and more things that we didn’t realise were out there.”

Journal reference: PNAS, DOI: 10.1073/pnas.1506520112; DOI: 10.1073/pnas.1505541112