Bacteria in sea ice are making mercury worse

(Credit: euphro/Flickr)

Scientists have discovered methylmercury—a potent neurotoxin—in sea ice in the Southern Ocean.

The results, published in the journal Nature Microbiology, are the first to show that sea-ice bacteria can change mercury into methylmercury, a more toxic form that can contaminate the marine environment, including fish and birds.

Mercury is a heavy metal pollutant that can be released into the environment through volcanic eruptions and re-released from vegetation during bushfires. It is also created through human activity, such as gold smelting and burning fossil fuels.

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If ingested, methylmercury can travel to the brain, causing developmental and physical problems in fetuses, infants, and children.

Methylmercury builds up in the food web through a process called “biomagnification,” says team leader Caitlin Gionfriddo, a PhD candidate from the School of Earth Sciences at the University of Melbourne. “Larger fish eat smaller contaminated fish, and continuously accumulate methylmercury at harmful levels for human consumption.”

The team wanted to understand more about how the most toxic form of mercury enters the marine environment as well as the food we eat.

Gionfriddo spent two months aboard the icebreaker Aurora Australis to collect samples of Antarctic sea ice during an expedition mounted by the Australian Antarctic Division.

Researchers analyzed the ice for different forms of mercury, including methylmercury, at the US Geological Survey in Wisconsin. The DNA and proteins from sea ice microorganisms were studied at the University of Melbourne and Lawrence Livermore National Lab in the US.

The results confirm the presence of bacteria in the sea ice with the genetic ability to convert mercury into the more toxic form, says John Moreau, team leader and geomicrobiologist at the University of Melbourne.

These findings highlight the importance of eliminating mercury pollution from the environment, and following current recommendations to limit consumption of certain types of fish, say the researchers.

“These results are the first to identify a particular genus of bacteria, Nitrospina, as capable of producing methylmercury in Antarctic ice,” says Moreau.

“The presence of these potential mercury-methylating bacteria raises an interesting question,” he adds. “Could they also play a role in forming the methylmercury observed in the oceans worldwide?”

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The team is keen to understand this process in the next steps of their research.

“Mercury has a long lifecycle in the atmosphere, up to a year,” says coauthor Robyn Schofield. “This means that mercury released through fossil fuel burning from countries over 3,000 km away goes up in the atmosphere and ends up in Antarctica.”

“The deposition of mercury into the sea occurs all year-long but increases during the Antarctic spring, when the sunlight returning causes reactions that boost the amount of mercury that falls onto sea ice and the ocean,” says Gionfriddo.

“We need to understand more about marine mercury pollution,” says Moreau, “Particularly in a warming climate and when depleted fish stocks means more seafood companies are looking south.”

Funding came from the Australian government’s Antarctic Science Grant Program and the University of Melbourne Joyce Lambert Antarctic Research Seed Funding Grant.

Source: University of Melbourne