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Climbing temperatures in the Arctic tundra are transforming inorganic mercury deposited by power plants and other industrial polluters, some of it inert for decades, into a neurotoxin that is accumulating in the region’s lake sediments, wetland ponds, soils and food chains.
Certain tiny anaerobic microorganisms are thought to play a role in accelerating the formation of methylmercury (MeHg), a neurotoxin. As the permafrost thaws, researchers posit, decaying soil releases an abundance of nutrients. Those nutrients fuel the metabolisms of anaerobes, which convert inorganic mercury into methylmercury, a form they can excrete.
Lijie Zhang, an assistant professor of chemistry and environmental science who studies the transformation of industrial pollutants in the environment, is conducting lab experiments to better understand the biogeochemical drivers of the toxin’s formation, a process known as methylation, in tundra soils.
“By elucidating these mechanisms, we can try to incorporate them into biogeochemical models that allow us to more accurately predict the fate and transformation of mercury in the Arctic under future climate conditions,” Zhang explains.
High levels of mercury that travel on air currents from power plants, mining and metal refining factories, from thousands of miles away in some cases, have been recorded in the tundra, she says. Concentrations of methylmercury, well above U.S. Environmental Protection Agency guidelines, have been observed in Northern communities where diets are composed predominantly of traditional foods such as Arctic char, ringed seal and beluga whales.
The agency describes methylmercury as a powerful neurotoxin that particularly affects children in the womb, who are vulnerable to developmental impacts to cognitive thinking, memory, attention, language, fine motor skills and visual- spatial skills. As many as 75,000 newborns in the U.S. each year may have increased risk of learning disabilities associated with in-utero exposure to methylmercury.
“Although the EPA issued regulations in the late 20th century to limit or ban mercury emissions, it had already impaired thousands of freshwater ecosystems in the United States,” Zhang says, noting that most human exposure to mercury is from eating contaminated fish and shellfish. “Populations with high consumption of fish and marine mammals, such as Northern Peoples, are particularly at risk of elevated exposure to methylmercury that biomagnifies in food chains.”
Zhang’s first step was to identify the principal producers of methylmercury to determine how they are related to geochemical characteristics in specific soils.
Using anaerobic incubation, her team studied microbial production of the organic toxin in contrasting Arctic tundra soils collected near Nome, Alaska: an acidic bog soil and a neutral fen soil. She introduced two substrates that methanogens and sulfate-reducing bacteria subsist on — acetate, an organic carbon, and sulfate, a mineral salt, that are formed during the decay of organic matter, precipitation and seawater intrusion — to see how changing levels affected methylmercury formation.
The team discovered that methylmercury production in the neutral fen soil stalled entirely when chemicals that inhibit the metabolic activity of sulfate-reducing bacteria were introduced. Adding sulfate in the low-sulfate bog soil increased production five-fold, however. This suggested, Zhang says, that the bacteria that metabolize sulfate are a key group of methylators in the soil.
“Our results indicate that the dominant groups of methylators are dependent on soil geochemistry, which can be altered by climate change,” she notes. “This is an important step to advance our understanding of the transformation and bioaccumulation of mercury and methylmercury in the rapidly changing Arctic ecosystem due to global warming.”
Zhang also quantified these increases by measuring the methylation rates of the microorganisms she has identified as the predominant producers. Incorporated into geochemical models, this data will be useful to policy makers as they consider managing mercury emissions.
“It is difficult to remediate mercury pollution in the Arctic directly,” Zhang notes. “The best solution is to reduce the release of mercury from sources around the globe that do not currently control it, such as some coal-fired power plants and gold mining operations, among others.”
According to the EPA, some estimates indicate global sources contribute about 70% of mercury deposited in the contiguous U.S., while the percentage varies geographically.
In upcoming experiments, Zhang is planning to study the interactions among the decay of organic soil, the emission of greenhouse gases and methylmercury formation in various other natural ecosystems, including areas affected by wildfires.
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