Mercury, in liquid form, was once widely used in thermometers, dental fillings, and electronics. But the recognition that it can cause damage to the brain, immune system and developing embryos, has greatly reduced its use in most consumer products. Yet it remains a serious threat to human health because of the release of high levels of mercury into the environment by industry during the past 100 years. When mercury binds with organic molecules in nature, in a process that is still not fully understood, it becomes highly toxic. This type of mercury is known as methylmercury.
Mercury's Greek name hydrargyrum means "liquid silver," and it has long intrigued scientists seeking to understand its properties. Environmental engineer Liyuan Liang and her team at Oak Ridge National Laboratory (ORNL) in Tennessee are some of those scientists. They're getting closer to understanding just how mercury is transformed into methylmercury; knowledge that could lead to ways of limiting its production in the environment.
Liang and researchers have figured out how bacteria convert mercury into the neurotoxin methylmercury. It's a process known as microbial mercury methylation.
They discovered two genes in bacteria, hgcA and hgcB, were both needed to convert mercury into methylmercury. Those two genes are found in many different bacteria, and all are capable of producing methylmercury under the right circumstances.
Liang's selection as an AAAS Fellow in 2014 was tied to these discoveries, "for leadership in understanding mercury transformation in the environment, leading to the discovery of mercury methylation genes in anaerobic bacteria."
Methylmercury is especially nasty Liang says, because it biomagnifies, increasing in amount and concentration as it travels up the food chain. The most common risk to humans from methylmercury is in seafood. Global warnings are widespread, especially for pregnant women, to limit intake of fish with known high mercury levels -- tuna, swordfish, and mackerel.
"Methylmercury accumulates inside the cell. It looks like other amino acids, and in fish for instance, it is used as a building block for muscles. Once the methylmercury gets into the proteins, they stay, they don't break down so easily," said Liang.
The industrial discharge of mercury in Minamata, Japan, at its height in the mid 1950s, was one of the most horrific cases of toxic contamination. Thousands of humans and animals were killed or debilitated.
"In Minamata, people thought it was lead poisoning. Not until a few years later did people realize methylmercury was the cause," said Liang.
While such industrial catastrophes are less likely today, the danger remains. Its presence can come from coal-burning power plants, known to release high levels of mercury into the air, and rice paddies, which naturaly pull metals like mercury from soil. It can also occur in contaminated aquatic sediments, like coastal dead zones. It is also increasingly present due to the widespread thawing of permafrost triggered by climate change.
The international community now recognizes mercury's threat to human health and the environment. More than 140 countries signed an international treaty in 2013, limiting mercury use.
Liang takes great satisfaction in this treaty but her work and that of her team is far from over. They are now looking at how mercury moves through the atmosphere, both naturally, from volcanoes, and by coal-burning.
"We have found mercury in the tundra even in the summer. There's no pollution source there. It is simply long distance atmospheric transport," said Liang. "There are many people looking at how mercury transports as a gas, and then how it gets oxidized and incorporated into particles and water droplets, and then introduced into our aquatic system. You can see the cycling that just goes on and on," she said.
And she has her eye on studying the ocean.
"The concentration of mercury in the ocean is very low, yet there's so much bioaccumulation along the food chain that gets into fish. And people still don't understand how that happens," she said.