Scientists track a deadly environmental poison in Earth’s oceans.
The mystery of methylmercury formation in the environment has baffled scientists for more than half a century. The ancient Romans knew mercury as hydrargyrum, or liquid silver, from which it pulls its chemical designation, Hg.
A glistening, metallic fluid at room temperature, it fascinated alchemists and played a suspect role in medicine and agriculture over the centuries. When elemental mercury is released into the environment — coal burning, gold mining and volcanic eruptions are significant sources — and bonds with carbon, it becomes methylmercury, a potentially fatal poison.
Fish and animals ingest methylmercury from the environment, where it becomes concentrated in their tissues in increasing amounts along the food chain, a process known as bioaccumulation. When humans eat contaminated food, it damages the brain, nerves and unborn babies.
In 1956, people in and around Minamata City in the Kumamoto Prefecture of Japan suddenly developed frightening symptoms: tremors, loss of speech and vision, paralysis, seizures and coma. Hundreds died and children were born with severe birth defects. Investigators determined the cause to be methylmercury poisoning from contaminated seafood. They found the Chisso chemical factory had been dumping wastewater containing the toxin directly into Minamata Bay for years. The amount of mercury in the fishing waters at Minamata was so great the chemical company later recovered it from the bay’s sediment to re-sell for use in manufacturing.
The Japanese government has recognized less than 3,000 patients diagnosed with Minamata disease, but the Chisso Corp. settled claims with more than 10,000. There may be tens of thousands more victims, including those who died before the disease was diagnosed, whose claims have since been denied by the government or who chose not to come forward because of the social stigma associated with the disease. The scale of human suffering at Minamata put methylmercury poisoning in the spotlight and galvanized the scientific community. The United Nations Environmental Programme acknowledged the magnitude of the global health hazard by adopting an international treaty, the Minamata Convention, in 2013.
There have been other cases of large-scale methylmercury poisonings from industrial uses of the compound, but most of the methylmercury in the oceans comes from inorganic mercury released by human-made and natural sources that are converted to methylmercury. Until recently, how that conversion happens remained a mystery.
Enter, scientific sleuth Dr. Liyuan Liang. In 2013, Liang, a molecular chemist, and her team of researchers, working at Oak Ridge National Laboratory in Oak Ridge, Tennessee, tracked down the specific genes in bacteria that are responsible for converting methylmercury.
ÛÏIn a sense, I see myself as a kind of detective, a science detective,Û said Liang. ÛÏThe research process is both a painstaking and an exciting thing. Sometimes you ask yourself, Û÷Is this solvable?’ Because it is very difficult. So when you do make progress, it’s a moment of joy.Û
Established research showed that certain bacteria, like Desulfovibrio desulfuricans, that thrive in low oxygen environments can convert mercury to methylmercury. Liang and her team suspected that two genes were responsible. They removed the two genes, named HgcA and HgcB, from sample bacteria.
Next, the mutated bacteria without the genes were fed mercury to see whether they still produced methylmercury. They didn’t. The news caused elation among Liang’s team, but one more step remained to confirm their discovery. The genes were inserted back into the bacteria to see if methylmercury conversion would resume. Liang said her team members held their breath. The bacteria again produced methylmercury, and after 10 months of rigorous experimental work their theory became fact.
ÛÏIt was painstaking, but because we were after something; it was exciting,Û Liang said. ÛÏIn the end, you really want to understand and solve the problem. I think that’s the driver.Û The research marked ÛÏa defining momentÛ for Liang, who calls the work the most important of her career. The team published another paper in January 2016, demonstrating the ability of certain bacteria to demethylize mercury. Liang says the full implications of their laboratory findings aren’t yet understood and many questions remain.
One question is how mercury becomes methylated in the ocean, where mercury levels are disproportionately low compared to the amount of methylmercury found in fish. Because most cases of human methylmercury poisoning occur through eating seafood, Liang said it’s imperative to understand how mercury released into the environment becomes methylated in the oceans and whether there are sources of mercury below the surface of the oceans. While low oxygen zones exist in ocean water columns and are a possible avenue for methylation by bacteria, scientists are still gathering data to determine where and how the process takes place in marine environments.
Drs. Katlin Bowman and Kathleen Munson were among a group of researchers who analyzed mercury levels in the Atlantic, Pacific, Southern and Arctic oceans gathered during several U.S. GEOTRACES research cruises. Their results, published in 2014, showed that human activities have tripled the amount of mercury found in surface waters and increased the amount in the thermocline — the layer of rapidly decreasing temperature between warmer surface water and the colder deep ocean — by 150 percent compared to levels prior to human impact.
Scouring Earth’s Oceans for Methylmercury
Rising mercury levels in the Earth’s oceans means a greater potential for methylmercury to form and accumulate in seafood. Two highly publicized cases in the United States brought methylmercury poisoning into the public conversation. In 2005, Richard Gelfond, CEO of IMAX Corp., found himself stumbling when he ran or played tennis. Gelfond’s balance and coordination continued to deteriorate until he couldn’t walk unsupported. He also developed a strange numbness in his lips and feet. After months of tests to rule out brain tumors and other illnesses, his doctor asked him if he ate a lot of fish. Gelfond, who ate fish twice a day, was diagnosed with methylmercury poisoning after tests showed he had a shockingly high level of it in his blood.
Actor Jeremy Piven developed symptoms of the disease in 2008, which put him in the hospital and ended his starring role in a hit Broadway play. The controversy surrounding his illness and sudden departure from the play both raised awareness about methylmercury poisoning and highlighted the lack of information about the disease and its connection to consumption of fish.
Although Gelfond recovered to a large extent, he said during a lecture for the Department of Medicine at Stony Brook University Medical Center in November 2010 that he still suffers from some effects of the disease. Piven continues to undergo chelation therapy to remove the toxin from his body, according to media reports.
The connection between mercury in the oceans and human health drives Bowman, one of the researchers who worked on the global ocean inventory, to scour the seas for the heavy metal and its toxic cousin, methylmercury. Bowman is a postdoctoral researcher in oceanography at the University of California, Santa Cruz, and sailed with nine research expeditions, including the ongoing GEOTRACES study, to measure mercury concentrations in large sections of the ocean and deep-sea hydrothermal vents.
ÛÏOne of my research goals has been to quantify how much mercury in the ocean originates from natural- versus human-caused sources,Û said Bowman. ÛÏDeep-sea hydrothermal vents may be a source of mercury to the sea floor, but we don’t have enough data to understand how far-reaching or significant this source is.Û
Earlier this year, Bowman spent a week at Oak Ridge National Laboratory, where Liang’s work was done, to learn DNA extraction methods and work with newly developed tools to search for relevant mercury gene sequences. Using DNA samples from the Arctic Ocean where Bowman found methylmercury, she will apply her new skills to search for methylating genes.
During her most recent cruise onboard the R/V Atlantis out of Woods Hole Oceanographic Institution, Bowman took her first dive aboard Alvin, a Human Occupied Vehicle (HOV) used for deep-sea exploration, and used Sentry, a Remotely Operated Vehicle (ROV), to collect coral that may show changes in the rate of mercury deposition over time.
ÛÏHard corals incorporate mercury in their skeleton along with calcium and carbonate,Û Bowman explained. ÛÏDeep sea corals can grow for hundreds of years, so we expect to see an increase in mercury during the industrial revolution 200 years ago.Û
Tracking Genes in the Ocean
Dr. Kathleen Munson, a chemical oceanographer currently conducting research on mercury in the Hudson Bay System with the University of Manitoba, said Liang’s discovery of methylating genes adds a new dimension to her ocean research.
ÛÏThe identification of the genes lets us look at exactly the diversity of the organisms that are capable of this, and also it is kind of forcing us to look for these genes in different environments,Û said Munson. She earned a bachelor’s in biochemistry and a Ph.D. in chemical oceanography from Massachusetts Institute of Technology and Woods Hole Oceanographic Institute (WHOI), and is currently conducting research on mercury levels in the Hudson Bay system in northeastern Canada. Munson has extensively measured mercury and methylmercury at WHOI and aboard the CCGS Amundsen. ÛÏI think a very big question right now is: Are these genes present in the water column as well, or is it a different process that’s methylating mercury in the water column?Û
In Richland, Washington, Liang, now the director of the Environmental Molecular Sciences Laboratory at the Department of Energy’s Pacific Northwest National Laboratory, is excited about the future implications of current research, in the lab and at sea. She said the Oak Ridge team’s latest demethylation research could lead to possible avenues to limit or decrease the amount of methylmercury in the environment.
One such avenue might be to promote more competition in the environment from bacteria that break the bond between mercury and its methyl group, to reverse the methylation process. Another is to understand how the demethylizing bacteria breaks the bond between the methyl group and the mercury, and somehow alter the bacteria.
ÛÏThe current research is really trying to see exactly how this whole thing works,Û she said. ÛÏThen maybe we can manipulate this amino acid, we can manipulate whatever, to make it go in the reverse.Û
While scientists search for the answers to methylmercury formation in the oceans, others continue to work toward limiting mercury emissions. Due in a large part to the Minamata Convention, emissions from coal burning are declining, but small-scale and artisanal gold mining continues to be a significant source of mercury.
According to the U.N. Environmental Programme, about 1,400 tons of mercury contaminates the environment each year to meet the demand for manufacturing consumer electronics. Liang said it will take earnest efforts on all fronts to mitigate the problem but believes her team’s discoveries have allowed research to accelerate, bringing solutions to this global health hazard within reach.
Lorelei Goff writes about science and the environment from Greeneville, Tennessee.