Blown away

By Fred Pearce REMOVING toxic mercury from landfills may be as easy as planting trees. By borrowing a gene from bacteria, researchers have produced yellow poplar trees that suck mercury from the soil and convert it into a less toxic form that simply blows away. But there are worries about where this atmospheric mercury will end up. Ionic mercury is one of the most dangerous poisons lurking in the toxic waste sites—and one of the most difficult to remove. A cheap way of eliminating it would allow large areas of abandoned land to be redeveloped. Some bacteria and plants have a natural ability to take up and detoxify metals from polluted soils, but they are small and slow-growing, and would take decades to clean a typical site. Now Clayton Rugh and his colleagues at the University of Georgia, near Atlanta, have transferred a gene from mercury-resistant bacteria to a fast-growing tree, the yellow poplar. The bacterial gene, merA, produces an enzyme that reduces toxic mercury ions to insoluble mercury metal. Elemental mercury is volatile and evaporates from the tree leaves. The advantage of trees is that they are large, grow fast and have extensive root systems that penetrate deep into contaminated soil. Preliminary studies of seedlings show that those with the bacterial enzyme can grow in soils with levels of mercury that are toxic to ordinary poplar seedlings. The gene increased the plants’ natural ability to take up mercury by a factor of 10 ( Nature Biotechnology, vol 16, p 925). Field tests are due to begin next spring. One potential problem is that the trees can take up only soluble forms of mercury. Most contaminated sites, however, contain insoluble mercury sulphide. But mercury sulphide slowly breaks down into soluble forms—which is when it becomes dangerous, Rugh points out. “Our merA engineered plants would theoretically be able to remove this mobile fraction and avoid its transformation into hazardous methyl mercury,” he says. Another problem is that although the trees will remove the extremely toxic ionised mercury from the ground, elemental mercury is itself poisonous. “I would not plant these trees in a heavily populated area, for this reason,” says team member Scott Merkle. “For anybody not close, the amount of mercury would be minuscule, a drop in the bucket compared to natural sources such as bacteria in soils.” There is growing international concern, however, about the long-range transport of mercury in the atmosphere. Mercury, which is emitted by fossil fuels as well as landfill sites, has turned up in the Inuit of the Canadian Arctic in concentrations above the WHO’s guidelines. It apparently travels thousands of kilometres in the air before condensing out in the cold of the Arctic (This Week, 26 September, p 13.) David Salt, a bioremediation expert at Northern Arizona University in Flagstaff, describes the modified trees as “a very important first step to producing a useful plant for removing mercury from the soil”. But he says the airborne mercury is an issue that regulators will have to address. “We have no jurisdiction over trees at all. But we do over landfills. So this could be kind of a grey area,
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