Metal maniacs
IN THE last few years, scientists have come up with a novel method to fight pollution called bioremediation, or using biological means to clean up polluted lands and water bodies. It usually involves plants that have been geneticaily-modified to absorb a certain pollutant. Though still in its infancy, bioremediation can be a valuable weapon against pollution in the near future.
In a recent development,US scientists claim that they have 'borrowed' a gene from bacteria and produced yellow poplar trees that soak up mercury from contaminated soil and convert it into a less toxic form.
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 be to allow large areas of abandoned land to be redeveloped. Some bacteria and plants have natural ability to take up and detoxify metals from polluted soils, but these are small and slow, and would take decades to clean a typical site.
Now, Clayton Rugh and his colleagues at the University of Georgia near Atlanta, USA, have transferred a gene from mercury-resistant bacteria to a fast growing tree, the yellow poplar. The bacterial gene, mem, produces an enzyme that reduces toxic mercury ions to insoluble mercury metal. Elemental mercury is volatile and evaporates from the tree leaves. But there are worries about where this atmospheric mercury will end up.
The advantage of trees is that they are large, grow fast and have extensive root systems that go deep into the 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 as much as 10 times. Field tests are due next spring, say the researchers.
One potential problem is that the trees can absorb only soluble forms of the metal. Most contaminated sites, however, contain insoluble mercury sulphide. But this mercury sulphide slowly breaks down into soluble forms -which is when it becomes lethal, Rugh points out. "Our merA-engineered plants would theoretically remove this mobile fraction and avoid its transformation into hazardous methyl mercury,' he says. Another problem is that although the trees will remove 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 World Health Organisation (WHO) guidelines. It apparently travels thousands of kilometres in the air before condensing out in the cold of the Arctic. David Salt, bioremediation expert at Northern Arizona University in Flagstaff, USA, describes the modified trees as "a very important first step to producing a usefull plant for removing mercury from the soil". However, he says the airborne mercury is an issue that regulators will have to address (New Scientist, Vol 160, No 2154).
"We have no jurisdiction over trees at all. But we do over landfills. So this could be kind of a grey area," says us Environmental Protection Agency (FPA) spokesperson David Ryan. "To be honest when we got this question we laughed. It reminded us of the time when former us president Ronald Reagan used to say that trees caused pollution." In another interesting development, researchers based in New Zealand have discovered a rather unique use for plants. They say certain plants can be used to 'mine' for precious metals. They have selected Indian mustard plants to purify gold ore from the soil.
Researchers led by Robert Brooks at Massey University in Palmerston North, New Zealand, found that plants readily absorb gold dissolved in ammonium thiocyanate, a liquid used in traditional mining to dissolve gold. In lab tests, they grew Indian mustard plants (Brassica juncea) in soil containing four parts per million of gold. When absorbed by the stems and leaves of the plants, the gold concentration increased to an average of around 10 parts per million, in one case reaching 57 parts per million. The plants only lived for about a week. "They quickly die from absorbing the reagent, because it contains cyanide," says Brooks. "But that doesn't matter because once the plant has done its job you don't need it anymore." After the dead plants were burned, the ash contained 150 parts per million of gold.
The ash can then be refined using traditional methods to yield commercial quantities of gold. "Any plant can be used in this process because the gold is in solution, so it's absorbed by the roots when they take up water," Brooks adds. "We used Indian mustard simply because it produces a large biomass very quickly."