Engineered organisms could make toxic clean-ups safer and speedier.
7 October 2002
Researchers have created a plant that safely takes up the toxic element arsenic, and hope to use it to restore soils that are too contaminated for human use. Genetic techniques could also produce new biological tools to absorb other chemical pollutants, they say.
Although using poison-absorbing plants to mop up polluted soils - a technique called phytoremediation - is not new, naturally occurring plants that thrive in toxic sites are few and far between.
By inserting two bacterial genes into thale cress, Arabidopsis thaliana, researchers at the University of Georgia in Athens have created a plant that not only tolerates arsenic-contaminated soils, but sucks up the toxin and stores it in its leaves1. The genes, from the bacteria Escherichia coli, make enzymes that digest arsenic compounds so they can be absorbed.
"Some plants grow 100 million miles of roots in an acre in one year, which could draw and concentrate arsenic above ground," says Richard Meagher, who headed the team. He says that eventually plants could be developed that might clean a contaminated site in just two or three years.
Arsenic occurs naturally but gets concentrated to dangerous levels by mining, industrial wastes and groundwater wells. Even small amounts can cause cancer and nervous-system damage. Contamination from groundwater is particularly severe in Bangladesh and the Indian state of West Bengal, where millions of people suffer from arsenic poisoning.
Metal pollutants such as arsenic, cadmium, mercury, copper and zinc cannot be broken down to non-toxic forms, so removing them from soil traditionally means excavating a site and burying the toxic soil somewhere else. But this is messy, degrades the environment of the site, and costs millions of dollars per acre.
Once arsenic has been concentrated in the leaves or stems of plants however, they can be harvested cheaply and incinerated safely.
Meagher hopes to use his technique to transform large plants such as cottonwood, rice, willow, sweet gums, wetland grasses and water lilies into soil-cleaning machines. But he adds that toxic vegetation must be monitored closely and kept off-limits from foraging animals.
Some researchers plan to use genetic screening techniques to find bacteria, which on their own can be recruited to do our dirty work - perhaps by converting pollutants dissolved in water to insoluble forms that can be filtered out.
Shewanella oneidensis, a freshwater sediment bacteria, "has an enormous laundry list" of things it can clean, removing two very toxic metals, chromium and uranium, says John Heidelberg, a microbial geneticist at The Institute for Genomic Research in Rockville, Maryland.
But some of the conversions could result in pollutants being released, so Heidelberg and his colleagues sequenced the roughly 5,000 Shewanella genes2. Heidelberg says that the researchers are now tracking down and removing the toxin-releasing ones.
Because of the costs of conventional clean-ups, genetically engineering plants and bacteria "may be the only way of going about things", says University of Pennsylvania biochemist Phil Rea. Last week he organized the first conference dedicated to heavy metal removal by plants in Philadelphia.
Dhankher, O.P. et al. Engineering tolerance and hyperaccumulation of arsenic in plants by combining arsenate reductase and (-glutamylcysteine synthetase expression. Nature Biotechnology,
Heidelberg, J. F. et al. Genome sequence of the dissimilatory metal ion-reducing bacterium Shewanella oneidensis. Nature Biotechnology
© Nature News Service / Macmillan Magazines Ltd 2002
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