A team of researchers has developed the first transgenic system for removing arsenic from the soil by using genetically modified plants, reports Cat Lazaroff of the Environment News Service. This new system could help remove the toxic metal from naturally and artificially polluted soil and water, reducing their threat to the environment and to human and animal health around the world.
The scientists inserted two genes from the common bacterium Escherichia coli (E. coli) that allow the test plant, a member of the mustard family called thale cress, to tolerate arsenic, which is normally lethal to plants. The plant removes arsenic from the soil, storing it in its leaves in a form that is less available to the environment, and easier to remove and eliminate.
"Our data demonstrate the first significant increase in arsenic tolerance and what we call 'hyperaccumulation' by genetically engineered plants," said Dr. Richard Meagher of the University of Georgia, who led the research effort. "This new system is a major step in developing methods of cleaning up the environment using plants."
Phytoremediation the cleaning of polluted soils through the use of plants has the potential to be of use on millions of acres of arsenic polluted lands. After plants absorb toxic materials, they store them above ground, away from soils and groundwater, and where they can be harvested and destroyed in a safe manner.
Still, the research team faced some problems. Arsenic is toxic to most plants, so the idea of using a plant to withdraw arsenic from the soil seemed counterintuitive. But the team knew from other experiments that certain genes can make plants tolerate substances that would normally sicken or kill them.
Barry Rosen of Wayne State University in Detroit, Michigan, a coauthor of a report that appears Monday in the journal "Nature Biotechnology," was the first to characterize the genes for arsenic resistance in bacterial and fungal systems, making phytoremediation of arsenic possible.
That knowledge was combined with the special expertise of the team's other members, including postdoctoral associates Om Parkash Dhankher and Yujing Li of the University of Georgia (UGA); former UGA students Julie Senecoff and Nupur Sashti; Jin Shi of Wayne State University; and David Salt of Purdue University.
"Our working hypothesis was that controlling the electrochemical state of arsenic in the aboveground tissues and increasing organic sulfur 'sinks' throughout the plant would result in both resistance and increased accumulation of arsenic," said UGA's Meagher.
Most arsenic in surface soil and water exists in its oxidized form, arsenate. Plants actively take up arsenate mistaking it for the nutrient phosphate and transfer it to their leaves.
The team was able to insert two unrelated genes from E. coli called arsC and ECS into the model plant, thale cress or Arabidopsis thaliana. The team engineered the arsC gene to be activated by exposure to light, a technique that has been around for at least two decades.
The arsC gene reduces arsenate to a more toxic compound called arsenite, but only in the plant's leaves. The second gene, ECS, creates compounds in the plant that bind tightly to arsenate, making it less available to poison either the plant or to leach back into soil or water.
In essence, the altered plants remove arsenic from the soil, concentrate it, and then send it to their leaves. Instead of dying from exposure, the new plants thrive on the arsenic exposure, and when the plants are harvested, much of the arsenic pollution, once in the soil, can be removed from the site.
When grown on arsenic, the transgenic plants accumulated 17 times more weight in fresh shoots, and two to three times more arsenic per gram of tissue, than in common or wild type plants. Laboratory tests showed that 96 percent to 100 percent of the arsenic in the plants' leaves was reduced to arsenite and bound by sulfur.
"One of the most important aspects of the research is that this new system should be applicable to a wide variety of plant species," said Meagher. "My colleague Scott Merkle, in UGA's Warnell School of Forest Resources, is already working on putting the genes into cottonwood trees, which have a large root system and could be useful in the phytoremediation of arsenic."
Other researchers have already found that a fern native to the southern U. S. can accumulate arsenic at very high levels, but the genetic basis for this activity is unknown, and the narrow growing conditions for most fern species make these plants less likely candidates for phytoremediation.
Plants genetically engineered to remove arsenic could be used now, the study's authors say, but they expect dramatic improvements in the amount of arsenic they can extract as this current strategy is expanded in future experiments.
Using plants to remove arsenic from contaminated soil could be useful to almost every nation. Inorganic arsenic compounds are classified as Group A human carcinogens, and last year, a U.S. National Academy of Sciences panel found that the risks of cancer from high levels of arsenic in drinking water was even greater than previously thought. Exposure to arsenic also can cause skin lesions, lung, kidney and liver cancers, and damage the central nervous system.
While soil in some areas is contaminated by natural arsenic deposits, many other sites are contaminated by spills and drainage from chemical and manufacturing plants.
This research paper notes that hundreds of polluted sites in the United States are listed on the National Priority List, or Superfund list, because they contain high levels of arsenic. Although these sites are recommended for cleaning, most have not been cleaned yet because digging up the soils and removing them to storage sites is both expensive and environmentally destructive.
But the most serious human health problems from arsenic involve drinking water. In the Indian state of West Bengal and in Bangladesh, where naturally occurring arsenic contaminates water at concentrations far above the recommended safe levels set by the World Health Organization (WHO), researchers estimate that more than 112 million people are afflicted with various levels of arsenic poisoning.
WHO estimates that 200,000-300,000 people in India have arsenic induced skin lesions and cancer, and an estimated 200,000 to 270,000 cancer deaths in Bangladesh will be due to high levels of arsenic in drinking water.
Study author Om Dhankher, a native of India, said health officials in that country consider arsenic pollution, particularly in West Bengal, to be a catastrophe.
"In all, this is several fold worse than Chernobyl and Bhopal, and it is getting little attention," said Dhankher. "There has been much more attention to the problem in Bangladesh, but in India, the situation is extremely serious."
The problems of arsenic contamination have received relatively little international publicity. While WHO and the European Union have adopted a drinking water standard of just 10 parts per billion (ppb) of arsenic, standards in other nations are mixed.
The United States is scheduled to replace its 60 year old standard of 50 ppb with a new, 10 ppb standard, effective January 23, 2006.