Using Peter Rabbit to Clean Peter's Pond

Purdue University researchers have implanted poplar trees with genetic material from rabbits. The trees are destined for a Herculean task: cleaning up a contaminated site that housed an oil storage facility. The site, called Peter's Pond, was tainted by contaminated oil stored there nearly 40 years ago. The process, called phytoremediation, allows transgenic trees to slurp up underground contaminants.
By Sarah Smith
By the U.S. EPA's own estimate, there are "tens of thousands" of Superfund sites scattered throughout the United States. Complex, long-term, formidable processes to clean up those abandoned hazardous waste sites are taking place throughout the nation, parsed among 10 EPA districts. Superfund was born in 1980 in response to the discovery of numerous environmental catastrophes like the Love Canal. Those toxic waste dumps gave rise to the Comprehensive Environmental Response, Compensation and Liability Act.

Superfund areas carry the dubious designation as the nation's worst toxic waste sites. Currently more than 1,300 sites are on the National Priorities List and EPA estimates that they affect 11 million people. But numerous other sources of land contamination, such as state Superfund sites, brownfields, nonhazardous waste disposal facilities and other land contamination sources are not currently tracked in national databases, leading to EPA's ballpark estimate of tens of thousands of areas that need cleanup attention. That's where Purdue University researchers come in, participating in the EPA's Return to Use program.

One of the most prevalent pollutants, trichloroethylene, or TCE, has been found to be a susceptible victim of transgenic poplars-trees that possess a gene transferred from another species. The Purdue researchers, collaborating with scientists from the University of Washington, have found that the poplars are capable of absorbing TCE and other pollutants, then processing them into harmless byproducts. "The poplar has an innate ability, in plants that have not been genetically modified, to absorb and metabolize TCE to a certain extent," says Richard Meilan, associate professor of molecular tree physiology at Purdue. "But it's not particularly efficient."

Enter the rabbit to speed things along. Mammalian livers are "highly evolved detoxifying organs," Meilan says, and contain enzymes that are capable of metabolizing a variety of potential hazardous compounds, including TCE. When the gene encoding this enzyme is introduced into a poplar, the tree recognizes the rabbit DNA and it's capable of breaking down TCE and other harmful chemicals, including chloroform, benzene, vinyl chloride and carbon tetrachloride. The EPA thinks TCE is the most common groundwater pollutant at Superfund sites, and it's a suspected carcinogen. At Peter's Pond, TCE lies within 10 feet of the surface-an easy target for transgenic tree roots.

Meilan co-authored a study, published in October 2007 in "Proceedings of the National Academy of Sciences," which found that the genetically altered trees were able to absorb and metabolize a variety of toxic compounds much more rapidly than unaltered poplars. "Livers serve a protective role to detoxify these things and keep them from having negative effects" in mammals, Meilan says.

TCE is considered a halogenated product because it contains nonmetallic elements from what Meilan refers to as the "dreaded" Periodic Table, which was introduced to most people in junior-high science classes. Halogens include fluorine, chlorine, bromine and iodine. They're highly reactive and in their natural state generally have fleeting existences as gases. "Provided they're not ingested in huge quantities, some halogens such as the chloride ion in table salt aren't harmful to us," Meilan says. "But when you attach the chloride ion to other molecules, depending on their configuration, they can be incredibly harmful molecules. There are all kinds of organic compounds and when they have these halogens attached, they can be nasty stuff." That's where the hare-poplar combination proves useful. The enzyme produced by the expression of this rabbit gene in poplars, cleaves off the halogens and liberates the organic material, producing an innocuous product with the toxic properties removed. Essentially, the trees metabolize the harmful contaminants and emerge unfazed, and unpolluted.

Peter's Pond is actually a Chrysler site. "It was never grossly contaminated and Chrysler removed the majority of the contamination in work done in the 1980s and then again in the early part of this decade," says Max Gates of the automaker's Safety and Regulatory Communications Department. "At Chrysler, we got involved with EPA and the Return to Use program at a site in a rural region near our headquarters north of Detroit. The work at Peter's Pond is an extension of that involvement, which we think has great potential for recapturing the benefits from the investment made in environmental cleanups and creating potential sites for growth of fuel crops for biodiesel, ethanol and perhaps other alternative energy sources."

Chrysler has committed $313,000 to funding the project, Meilan says.

Mutated Plants Spawn Controversy
Meilan and other researchers must undergo a lengthy permitting process with the U.S. Department of Agriculture before they receive permission to plant genetically altered trees anywhere but in a laboratory or a contained greenhouse. Before any deployment takes place and after a permit has been issued, genetically altered plants are strictly regulated by a U.S. government agency called the Animal Plant Health Inspection Service (APHIS). The researchers are taking comprehensive steps to assure that no transgenes escape into the environment. Hence the three-year life span of the project.

Meilan says although many genetically modified crops such as corn are being grown for commercial purposes, "because they've been so heavily domesticated they have few or no wild relatives with which to mate, thus minimizing environmental risks," he says.

Trees are different because they have long juvenile periods. They can grow for many years without producing seeds. Meilan says some trees don't become sexually mature until they are 25 years old. "We have not domesticated trees to the extent we have agronomic crops," he says. "As a result all these trees we've genetically altered have wild relatives with which they can mate. Pollen from a 15-year-old tree can disperse over miles and there may be wild relatives downwind that it can fertilize." That's why genetically engineered trees are strictly regulated. "Currently no transgenic trees can be grown for commercial purposes, but they can be grown for research purposes," he says.

The trees will be harvested after three years because poplars typically flower after about five years. If they are harvested before they flower, there's no chance of any genetic material escaping. But the three-year growth period will allow the poplar roots to grow to the necessary depth to access the contaminants in the suspended water table at Peter's Pond.

Contaminants similar to those in Peter's Pond are prevalent throughout the United States, Meilan says, particularly at military installations. "There were gobs of this stuff," he says, referring to the TCEs. "Many sites need to be cleaned. Ultimately there's some potential down the road."

Afterlife of a Genetically Altered Tree-Devitalization
"There are a lot of contaminants out there-mercury, lead, cadmium-and plants such as ferns can sequester these contaminants," Meilan says. "They bio-accumulate them in their tissues. Using these plants to accumulate heavy metals will take up this stuff but you now have a contaminated plant. What do you do with it? Do you burn it and release it into the atmosphere? Do you dispose of it in a landfill?"

Unlike these other plants used for phytoremediation, poplars metabolize the harmful substances so there are no worries about disposal, he says. But any biomass applications for the harvested trees cannot be used in for-profit ventures. Regulators won't allow poplars containing the hare gene to be grown commercially until containment strategies have been perfected to APHIS's satisfaction so genetically engineered trees don't release any "introduced genes" into the wild.

Destruction of the transgenic trees, at the end of the field test, involves what APHIS calls devitalization. Suitable methods include autoclaving, burning, chipping and an herbicide treatment. But the key is commercial applications. They cannot be harvested for commercial purposes, unless a commercial entity has a permit to grow transgenic trees for experimental purposes. And they still cannot be sold on the open market. "In the future poplars have the potential to be used as a bioenergy crop, as a cellulosic feedstock," Meilan says. "Because these trees are able to detoxify the contaminants, they may make for an ideal feedstock for energy production." They can also be grown for a variety of other purposes, including fiber for making paper and carbon credits. "Thus, in the future, maybe we can double-dip," he says.

Meilan just hopes that regulators will soon allow commercial applications for transgenic poplar biomass-especially for trees carrying a lucky rabbit's foot-or liver.

Sarah Smith is a Biomass Magazine staff writer. Reach her at or (701) 663-5002.

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