Science Editor

WASHINGTON - For the cleanup of contaminated earth, "current methods depend on excavation, the removal of the soil. Then you have to burn it, and then you have to get rid of it somehow," said Donald Cheney, associate professor of biology at Northeastern University in Boston. "Phytoremediation is a much greener approach."

Phytoremediation uses plants, genetically engineered or not, to clean up pollution. Basically, the plants absorb the pollutant. Ideally, they metabolize it into something more benign, but even if it is just sequestered, dealing with the pollutant in concentrated forms in plants is easier than cleaning up, literally, tons of soil. (See BioWorld Today, Dec. 31, 2001.)

Cheney was co-organizer of a symposium called "Phytoremediation: New Solutions to Pollution on Land and in the Sea" at the annual meeting of the American Association for the Advancement of Science, held here Feb. 17-21.

At that symposium, Neil Bruce, professor of biotechnology at the University of York in Great Britain, talked about using genetically modified plants to clean up one kind of organic (that is, carbon-containing) pollutants - explosives. The basic principle is to clone the enzyme PETN reductase, which is found in certain bacteria, and express the enzyme in a plant more often thought of as a polluter than a cleanup expert, namely tobacco.

PETN reductase can break down several explosives, including TNT and nitroglycerin, and use the resulting nitrogen as an energy source. Weapons are a surprisingly pervasive source of pollution. On the one hand, there is unexploded ordnance from former wars. On the other, military training exercises tend to be held in areas that are somewhat remote, but civilians have slowly encroached on those areas. Bruce cited spots near Cape Cod, Mass., as an example.

"Microbial degradation of explosives works, but lots of these compounds persist in the soil," Bruce explained. Just by virtue of their larger size, genetically modified plants are able to deal with larger volumes of pollutants. He called his work "the first example of genetic engineering of a plant to degrade an organic pollutant."

His research showed that transgenic tobacco plants can grow on levels of explosives that are toxic to tobacco used as a control, and that transgenic tobacco sequesters TNT from the growth medium. Bruce said the explosives end up sequestered in the plant cell wall, but research to prove or disprove that hypothesis is ongoing.

Would You Like Some Arsenic With That?

Inorganic pollutants also are on the menu for some plants. At the symposium, Lena Ma, professor of biogeochemistry of trace metals at the University of Florida at Gainesville, spoke of using the Chinese brake fern to reduce arsenic pollution.

Chinese brake fern makes a number of unhealthy lifestyle choices: It likes sun and arsenic. Arsenic contamination can cause cancer, mutations and birth defects. It is detrimental to the immune system and has been associated with diabetes. Arsenic also is very stable in both water and soil. In 2001, in recognition of arsenic's toxicity and stability, the United States Environmental Protection Agency reduced the allowable level of arsenic in drinking water from 50 parts per billion (ppb) to 10 ppb, effective in January 2006 - a number of local water systems are still scrambling to meet the new standard.

Ma's research showed that Chinese brake fern can take up arsenic from both soil and water - in some cases, up to a whopping 2.1 percent of its body weight. Many details remain unclear, but it seems Chinese brake fern preferentially send the arsenic they take up into the leaves, while in related ferns, the arsenic remains in the roots and interferes with phosphorus uptake, thus poisoning the plant.

Chinese brake fern's preference for, or at least indifference to, sun and arsenic make it a good candidate for cleaning up golf courses, which use herbicides that include arsenic. The fern also is able to extract arsenic from drinking water, and it is being tested for commercial applications in that area.

The University of Florida has patented the use of fern plants to clean up arsenic, and the patents are licensed to Edenspace Systems Corp., a Dulles, Va., enterprise that describes itself as a "systems technology" company focused on environmental phytotechnology. In hydroponic model systems, Edenspace has shown that the fern is able to routinely reduce water contamination to less than 10 ppb, from as much as 100 ppb; in some cases, the contamination was as low as 2 ppb after treatment. Given that the EPA also has proposed a "health-based, non-enforceable goal" of zero micrograms per liter for arsenic contamination, meaning that it does not believe there is a threshold level below which arsenic exposure is safe, any reduction below the new standard is likely to be beneficial.

Any technology has its drawbacks, and in the case of the Chinese brake fern, the problem is that it's easy to get the arsenic into the leaves, but it's hard to get it back out. "Ideally, you want to recover the arsenic," Ma told the audience. Her group is working on recovery technology, "but that technology is not mature yet. So right now, you have to send it to a landfill."