Mirror, mirror. Tests with the freshwater crustacean Daphia (inset) suggest that a pesticide's toxicity may change as it lingers in the environment.

A Chiral Wildcard

Many widely used pesticides may have unpredictable effects on wildlife as they degrade in the environment, a new study suggests. The chemicals that make up these pesticides are chiral, meaning they occur as two identical, but mirror-image, structures. The researchers who conducted the new study found a "dramatic difference" in toxicity and persistence between chiral forms, and they argue that regulators should therefore consider chirality when assessing the risk of pesticides.

Although the mirror-image versions, or enantiomers, of a molecule have the same chemical properties, they can interact with enzymes in different ways. About a decade ago, researchers noticed that microbes tend to preferentially break down particular enantiomers of persistent pesticides, such as DDT, and other pollutants. As a result, the enantiomer ratio for these pesticides became skewed in some food chains. Researchers also showed that enantiomers could vary in their toxicity, although most of this work was done in banned chemicals.

Chemist Jay Gan and his colleagues at the University of California, Riverside, decided to look at pesticides that are widely used today. They examined five common insecticides, including organophosphates such as profenofos, and synthetic pyrethroids, such as permethrin. For all these compounds, one of the enantiomers was at least 10 times more toxic than the other to Daphia, a small crustacean often used to assess toxicity.

The researchers also found that the enantiomers of two organophosphates differ in how they linger in sediment. For example, one enantiomer of permethrin became almost twice as prevalent as the other over the course of a year, they report today online in the Proceedings of the National Academies of Sciences. This means that the environmental impact of these pesticides may depend on the behavior of a particular enantiomer, the team concludes. "The difference in terms of risk assessment and regulation could be pretty drastic," Gan says.

Derek Muir of the National Water Research Institute in Burlington, Canada, agrees that variation in enantiomers can play a big role in determining ecotoxicity. Selective degradation "is important for assessing pesticide risk to non-target organisms," he says. The bottom line is that regulators may not get the true picture if they don't consider changes in the ratio of enantiomers and their individual toxicity, says environmental chemist Charles Wong of the University of Alberta in Edmonton, Canada.

Related sites
The basics of isomers, enantiomers, and their ilk
A recent meeting session on chiral pollutants
Information on pesticide regulation