Pesticides Change `Hands' and Risks.

Evolving climates and farming practices may undermine scientists' efforts to predict the toxicity and persistence of many long-lived pollutants, a new study concludes.

Some one-quarter of all commercial pesticides are chiral, that is, their molecules come in mirror-image twins, or enantiomers. The variations arise during manufacturing and can be described as left- or right-handed.

The activity of chiral structures can vary dramatically--as can their palatability to pollution-degrading microbes. When chemists and regulators evaluate pollution risks, they try to account for the preferential removal of one twin. However, which twin a microbe prefers can unexpectedly switch as environmental conditions change, according to a study in the Oct. 28 NATURE.

Such a change can have important implications for public health and the environment, observes study leader David L. Lewis, an Environmental Protection Agency microbiologist at the University of Georgia in Athens.

In some cases, he notes, one twin may kill pests but leave humans and other nontargeted species unharmed, while its chiral sibling does the opposite. If microbes switch their preference, "the residues they leave behind, and which can end up on food, may become more toxic than tests had predicted," Lewis says. However, if only one of the twins is toxic and it suddenly becomes the one that appeals to microbes, pollutant residues might turn harmless.

In the new study, Lewis and his colleagues sampled soils from sites of long-running studies. In Norway and New England, scientists have artificially heated some fields for up to 7 years to model global warming. In Brazil, other researchers are comparing woodlands with adjacent deforested pastures.

Back in Georgia, Lewis' team applied one of three pesticides to each soil sample. To half the soils that had been fertilized in situ with inorganic amendments, the scientists also added an organic fertilizer.

Soils from the warmed sites harbored microbes that showed a preference for one pesticide twin while those from adjacent unheated sites preferred the other, the researchers report. In some cases, the organic fertilizer also triggered a breakdown of the twin that microbes had previously ignored. This was especially evident, Lewis notes, in soils treated with longer-lived pesticides: the insecticide ruelene and the weed killer dichlorprop.

The new findings point out how unreliable studies conducted with a mix of enantiomers can be, Lewis says. He argues that pesticide manufacturers should take a cue from drug makers and eliminate inactive twins from the final product--especially if they prove toxic.

Microbial communities that can switch-hit from one enantiomer to the other "make sense but are not something that I had considered," says Daniel M. Sheehan of the National Center for Toxicological Research in Jefferson, Ark. Certainly, they greatly complicate safety evaluations, he says.

This study indeed suggests that scientists can't expect data from the lab to model what will happen just anywhere in the environment, says Liisa M. Jantunen of Environment Canada in Downsview, Ontario.

Recently, some manufacturers have begun producing single-enantiomer versions of a few chiral pesticides. However, microbes can sometimes sabotage such developments, notes Hans-Rudolf Buser of the Swiss Federal Research Station in Wadenswil. His team finds that microbes can convert a single-twin form of the herbicide mecoprop into its missing enantiomer.