Environmental▼News Pesticide mixture enhances frog abnormalities Developmental endocrinologist Tyrone Hayes of the University of California, Berkeley, has set amphibian studies hopping with his findings that tadpoles exposed to low doses of atrazine can grow up to be hermaphrodites (see story on p. 46A this issue and Environ. Sci. Technol. 2002, 36, 444A). Now, Hayes and his team have moved their research even closer to the real world by exposing tadpoles to mixtures of agricultural chemicals that are used together on cornfields. The mixtures have adverse effects on the frogs that far exceed the effect of any one chemical, Hayes reported at the American Society for Cell Biology conference in San Francisco on December 14. Hayes’ team evaluated the effects of mixtures of 10 herbicides, pesticides, and fungicides that are used in the Nebraska Corn Belt. The mixtures included atrazine, metolachlor, and alachlor. A total of 3000 tadpoles were exposed to either each chemical individually, or to two environmentally relevant combinations—one representing a typical springtime mixture found in runoff and the other summer surface water. These tadpoles were raised in individual plastic containers to measure growth and development. In addition, half of the tadpoles were raised to frogs to study postmetamorphic changes. They found that tadpoles exposed to 10 parts per billion
(ppb) of atrazine metamorphosed into adults 10 days later than tadpoles not exposed to pesticides. Adding 10 ppb of metolachlor doubled that delay, while the full pesticide cocktail more than tripled it. Each dose also led to an increase in developmental abnormalities, such as contorted and distorted legs. However, the researchers warn that these data have not been fully analyzed. Although each chemical on its own may have a small or barely significant effect, the combination enhances the adverse consequences of exposure, says Hayes. Hayes currently cannot say whether these combined effects are additive or even synergistic, although it is clear that these chemicals have different mechanisms of action. For example, he says, one chemical may slightly delay metamorphosis, atrazine makes some hermaphrodites, and another depresses the immune system. In the mixture, it appears that all of these effects occur at once. “That’s reality,” says Hayes. “No environment has just one chemical.” This reality, he adds, “is a nightmare for regulatory agencies that regulate single chemicals, not mixtures.” This study represents the, “next generation” of environmental investigations, says Lou Guillette. “Creatures are practically never exposed to just one chemical. We all should be doing environmentally relevant exposures,” says the University of Florida, Gainesville endocrinologist. —REBECCA RENNER
Iodine counteracts perchlorate effects in frogs New data from several groups show that iodine can mitigate the detrimental effects of perchlorate on frogs in laboratory settings. The results raise the possibility that iodine could possibly protect amphibians in perchlorate-contaminated sites, according to the researchers who presented their results at the November meeting of the Society of Environmental Toxicology and Chemistry in Salt Lake City, Utah. High levels of perchlorate ions appear to pose a threat to normal development and growth in amphibians because they inhibit metamorphosis (Environ. Sci Technol. 2002, 36, 186A). Perchlorate blocks uptake of iodine into the thyroid and reduces production of thyroid hormones, which play crucial roles in the timing of amphibian metamorphosis. “Having an obvious antagonist for an environmental contaminant is pretty rare,” says Greg Harvey, a U.S. Air Force toxicologist based at Wright Patterson Air Force Base in Dayton, Ohio. Because perchlorate
sites are likely to be difficult and expensive to remediate, it may be possible to use iodine. “But we have to be careful. We want to find out how much is needed and then find out how to apply it properly,” says Harvey, who notes that the effects of adding iodine to other ecosystem denizens also need to be explored. Together with Harvey, wildlife biologist Donald Sparling at the U.S. Geological Survey’s Patuxent Wildlife Research Center in Laurel, Md., collected wild grey treefrog tadpoles (Hyla versicolor) and exposed them to perchlorate ranging from concentrations of 2.2 to 50 parts per million (ppm). Perchlorate inhibited metamorphosis in about 50% of these frogs at concentrations as low as 3.29 ppm. But the addition of just 10 parts per billion of iodine mitigated the effect of 50 ppm perchlorate. Although he expected to see an effect, Sparling says he is surprised that such a low concentration of iodine works. This may mean that amphibian thyroid glands take up
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iodine in preference to perchlorate, he says. Such a preference differs from human thyroid glands in which perchlorate is preferred. Toxicologist James Dumont at Oklahoma State University in Stillwater also presented results of perchlorate and iodine lab experiments with Xenopus and three different species of wild frogs: Southern leopard (Rana utricularia), bullfrog (Rana catesbelanna), and grey treefrogs (Hyla versicolor). He reported that higher concentrations of iodine were needed to mitigate the effect of higher perchlorate concentrations. For example, perchlorate concentrations of 200–300 milligrams per liter (mg/L) stopped the metamorphosis of Southern leopard frogs. However, the addition of 1 mg/L iodine as potassium iodide allowed metamorphosis to continue. In addition, frogs’ sensitivity to both perchlorate and iodine appears to differ with species. “These differences in sensitivity to both perchlorate and iodine are significant. So, generalizations about the protective effect of iodine seem to be difficult to make,” warns Dumont. —REBECCA RENNER
