TUCSON, ARIZONA--Combinations of chemical pollutants harm aquatic creatures in ways that can't be predicted by studying their effects independently, new research shows. The numerous pharmaceuticals in the world's waterways, it suggests, might be harming freshwater life in unknown ways.
Much of the $300 billion of pharmaceutical products purchased globally each year is flushed down toilets--either as human waste or because the drugs have expired. Because sewage treatment plants do not treat such chemicals, many waterways are laced with antibiotics and other drugs. Ecotoxicologists routinely demonstrate effects of certain chemicals on particular organisms, but they are only now beginning to test effects of combinations of chemicals. The tiny crustaceans called Daphnia are widely used to determine the effects of aquatic pollution.
So when ecology grad student Colleen Flaherty of the University of Wisconsin, Madison, found no effect on Daphnia in the lab from either Prozac or clofibric acid, a drug used to regulate blood lipids, at 100 parts per billion, “on a whim,” she mixed them. What she found shocked her. When Daphnia were exposed to both chemicals, 90% died. Flaherty speculates that the pharmaceuticals together might have created some chemical that mimics a toxin. A combination with less clofibric acid resulted in deformed carapaces and skewed sex ratios.
Then Flaherty looked at five common antibiotics, including erythromycin and triclosan, an additive in soap and toothpaste. Each of them independently affected Daphnia, either changing the sex ratio, altering reproductive ability, or stunting growth. But combinations had unusual effects. When Flaherty mixed three antibiotics that don't affect sex ratio on their own (erythromycin, triclosan, and trimethoprim), the combo upped the percentage of males. She presented her results here yesterday at the meeting of the Ecological Society of America and the Society for Ecological Restoration.
It's crucial to study pharmaceuticals and other pollutants in combination like this, says ecotoxicologist Stephen Threlkeld of the University of Mississippi: “Mixtures are where the real action is, where the real knowledge gaps are.” Because there are so many potential interactions among pollutants in a typical waterway, Flaherty says, it's important to expand such work; Daphnia are a key link in the aquatic food chain. How they fare can predict the health of fish and, ultimately, people.