Pharm pollution: excreted antibiotics can poison plants

Science News, June 29, 2002 by Janet Raloff

Patrick K. Jjemba was curious about the interplay of protozoa and the bacteria they eat in soil. As part of his research, he began altering the organisms' environment. When he applied large amounts of protozoan-killing antibiotics to dirt around the roots of soybeans, Jjemba was amazed at what happened. The drugs--widely used in human and veterinary medicine--did far more than subtly alter the balance of microbial predators and prey. One drug stunted soybeans, and another killed the plants.

The University of Cincinnati microbiologist realized he was sailing into largely uncharted waters. Though a growing body of research has documented the presence of antibiotics in the environment, most concerns have focused on what harm these antimicrobials might pose to people, fish, or aquatic birds (SN: 3/23/02, p. 182). These drugs, excreted by people and livestock treated with them, enter open waters primarily from sewage effluent and farm runoff.

The widespread environmental presence of such antibiotics raises the prospect that an increasing share of microbes will become resistant to them (SN: 6/5/99, p. 356). However, there's been little discussion of whether these drugs could harm plants.

Farmers apply large quantities of sewage sludge and manure to their fields, yet only a few studies have probed possible crop impacts of the accompanying antibiotics. Preliminary findings from those investigations, most conducted in Europe, paint a disquieting picture: A wide range of plants--from weeds to field crops--are susceptible to antibiotic poisoning.

Furthermore, plants that aren't severely injured by the drugs might convey antibiotics from the soil into the food supply.

POOPED OUT People and livestock collectively shed torrents of urine and mountains of feces into the environment daily. Sewage from some 1 million U.S. households enters the environment essentially untreated (SN: 4/1/00, p. 212). Animal wastes--sometimes with little processing--are applied to farm fields as fertilizer.

Manure and human waste products both can contain residues of drugs. Because the engineers of North America's sewage-treatment plants never designed these facilities to remove excreted drugs, the effluent from such facilities--and even some drinking water--bears traces of antibiotics and other pharmaceuticals (SN: 11/3/01, p. 285). Like manure, leftover solids from sewage treatment plants can carry drug residues, scientists reported in April at an American Water Works Association meeting in Cincinnati.

According to a 1999 report by the Environmental Protection Agency, farmers spread some 7 million tons of sewage sludge--known in the trade as biosolids--onto fields each year. A recent National Academy of Sciences report indicates that U.S. growers also annually recycle an estimated 3 million tons of manure this way. Not only are biosolids and manure low-cost soil amendments, but their use provides important means of recycling wastes.

The surface water used for irrigation can also host a dozen or more drugs. So, there are plenty of routes by which pharmaceuticals can reach crops. Nobody has yet quantified this contamination.

A quarter-century ago, soil microbiologist Arthur R. Batchelder was disturbed at the thought that people were unwittingly seeding crop soils with antibiotics. Then working for the Agricultural Research Service at Colorado State University in Fort Collins, he focused on farmers who were fertilizing their crops with manure from cattle feedlots. Then, as today, U.S. feedlot operators fed animals antibiotics to promote their growth. Most European countries have banned drug use that's strictly for promoting growth in livestock.

In the early 1980s, Batchelder laced the soil of young greenhouse plants with up to 180 parts per million (ppm) of chlortetracycline or oxytetracycline, which are common livestock-growth promoters. Though radishes, wheat, and corn were unaffected by these antibiotics, pinto bean plants showed ill effects.

Compared with bean plants grown drugfree, those planted in sandy loam soil containing an antibiotic were shorter, weighed less, produced smaller yields of beans, fixed less soil nitrogen, and picked up fewer nutrients from their environment. However, when grown in clay soil, the beans exhibited no effects from the antibiotics. Batchelder speculates that the drugs bound to the clay and remained unavailable to the roots.

In a published report, he concluded that "manure that contains either [antibiotic] should not be applied to sandy loam soil just before pinto beans, and possibly other seed legumes, are planted."

Batchelder had hoped to follow up with analyses of the drugs' uptake by plants. "I worried that you might be feeding antibiotics to people," the retired scientist recalls, "but I was pulled off the work."

If Jjemba had been aware of Batchelder's findings, he might have been less startled by the outcome of his studies with soy, another legume. Using chloroquine, quinacrine, or metronidazole, he heavily polluted soil in his lab with drug concentrations between 0.5 and 16 parts per thousand. In the February Chemosphere, he reported that each drug killed the plants, though at different doses.