Lake Slime Loaded With Pollutants

Pesticides, plasticizers, pharmaceuticals and other hormone-disrupting chemicals soak into the slime that coats rocks at the bottom of lakes and streams, found a new study. Fish and aquatic insects then feed on those contaminated slimes, also known as biofilms.

By documenting biofilms as covert hiding places for toxic chemicals, the study offers the potential for aquatic slimes to help remove pollution from wastewater effluents. For now, the findings also raise new concerns about how the chemicals in our drugs and personal care products work their way through food chains.

"The compounds we produce don't just go away," said Jeff Writer, an environmental engineer with the United States Geological Survey in Boulder, Colo. "We're a highly medicated society, we use a lot of compounds and we see them no matter where we look. It's important to understand where they go and where they end up."

Concern has been building for years about the environmental effects of endocrine disrupters, a class of chemicals that can interrupt the hormonal systems of both people and animals that are exposed to them. These chemicals, which include hormones from birth control pills and ingredients of many plastics, end up in the discharge that flows out of wastewater treatment plants all over the developed world.

In streams on the receiving end of wastewater treatment plants, between 25 and 75 percent of the water consists of discharged effluents. And previous research suggests that chemical-filled effluents can cause major problems for wildlife. Studies have found high rates of feminization in fish that live downstream from treatment plants. Ovarian tissue has even shown up in the testes of some of male fish.

Earlier studies have looked for evidence of endocrine disruptors in sediments and in the water column, Writer said. But he was more interested in the biofilms, which are ubiquitous in streams everywhere and are already known to soak up heavy metals, nutrients and other materials.

Biofilms are a combination of algae and bacteria, which make up about 10 percent of the green-brown slime. The rest of the material is a matrix of sugars and other materials that the organisms produce.

To see if biofilms might also be collecting endocrine-disrupting chemicals, Writer and colleagues scraped a lot of slime off of rocks in Boulder Creek, which starts in a remote area along the continental divide and flows right through the City of Boulder's municipal waste- water-treatment plant.

The researchers also allowed biofilms to grow for several weeks on stainless steel pieces of mesh that they put in the water. And they collected sediments and water samples. In the lab, they mixed the stream materials with chemicals of concern.

Their results, published in the journal Environmental Science & Technology, showed that biofilms absorbed about 10 percent more endocrine disruptors than sediments did. The slimes also held on more tightly to the chemicals.

After a week, bacteria in the sediments had degraded up to 30 percent of contaminants, compared to less than 10 percent in the biofilms.

Some wastewater plants already use certain biofilms to remove chemicals from their discharge, and the new finding suggests that stream slimes might be able to help, too.

Beyond the engineering implications, the study offers important new insights into the pathways and fates of harmful chemicals in the wild, said Bryan Brooks, an aquatic toxicologist at Baylor University in Waco, Texas.

Scientists might also want to start measuring chemical levels in biofilms, he added, before determining whether streams are safe sources for acquiring drinking water or setting up fisheries.

"If you put something into the environment," Brooks said, "you need to find out what happens to it, where it goes and how long it sticks around, because ultimately, without that understanding, we may not be fully able to characterize how wildlife would be coming into contact with it."

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