Wednesday, March 26, 2014
Much of the research on PPCP pollution focuses on individual chemicals. Knowing how triclosan, carbamazepine, or caffeine effect aquatic wildlife is undeniably important. But it doesn't necessarily tell us much about the actual risks to algae, fish, and other species. Our lakes, rivers, and oceans are home to much more than triclosan—much more even than PPCPs. To really understand the danger these chemicals pose to aquatic ecosystems, we need to know how PPCPs interact with other pollutants commonly found in our waterways.
That is exactly what a group of Norwegian researchers set out to do in a recent study published in Aquatic Toxicology. In a series of lab tests, the group investigated how 11 chemicals from different categories—PPCPs, biocides, PAHs, and organic compounds—effect the growth of S. pseudocostatum, a common marine algae. In addition to individual chemicals, researchers tested mixtures from the same category and across categories at different concentrations, allowing them to track changes in toxicity as the common contaminants interacted with each other.
Their results revealed that each of the tested mixtures had an additive effect on algal growth—the toxicity of the combined chemicals was simply the individual toxicities added together. Imagine that a specific concentration of an antidepressant reduced growth by 25 percent and the same concentration of a biocide cut growth by 50 percent. An additive effect means that those chemicals combined would reduce algal growth by 75 percent.
This chemical interaction may sound pretty straightforward, but the causes aren't quite as clear. The researchers did find an additive effect when they tested a mixture of two biocides, four PPCPs, and two organic contaminants. But the driving force behind the mixture's toxicity—what exactly was 'added up'—changed as individual concentrations got higher. At lower concentrations, the mixture slowed growth rates by attacking a diverse range of cellular functions in the algae, like interrupting DNA replication or inhibiting efflux pumps. At higher concentrations, though, the modes of action converged, meaning the mixture attacked fewer functions with more intensity.
This is the first study to report such a shift, and it is unclear exactly what caused it. One possibility, though, is that these chemicals attack a broader suite of functions at higher concentrations, increasing the likelihood that multiple chemicals in a mixture have the same mode of action.
Petersen, K., Heiass, H.H., and Tollefsen, K.E. 2014. Combined effects of pharmaceuticals, personal care products, biocides and organic contaminants on the growth of Skeletonema pseudocostatum. Aquatic Toxicology 150: 45-54.