Thursday, April 4, 2013
Perhaps the most interesting discovery from a new study on triclosan has little to do with the chemical or its impacts. The combined field and lab experiment was designed to shed some light on what happens when bacteria living in river sediment are exposed to triclosan, an antibacterial commonly used in soaps, cleaners, and even disinfected plastics. And it did. But it also revealed that wastewater effluent causes very different ecosystems to look alike overtime.
The process is known as biotic homogenization. The forces behind it are complex, but the result is much simpler: ecosystems that used to be different–housed different species, had different chemical concentrations, or shared different relationships with neighboring environments–become similar. Researchers at Loyola University of Chicago saw this process in action while testing the chemical characteristics of two Chicago-area rivers for their study on triclosan. Upstream of wastewater treatment plants, researchers found that the two rivers had distinct concentrations of chemicals like phosphorus and nitrogen and housed different communities of bacteria. But downstream of the treatment plants, the bacterial communities looked the same.
"We were surprised to see how effectively the treatment plant effluents homogenized the bacterial communities from these two very different rivers, but it make sense given the fact that the effluents affected the chemical composition of the two rivers in the same way," said John Kelly, Associate Professor at Loyola University Chicago. Dr. Kelly presented the findings of this study to the Illinois Sustainable Technology Center in March. "We are hoping to repeat these analyses at other sites throughout the U.S. to determine how widespread this phenomenon might be."
The long-term impacts of biotic homogenization are not clear. Although past studies on birds, plants, and fish indicate that less diversity across ecosystems may make it harder for species to recover from disturbances like fires, pollution, or spikes in insect populations, the relationship between bacterial community diversity and ecosystem stability has not been studied as extensively. For a review of some of these studies, visit fish.washington.edu/research/oldenlab/pdf/2008/ELS_2008.pdf.
But biotic homogenization was not the only unexpected result. The study also found that triclosan–an antibacterial–does not reduce the overall amount of bacteria living in river sediment, as researchers expected. The abundance of triclosan-resistant bacteria increased with triclosan exposure, and the mixture of species changed, but the total number of bacteria stayed roughly the same. One reason may be that bacteria adapt so quickly to triclosan that the species that survived had already had a chance to recover by the time the researchers collected their samples. Kelly told his audience at the Illinois Sustainable Technology Center that they hope to conduct more experiments to determine if there is a dip in bacteria population earlier on.
"The ability of the sediment bacterial communities to adapt to this potent antimicrobial compound was quite remarkable," Kelly said. "But we still have questions to answer regarding the potential implications of the increases in resistance and the changes in community composition that we observed."
To view Kelly's presentation, visit www.istc.illinois.edu/about/sustainability_seminars.cfm.
For further reading:
Drury, B., E. Rosi-Marshall, and J.J. Kelly. 2013. Wastewater treatment effluent reduces the abundance and diversity of benthic bacterial communities in urban and suburban rivers. Applied and Environmental Microbiology.