By Dustin Renwick
Gilbert and her team conducted research to test a new method for detecting contaminants in water supplies: tadpoles that glow in the presence of chemicals that disrupt the thyroid, a gland with critical functions related to growth and maturation in vertebrates.
Thyroid hormones influence growth processes and fetal brain development in many animals, including humans. Frogs have become science examples of choice because their thyroid hormones jumpstart the transformation from tadpoles to adult frogs.
“The beauty of it is that it represents a cumulative index of what the animal is being exposed to,” Gilbert said. “It’s a living organism, so it has a complete biological system that integrates all that information.”
As part of an EPA Pathfinder Innovation Project, Gilbert and her team worked with researchers at the University of Cincinnati, the University of Northern Arizona, and a Paris-based company, WatchFrog, to test the aptly-named FrogBox. Scientific American magazine recently featured the research.
The box contains the tadpoles in two water chambers. As tadpoles move from one reservoir to the other, they pass a detection device that measures how brightly they glow.
This type of technology represents the potential for a major shift in how scientists can test for water-based contaminants. Instead of searching for individual chemicals, the tadpole, referred to as a “biosensor,” provides a broader ecological picture.
Gilbert said much of what we know about the damage caused by thyroid hormone disruption comes from observing biological responses in mammals, often after a thyroid gland has been removed. Environmental contaminants affect our bodies in less obvious ways.
“There’s not much literature to inform EPA about what the consequences might be if there are more subtle perturbations of the thyroid system.”
The tadpoles have a gene that produces a fluorescent protein when activity stimulates the thyroid system, but the tiny, translucent creatures can’t show researchers which compound is present or how exactly the compound changes thyroid activity.
A change in glow only indicates a problem exists. Compared with a control measurement, a tadpole glowing dimmer indicates the presence of a thyroid-suppressing compound. Conversely, a brighter tadpole means the disruptor compound is activating the amphibian’s thyroid system.
This biosensor system holds potential for two reasons: (1) too many chemicals exist to test in traditional ways and (2) humans rarely come in contact with a single chemical at a time.
“We’re exposed to complex combinations at very low levels,” Gilbert said. “There may be adverse effects we would not be able to determine in exposure to a single compound.”
An integrated biosensor that detects low concentrations of chemicals presents opportunities for cleaner watersheds by identifying sources of contamination, testing the efficacy of wastewater treatments, and removing the mystery about whether a stream contains harmful contaminants.
About the author: Dustin Renwick works as part of the innovation team in the EPA Office of Research and Development.