Blooms of blue-green algae in freshwater lakes and reservoirs can produce toxic cyclic peptides called microcystins that sicken swimmers and people who drink the water. Researchers have now demonstrated a vibrational spectroscopy method that could eventually allow a quick test of whether these blooms are toxic. (Environ. Sci. Technol., DOI: 10.1021/es200255y)
Environmental researchers have several techniques to screen for the toxins but none are ideal. Enzyme-linked immunosorbent assays, better known as ELISA, can screen for microcystins, says Keith Loftin, a chemist and environmental engineer at the U.S. Geological Survey in Lawrence, Kansas, who was not involved in the new study. Such assays are sensitive and relatively inexpensive but they can’t identify specific forms of the more than 80 microcystins, which have varying toxicities. More expensive techniques can distinguish among the structures, but don’t work in the field.
Environmental engineer Peter Vikesland and graduate student Rebecca Halvorson of Virginia Polytechnic Institute & State University thought Raman spectroscopy, a vibrational technique, might do the trick since it allows researchers to classify molecules based on their functional groups. Vikesland and Halvorson thought that Raman could detect and distinguish among different microcystins in the environment.
To mimic field conditions, the researchers spiked water samples with known concentrations of a particularly toxic microcystin called microcystin-LR. The water samples were from the New River in Virginia, and from rehydrated organic material extracted from the Great Dial Swamp in North Carolina. They then concentrated the samples using solid-phase extraction and placed droplets onto quartz slides. As the water evaporated, the droplets left ring-shaped deposits, like coffee stains, which concentrated the microcystins at their edges. Using Raman spectroscopy on the dried rings, the researchers could detect as little as 2 ng of microcystin-LR. The sensitivity approaches the World Health Organization’s guideline of 1 µg/L for the maximum safe concentration in drinking water.
Raman spectroscopy could be useful both in the laboratory and in the field, Loftin comments. “Any technique that can help us protect human and animal health from potentially dangerous toxins is significant,” he says. A field-worthy test would need greater sensitivity and a way for technicians with limited training to quickly interpret results, Loftin adds.
Vikesland and his colleagues are now working to expand the technique so they can spot multiple microcystins in a sample. A next step will be to improve sensitivity and avoid solid-phase extraction, which is both time-consuming and can complicate the spectra, Vikesland says.
