Retinoic acid levels high in waterways rich in cyanobacteria blooms

By Susan Milius

Web edition: May 30, 2012

Harmless as they may look, some of the microbes called cyanobacteria have the power to dose waterways with a range of chemicals that might cause deformities in frogs or other vertebrate wildlife.

Cyanobacteria, also called blue-green algae, could be an underappreciated but widespread source of compounds called retinoic acids in waterways, says environmental toxicologist Jianying Hu of Peking University. Out of 24 kinds of cyanobacteria grown in a lab, 13 produced some kind of retinoic acid or retinoic acid cousin, Hu and her colleagues report online May 29 in the Proceedings of the National Academy of Sciences.

Retinoic acids, formed from vitamin A, help sculpt developing body parts in vertebrates. Researchers puzzling over clusters of deformed frogs first highlighted in the 1990s have considered retinoic acids as one of several possible villains.

Hu and colleagues also analyzed samples from China’s third largest freshwater lake, Taihu Lake, which is rich in cyanobacteria blooms. The lake carries high concentrations of retinoic acids, the researchers found.

The new work “opens up a new area of potential research,” says plankton ecologist Karl Havens, who directs the Florida Sea Grant College Program in Gainesville. “I will be interested now to see if the compounds are found in other lakes with cyanobacteria blooms, or even if they continue to be documented in this lake,” he says.

The retinoic acid concentration found in Taihu Lake — up to 20 nanograms per liter — was 10,000 times what Kunimitsu Kaya of the University of Tsukuba in Japan and his colleagues have found in Japanese lakes. Last year Kaya reported cyanobacteria making a retinoic acid cousin (7-hydroxy retinoic acid); he says he finds it plausible that blooms create such extreme conditions in Taihu. Scum there probably blocks sunlight that otherwise would break down some of the very potent retinoic acids.

There’s not enough information yet to link cyanobacteria to high rates of amphibian deformities, says ecologist Andrew Blaustein of Oregon State University in Corvallis. He and other researchers have proposed a variety of other possible contributing menaces, including UV radiation and trematode parasites.

Parasites, a leading hypothesis for the cause of some limb abnormalities, certainly don’t explain all the cases, says disease ecologist Pieter Johnson of the University of Colorado Boulder. The new paper “may be a useful step forward in helping to explain abnormalities in wetlands without the parasite.