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Bald eagles, along with other wildlife, have succumbed to a mysterious neurodegenerative disease in the southern United States since the 1990s. New research from Martin Luther University in Halle-Wittenberg (MLU) in Germany and the University of Georgia in the United States identifies the cause of these deaths: a toxin produced by cyanobacteria that grow on invasive aquatic plants. The problem is potentially exacerbated by the herbicides used to control these plants. The results were published in Science.
In 1994, bald eagles died en masse in the US state of Arkansas. Animals were losing control of their bodies and holes were growing in their brains. A previously unknown neurodegenerative disease called vacuolar myelinopathy (VM) has been identified. “The origin of the disease was a complete mystery,” explains Professor Timo Niedermeyer of the MLU Institute of Pharmacy.
Later, American researchers found that not only eagles were affected, but also their herbivorous prey. Scientists have discovered a link with an invasive aquatic plant (Hydrilla verticillata) that grows in freshwater lakes in affected areas. However, there were still lakes with the aquatic plant where the disease did not manifest itself. In 2005, Susan B. Wilde, professor at the Warnell School of Forestry and Natural Resources at the University of Georgia, identified a previously unknown cyanobacterium on the leaves of Hydrilla verticillata, which appeared to be the cause of the disease. It turned out that vacuolar myelinopathy only occurs in places where cyanobacteria colonizes the invasive plant. She called the “killer eagle that grows on Hydrilla” bacteria: Aetokthonos hydrillicola.
Bacterial colonies of the cyanobacterium A. hydrillicola growing on a leaf of the invasive aquatic plant H. verticillata. Credit: Susan Wilde
“I came across a press release issued by the University of Georgia and was fascinated by these findings because I have been working with cyanobacteria for years,” says Niedermeyer. He had samples sent, grew the bacteria in the lab, and sent them back to the United States for further testing. But the tests came back negative: the bacteria in the laboratory did not induce the disease. “It wasn’t just the birds that were going crazy, we were too. We wanted to understand this, ”says Niedermeyer. Once again, he had colonization sheets sent to him. Steffen Breinlinger, a doctoral student in his research group, then used a new imaging mass spectrometer to study the composition on the plant’s leaf surface, molecule by molecule. He discovered a new substance that only appears on the leaves where cyanobacteria grow, but which is not produced in cultivated bacteria.
His research into the chemical structure of the isolated molecule showed that it contains five bromine atoms. “The structure is really spectacular,” says Breinlinger. The properties are unusual for a molecule formed by bacteria. And they explain why the toxin did not form under laboratory conditions. Standard culture media in which cyanobacteria grow does not contain bromide. “We then added bromide to our lab cultures and – the bacteria started producing the toxin,” Breinlinger explains. Wilde and his colleagues tested the isolated molecule in birds, and finally, after nearly a decade of research in the Wilde and Niedermeyer laboratories, they had proof: the molecule triggers VM. According to the name of the bacteria, the researchers call their discovery etokthonotoxin, “poison that kills the eagle.” “Finally, we not only caught the murderer, but we also identified the weapon that bacteria are using to kill these eagles,” says Wilde.
Bald eagle flying.
A research group participating in the Czech Academy of Sciences study also found sections of DNA containing genetic information for the synthesis of the new molecule. The reason why cyanobacteria form the toxin on aquatic plants in the first place, however, has not yet been investigated. One of the herbicides used to control the invasive aquatic plant could play a crucial role in the appearance of VM: it contains bromide and could therefore stimulate the production of toxins.
Neurological disease has not yet occurred in Europe and there have been no reports of toxin-forming cyanobacteria.
Reference: “Eagle Slayer Hunt: Cyanobacterial Neurotoxin Causes Vacuolar Myelinopathy” by Steffen Breinlinger, Tabitha J. Phillips, Brigette N. Haram, Jan Mare, José A. Martínez Yerena, Pavel Hrouzek, Roman Sobotka, W. Matthew Henderson, Peter Schmieder, Susan M. Williams, James D. Lauderdale, H. Dayton Wilde, Wesley Gerrin, Andreja Kust, John W. Washington, Christoph Wagner, Benedikt Geier, Manuel Liebeke, Heike Enke, Timo HJ Niedermeyer and Susan B. Wilde, 26 years old March 2021, Science.
DOI: 10.1126 / science.aax9050
The research was supported by the Deutsche Forschungsgemeinschaft (German Research Foundation, DFG), the Czech Science Foundation GA? R, the US Department of the Interior, the US Fish and Wildlife Service, the Florida Fish & Wildlife Conservation Commission, the Gulf States Marine Fisheries Commission, the National Institute of Food and Agriculture McIntire-Stennis Capacity Grant, and the American Eagle Foundation .
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