Bald eagles, as well as other wildlife, have been succumbing to a mysterious neurodegenerative disease in the southern United States since the 1990s. New research from Martin Luther Halle-Wittenberg University (MLU) in Germany and the University from Georgia, USA, identifies the cause of these deaths: a toxin produced by cyanobacteria that grow on invasive aquatic plants. The problem is potentially compounded by the herbicides used to control these plants. The results were published in Science.
In 1994, bald eagles were dying on a large scale in the US state of Arkansas. The animals were losing control of their bodies and holes were forming in their brains. A previously unknown neurodegenerative disease called vacuolar myelinopathy (VM) was identified. “The origin of the disease was a complete mystery,” says Professor Timo Niedermeyer of the MLU Institute of Pharmacy.
Later, American researchers found that not only eagles were affected, but their herbivorous prey as well. Scientists discovered a connection to an invasive aquatic plant (Hydrilla verticillata) that grows in freshwater lakes in affected regions. However, there were still some lakes with the aquatic plant where the disease did not manifest itself. In 2005, Susan B. Wilde, a professor at the Warnell School of Forestry and Natural Resources at the University of Georgia, identified a previously unknown cyanobacterium in Hydrilla verticillata leaves that appeared to be the cause of the disease. It turned out that vacuolar myelinopathy only occurs in places where cyanobacteria colonize the invading plant. She called the bacterium “eagle killer that grows on Hydrilla”: Aetokthonos hydrillicola.
“I came across a press release issued by the University of Georgia and was fascinated by these findings, because I have worked with cyanobacteria for years,” Niedermeyer says. He had samples sent to him, cultured the bacteria in the lab, and sent them back to the United States for further testing. But the tests came back negative: bacteria in the laboratory did not induce the disease. “It’s not just the birds that went crazy, we too. We wanted to solve this, ”says Niedermeyer. Once again, he had colonized the leaves that had been sent to him. Steffen Breinlinger, a doctoral student in his research group, used a new imaging mass spectrometer to investigate the composition on the surface of the plant leaf, molecule by molecule. He discovered a new substance that is only found in the leaves where cyanobacteria grow, but is not produced in cultured bacteria.
His investigations 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 made up of bacteria. And they provide an explanation for why the toxin was not formed under laboratory conditions. The standard culture media in which cyanobacteria grow do not contain bromide. “Then we added bromide to our lab cultures and the bacteria started producing the toxin,” says Breinlinger. Wilde and his colleagues tested the isolated molecule in birds, and finally, after nearly a decade of research in Wilde and Niedermeyer’s labs, they had the proof: the molecule triggers VM. Based on the name of the bacteria, the researchers call their discovery ethoctonotoxin, “the poison that kills the eagle.” “Ultimately, we not only caught the killer, but we also identified the weapon the bacteria use to kill those eagles,” says Wilde.
A research group that participated in the study of the Czech Academy of Sciences also found sections of DNA It contains genetic information for the synthesis of the new molecule. However, why cyanobacteria form the toxin in aquatic plants in the first place has yet to be studied. One of the herbicides used to combat the invasive aquatic plant could play a crucial role in the appearance of VM: it contains bromide and therefore could stimulate the production of toxins.
Neurological disease has not yet occurred in Europe and no cases of toxin-forming cyanobacteria have been reported.
Reference: “Hunting the Eagle Killer: A 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, March 26, 2021, Science.
DOI: 10.1126 / science.aax9050
The research has been supported by the Deutsche Forschungsgemeinschaft (German Research Foundation, DFG), the Czech Science Foundation GA? R, US Department of the Interior, US Fish and Wildlife Service, Florida Fish and Wildlife Conservation Commission, Gulf States Marine Fisheries. Commission, the McIntire-Stennis Ability Scholarship from the National Institute of Food and Agriculture, and the American Eagle Foundation.