In the state of Arkansas, sudden deaths occurred in 1994 among bald eagles, the iconic bald eagles with the white head (Haliaeetus leucocephalus). Around DeGray Lake, biologists found more than 70 dead eagles in two years. Their brains turned out to be affected by an as-yet-unknown condition: the tissue had become spongy from countless tiny vesicles.

Further research showed that all kinds of other animals around the lake also died from this brain disease. Mainly coots, ducks and geese, but also reptiles, amphibians and fish. In the years that followed, more and more reports came in, including from neighboring states, from Texas to Florida and North Carolina. The brain disease was found to be passed from prey to predators. Hence, the bald eagle, at the top of the food pyramid, suffers the most.

“This death has puzzled biologists from the start,” said Timo Niedermeyer, a professor of pharmaceutical biology at the University of Halle-Wittenberg in Germany, one of the authors of the study. “Experiments showed that waterfowl from elsewhere that you released at such a lake showed symptoms within five days. But the researchers found no trace of toxic substances or infectious diseases.”

reservoirs

The sickening lakes did have something in common: they were man-made reservoirs containing Hydrilla verticillata, an aquatic plant very similar to our waterweed and native to Southeast Asia. As an aquarium plant, it ended up in the US in the 1950s, where it was sometimes dumped in ditches. In the warm Southeast, it did so well that it quickly became the country’s most invasive species, growing and spreading rapidly, displacing other species, especially in standing water such as reservoirs.

“Susan Wilde of the University of Georgia discovered an unknown blue-green algae on that plant,” says Niedermeyer. “Funnily enough, it has only been observed in the southeastern US.”

There are many blue-green algae that produce toxins that affect the brain or nerves. Blue-green algae native to the Netherlands also do this. This immediately made the new blue-green algae suspect. That’s why he got the name Aetokthonos hydrillicola: ‘the eagle slayer of Hydrilla’. But a toxic substance had still not been found in or around the bacterium.

“Wilde sent me a sample of the blue-green algae to grow and examine them for toxins,” says Niedermeyer. “That is the usual way of doing this, because it is easier to propagate them in a culture medium than on the plant itself. But we found no toxins on the cultivated blue-green algae. And lab animals didn’t get sick from it. So we had to try to investigate blue-green algae that grew up Hydrilla, which is much more difficult. Then, using imaging mass spectrometry, we suddenly found an unknown molecule containing five bromine atoms.” That set off alarm bells, because bromine compounds are known to damage the brain.

Research has shown that the aquatic plant accumulates bromine compounds from the water – but only in some lakes and not all year round. “We don’t know where those bromides come from,” says Niedermeyer. “Maybe from the herbicides that, wryly, are used to control Hydrilla. But I mainly suspect the local coal-fired power stations. They use bromides to remove mercury from their emissions.”

These experiments

In places where bromides are in the water, Hydrilla absorbs them and passes them on to the blue-green algae, which uses them to make its mysterious toxin. The researchers called the new substance aetokthonotoxine: the poison that kills eagles. Animal experiments showed that this toxin does indeed cause the deadly brain disease. “Now we want to investigate the underlying mechanism,” says the German professor. “And also whether people can suffer from it if they eat fish or waterfowl.”

Nico van den Brink, ecotoxicologist at Wageningen UR, thinks the research is in Science “very well done”. Looking for an unknown toxin in the environment is like looking for a needle in a haystack, he knows. “And also a toxin that only arises in a complex interaction between one plant and a blue-green algae, and only under specific conditions. No wonder it took them thirty years to do that.”

Hydrilla is also found in Europe: in hot springs in Central Europe and in the wild in Poland, Great Britain and Ireland. But nowhere does the species behave invasively – and nowhere does it carry the eagle-killing blue-green algae. At least, as far as we know. “And we don’t know if the distribution changes with climate change,” says Niedermeyer. “We continue to investigate this. This story is too fascinating to let go.”