After 20 years, they found the weird toxin that’s been killing eagles.

Science News celebrates a discovery that may save the lives of hundreds of eagles, ducks, and other birds living on or near lakes in the southeastern United States every year. They’ve been getting killed by a toxic combination of bacteria and a specific water plant that directly attacked their brains – and now we can do something about it:

The mystery of the unknown toxin [AETX] began at an Arkansas lake during the winter of 1994–95 with the nation’s largest unexplained die-off of bald eagles. The eagles, coots and some other birds lost their motor coordination, struggled to fly or even walk, and had seizures. Checking the ill animals’ brains revealed swathes of unnatural microscopic holes, or vacuoles. By 1998, six states had confirmed bird die-offs with the same disease, now called VM, short for vacuolar myelinopathy.

Wilde noticed that lakes with die-offs grew dense expanses of the green bottlebrush-shaped invasive water plant called Hydrilla verticillata. In 2001, she and several generations of students and international collaborators began a long journey of exploring whether the plants and their ride-along cyanobacteria might sometimes destroy brains.

This cyanobacterium turned out to be a new species (Aetokthonos hydrillicola). It also turned out to be hard to grow in the lab. It took two years after collecting the stuff from a reservoir before the team raised enough cyanobacteria for their first trial feeding a watery solution of it to test animals.

This cyanobacteria soup didn’t seem to bother the test chickens at all, however. Microscopic analysis found no tiny holes in their brains.

The researchers then wondered if something weird was going on with the lab culture setup. They collected wild weeds glopped with cyanobacteria from lakes with confirmed VM outbreaks. Using an elaborate lab setup to detect what chemical compounds the bacteria were making, the team hunted for unusual suspects. And there, lurking in the splotches marking the bacterial colonies, was a previously unknown compound, with abundant bromide molecules.

Bromine can get into lakes from various sources, some natural and not, such as power plants. Researchers discovered that the invasive Hydrilla builds up extreme concentrations of bromine compounds, 20 times greater than the concentrations in the lake bottom mud and 500 to 1,000 times greater than in lake water. In late summer, the warm water stays at the top of the lake trapping chilly, dark, low-oxygen water below. The weedy Hydrilla stops flourishing and starts leaking bromine-rich compounds. That’s when the cyanobacteria have the ingredients to make their deadly toxin.