We may have an explanation for the mysterious death of hundreds of bald eagles(Haliaeetus leucocephalus) across the south-eastern US. They may have ingested bromide-laced prey plucked from lakes, although the source of the bromide is unclear.

In 1994, dozens of bald eagles died in Arkansas. Since then, nearly 200 others – all living near artificial lakes from Texas to the Carolinas – have been diagnosed with vacuolar myelinopathy, which creates holes in the brain and spinal cord.

Eagles with the condition have crashed into cliffs or starved as they lost control of their wings and other bodily movements, says Timo Niedermeyer at Martin-Luther-University Halle-Wittenberg in Germany.

US researchers have long suspected a connection with water-thyme (Hydrillaverticillata), a fast-growing underwater weed, and the slimy blue-green algae – technically cyanobacteria – that covers it. They noticed that fish and water birds became weak and uncoordinated after consuming the weed and that eagles were eating this contaminated prey as it was easy to catch.

Susan Wilde at the University of Georgia sequenced the cyanobacteria’s DNA and named the new species Aetokthonos hydrillicola, meaning “eagle killer living on water-thyme”.

Read more: Starving bald eagle chicks hint at ecosystem collapse in Florida

Despite their suspicions, though, Wilde’s team couldn’t find the poison in the cyanobacteria. So she sent samples to Niedermeyer, whose team grew the cyanobacteria in their specialised laboratory. But when the researchers fed their lab-grown cyanobacteria to chickens, the birds stayed healthy. “This was a huge setback,” he says.

Undeterred, Niedermeyer studied cyanobacteria samples from Wilde’s team again, using a more advanced technique – atmospheric pressure matrix-assisted laser desorption/ionisation mass spectrometry – and, this time, he found an unexpected ingredient: bromine.

Bromine in the form of its stable ion bromide isn’t uncommon in nature, he says, but it is rare in freshwater. Apparently, water-thyme loves it: measurements from the lakes it inhabits in the US show the weed soaks up so much bromide that its tissue contains almost 1000 times more than the water it is in.

It seems that when the cyanobacteria grow on the water-thyme, they pick up bromide from the weed.

Niedermeyer’s team grew more cyanobacteria, this time adding bromide to the test tubes, and within days, the cyanobacteria were producing star-shaped compounds chock full of bromine. Further testing confirmed that the compounds were toxic, he says.

Why bromide appears in lake water is the new mystery to be solved. Niedermeyer speculates that park managers might be using bromide-laden weedkillers to control the invasive water-thyme, which clogs up areas normally used for swimming, fishing and boating. It might also come from coal-fired power plant waste water.

Researchers plan to investigate possible sources as well as study whether the toxin has any effects on mammals, including humans.

Journal reference: Science, DOI: 10.1126/science.aax9050