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  • Thanks to Brilliant for supporting  this episode of SciShow.

  • Go to Brilliant.org/SciShow to learn how you can

  • take your STEM skills to the next level!

  • [♪ INTRO]

  • It was a mystery that went  unsolved for more than 25 years..

  • Across the southeastern United  States, bald eagles were dying.

  • This time, it wasn't DDTBut no one knew what it was.

  • Finally, in March of 2021, researchers  announced that they had found the killer:

  • a toxin from a recently  discovered species of bacteria.

  • And the scientists hope that this  knowledge can better protect eagles

  • and the ecosystems they depend on.

  • But how these scientists uncovered this  mystery is better than any murder TV

  • show that you've ever seen, so get  ready for some twists and turns.

  • It all begins in 1994 near a reservoir  in Arkansas called DeGray Lake.

  • Soon after the eagles had  arrived to spend the winter,  

  • they started turning up dead. Two years later, it happened again.

  • This time, another overwintering  species was affected as well.

  • Some duck-like birds called coots,  a prey item of eagles, by the way,

  • were having trouble swimming and flying.

  • In necropsies, the non-human  version of an autopsy,

  • the bodies of the affected  birds looked mostly healthy.

  • But in their brains, the  white matter looked spongey.

  • The axons, the skinny tubes that carry messages

  • between brain cells, were swollen.

  • And their fatty myelin wrappers were riddled  with microscopic, fluid-filled bubbles.

  • Researchers ruled out some possibilities,

  • like prions, or misfolded proteins, and viruses.

  • And by 1998 they classified  it as a new brain disease:

  • Avian Vacuolar Myelinopathy, or AVM.

  • No one had ever seen anything  like it in wild birds

  • But it did look a lot like the damage  in mammals from certain neurotoxins.

  • Like, from the rat poison bromethalin.

  • Bromethalin works by blocking mitochondria

  • from producing energy in the form of ATP.

  • Neurons use a lot of ATP to set up their signals.

  • And when this breaks downthey sort of fall apart.  

  • Trouble was, testing for these and  other toxins turned up nothing.  

  • Meanwhile, the problem was growing.

  • AVM began showing up across the southeastern US.

  • Over eight winters, it killed at leasthundred bald eagles and thousands of coots,

  • and it sickened members of  several other bird species.

  • There was growing evidence that  the cause was a natural toxin

  • not a chemical that humans had  dumped into the environment.

  • But despite years of tryingresearchers couldn't identify it.  

  • Then, in 2005, we have our first breakthrough.

  • Scientists linked the poisonings  to artificial bodies of water

  • where a certain invasive weed was growing.

  • And where a certain type of  cyanobacteria was growing on the weed.

  • The nuisance weed is called  Hydrillaor waterthyme.

  • And these lakes had a lot of it.

  • Hydrilla also grew in similar  lakes without these cyanobacteria.

  • And even though the same types  of birds spent time there,

  • they were not getting sick. So all signs were pointing to the bacteria.

  • No one had seen this species beforebut others like it are notorious

  • for making potent toxins, including  ones that affect the brain.

  • You may have heard about toxic  algal blooms that poison wildlife

  • and close beaches?

  • Well, that is actually cyanobacteria,

  • sometimes called blue-green  algae, and they can be nasty.

  • Researchers suspected these new  cyanobacteria were making a toxin

  • that accumulated as it moved up the food chain.

  • This is sort of like what happens  with other toxins like mercury or DDT,

  • the pesticide infamous for  thinning bird eggshells.

  • Like, waterbirds eat the weeds.

  • Then eagles eat the waterbirds.

  • Especially the sick onesbecause they're easy to catch.

  • Since eagles are at the top of the food chain,

  • they likely get the highest dose of toxin.

  • Which may have made AVM  especially deadly for them.

  • But it affected other critters in these lakes too,

  • like snails, tadpoles, fish, and turtles.  

  • Problem was, when researchers  grew the cyanobacteria in the lab,

  • itself, turns out, no easy task  — they didn't make any toxin.

  • So even though there was all this  evidence that they were the killer,

  • it was all circumstantial evidence.

  • Now to be clear, it's not that the  scientists were missing something.

  • The bacteria seem to produce very low  levels of the toxin most of the time,

  • and they need a secret ingredient to do it.

  • The final clue fell into place when  researchers used a super fancy tool

  • called atmospheric-pressure  matrix-assisted laser desorption/ionization

  • mass spectrometry imaging.

  • ...is what it was called.

  • What is it?

  • Well, it uses a laser to vaporize the  molecules on the surface of an object.

  • In this case, the objects in  question were bacteria-covered weeds

  • gathered from the wild.

  • Then it weighs and analyzes the molecules.

  • What stood out was a molecule  with five bromine atoms.

  • Sort of like the ones on  bromethalin, the rat poison.

  • Now, bromine isn't all that  plentiful in living things.

  • And it is not something you'd necessarily  think to add to culture medium

  • if you were trying to grow cyanobacteria in a lab.

  • But when the researchers fed  the bacteria some bromine

  • in the form of potassium  bromide, they made the toxin!

  • They named it AETXshort for aetokthonotoxin,

  • which meanspoison that  kills the eaglein Greek.

  • Though, as it turns out, it can  kill plenty of other animals too.

  • And with that, the mystery was solved.

  • They also made some important connections

  • to what was happening in the reservoirs.

  • Like, when bacteria are stressed, they  crank out a hundred times more toxin.

  • And they get stressed by things  like water temperatures dropping

  • and the water being stirred up.

  • Researchers think those same  conditions precede bird poisonings.

  • Plus, it turns out Hydrilla accumulates  bromine from the environment.

  • So the cyanobacteria probably  suck bromine out of the weed.

  • And now we know we should  probably stop using a certain

  • bromine-containing herbicide  to try to control the Hydrilla,

  • because it could be feeding  bromine straight to the bacteria!

  • We can also monitor bromine in the lakes

  • and look for ways to make it  less available to cyanobacteria.

  • And finally, we can look for better ways  to combat Hydrilla, the invasive weed,

  • and the toxic bacteria.

  • Meanwhile, by studying how the toxin works,

  • we might even be able to find a treatment.

  • Solving this mystery should help  us protect not only bald eagles,

  • but these entire freshwater ecosystems  and the people living near them.

  • Decades of detective work have finally paid off.

  • Scientists have to be  comfortable with the unknown,

  • they tackle it all the time.

  • And Brilliant's course Knowledge and Uncertainty

  • can teach you not just how  researchers understand uncertainty,

  • but also how you can deal  with it in your own life.

  • Brilliant is an online learning platform  that offers tons of interactive courses in

  • science, math, engineering, and computer  science, all designed by top educators.

  • If you'd like to give them a try, you  can check out Brilliant.org/SciShow

  • to sign up and save 20% on an  annual Premium subscription.

  • [♪ OUTRO]

Thanks to Brilliant for supporting  this episode of SciShow.

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We Solved the Mystery of the Dying Birds

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    joey joey に公開 2021 年 06 月 28 日
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