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  • Hi, this is Julián from MinuteEarth.

  • Three billion years ago the land was lifeless and the air oxygen-free but rich in CO2.

  • The oceans were hot and loaded with nitrogen and phosphorus, and aquatic microbes called

  • cyanobacteria were loving it.

  • These microbes would later turn out to be our enemies, but at this point in time humans

  • didn't exist yet.

  • In fact, cyanobacteria actually helped make our existence possible in the first place.

  • But back to early life on Earth...

  • In addition to being heat tolerant, the cyanobacteria grew in thin mats that were good at soaking

  • up light and nutrients like nitrogen and phosphorus, and built nasty toxins to poison their competitors.

  • And in one of the most profound steps in all of evolution, they figured out how to combine

  • carbon dioxide with water to make tasty sugar - a process called photosynthesis.

  • But that fancy new photosynthesis also happened to release oxygen which was poisonous to organisms

  • that had evolved under oxygen-free conditions, which meant pretty much all life on Earth

  • at that time, including most of the cyanobacteria themselves.

  • But over time the surviving cyanobacteria evolved, to not just tolerate oxygen, but

  • to use it, with a sort-of reversal of photosynthesis, which we now call aerobic respiration.

  • This helped cyanobacteria survive, and in fact, grow to dominate the Earth's oceans

  • for another billion years.

  • Eventually though, as the Earth began to cool and nutrient supplies got used up, some algae

  • well adapted to those conditions also stole cyanobacteria's metabolic secrets.

  • The algae out-competed cyanobacteria, pushing them into the shadows across much of Earth's

  • waters for the next billion years or so.

  • This long interval also saw the evolution of more complicated life-forms, including

  • oxygen-breathers like usand we have held center stage from cyanobacteria ever since.

  • But our success today is now making things awesome again for cyanobacteria.

  • We've done this by pumping CO2 into the air, which has warmed the atmosphere and oceans,

  • which cyanobacteria like.

  • Also, because we over-fertilize our farm fields, the rain washes a lot of that fertilizer into

  • rivers and oceans, providing a level of delicious nutrients that cyanobacteria haven't seen

  • for perhaps billions of years.

  • And that's bad for us, because these heat-loving, nutrient-gobbling microbes are once again

  • forming sludgy gross-smelling mats, that release nasty toxins that keep their algal competitors

  • at bay - but also make animals and people sick.

  • And since cyanobacteria live short lives and die in large groups, floating mats of their

  • dead bodies serve as food for oxygen-breathing decomposers, who temporarily use up all the

  • available oxygen in the water, killing fish, shrimp, insects, and plants in sometimes dangerously

  • massive dead zones.

  • To keep cyanobacteria at bay, we need to stop warming the planet and to farm in a way that

  • doesn't send nutrients into waterways.

  • Until we do, the little creatures that first gave us oxygen are going to keep on blooming.

  • And dying.

  • And turning our oceans and lakes to the dark side.

  • This video was sponsored by the St. Croix Watershed Research Station, with financial

  • support from the McKnight Foundation.

  • The Research Station is part of the Science Museum of Minnesota, and its scientists study

  • lakes and rivers across Minnesota to better understand when, where, and why cyanobacterial

  • blooms occurand how we can prevent cyanobacteria from harming lakes and rivers and the people

  • and wildlife that depend on them.

  • Thanks, St. Croix Watershed Research Station!

Hi, this is Julián from MinuteEarth.

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The Bacteria That Made Life Possible Are Now Killing Us

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