It would be easy to assume that pond scum is harmless, but it would also be a mistake. This aquatic bacteria is often known as blue-green algae, although it’s not really algae at all. In small quantities, it can be pretty, and pretty harmless. But when it blooms, as you’ll see in the video above from the American Chemical Society, cyanobacteria can kill. 

A thick mat of blue-green bacteria on the water’s surface absorbs all the sunlight for itself, blocking any of the Sun’s energy from reaching the ecosystem below. It can also produce a number of pretty terrifying toxins, as host Sophia Cai and chemist Lauren Wolf illustrate. These toxins can act on a number of different organ systems, including the brain, the liver, and the lungs. Anatoxin-a, for example, causes such rapid respiratory failure that it’s sometimes known as VFDF—very fast death factor

Cyanobacteria blooms are a natural phenomenon, but they’re also helped along by a lot of human factors, including water pollution and climate change. If you’ve been looking for a selfish reason to start caring about the environment, non-lethal drinking water is a good place to start.

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