Have you ever seen pond scum—that thick, smelly layer of slime floating on the surface of the water? That's probably blue-green algae, known to scientists as "cyanobacteria." These tiny bacteria grow in soil, freshwaters, and seas around the world. They are a normal part of ecosystems on land and in water. But when they form in large groups, they can spoil the water and can even be toxic!

Image credits: main image, Hans Paerl, author. Licensed for use, ASM MicrobeLibrary.