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!

Common name: blue-green algae
Age: species similar to the ones we see today first
appeared on Earth about 3.5 billion years ago
Size: you need a microscope to see it; groups are
big enough to see with the human eye
Habitat: shallow water, marine and freshwater; damp areas; tree trunks; and in surface soils
Characteristics: pollution can help it grow; can cause a smelly, thick scum in water

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