An alga (plural: algae) is a simple plant-like organism that lives in water or damp places and uses sunlight to make its own food. Algae don't have roots, stems, or leaves like the plants you see in a garden, but many of them contain chlorophyll, the green substance that lets them capture energy from the sun.

You've probably seen algae without knowing what to call it. That green slime on the side of a fish tank? Algae. The slimy green coating on rocks in a stream? Algae. The dark green seaweed washing up on a beach? That's algae too.

Algae range in size from microscopic single cells you can't see without a microscope to giant kelp forests stretching over 100 feet tall in the ocean. Though they might seem unimportant, algae are actually crucial to life on Earth. They produce much of the oxygen in our atmosphere, and countless ocean creatures depend on them for food.

Some algae cause problems, like when they grow too fast and create thick mats called algal blooms that can harm fish and make water unsafe for people and animals. But scientists are also discovering useful applications for algae, from creating biofuels to producing nutritious food supplements.