Algae is the common, or non-scientific, name given to a large group of eukaryotic organisms that use photosynthesis to produce energy. Different species of algae may actually only be distantly related, but they tend to share similar characteristics.
Included in this group are microscopic organisms ranging from the single-celled microalgae and diatoms all the way to the enormous giant kelp, that can grow to 45 m or more. Algae can grow in both freshwater and saltwater and are different from land plants because they don't have roots, stems, and leaves.
Scientists have estimated that the first land plants evolved from a group of green algae, just under 1 billion years ago. The closest living relatives of land plants are a group of freshwater algae called Charophyta.
Agar, for example, is a gel made from red algae (often called red seaweed). It's a super important substance with uses all the way from DNA sequencing to making candy!
There are about 27,000 different species of algae all over the planet.
Algae is the primary source of food for most fish and other aquatic life. Additionally, because all land plants evolved from algae, there would be no life and land without it. And if all this wasn't already enough, scientists also estimate that algae produce up to 80% of Earth's oxygen.