Algae are simple plant-like organisms that grow in or near water. Algae have many similarities to plants, but they are not true plants. Pond scum, seaweed, and giant kelp are all examples of algae.
Algae do not have a true root system like plants. Neither do they have stems or leaves as true plants do. Algae cannot sustain themselves on land. Most algae species live underwater; however, some species live in very damp places.
Like plants, algae contain a green pigment called chlorophyll. This makes many kinds of algae green. However, different kinds of algae can be red, yellow, brown, or even blue.
Algae use sunlight to convert water and carbon dioxide into oxygen and into nutrients called sugars in a process called photosynthesis. In this regard, algae is similar to true plants.
Algae can grow in a variety of environments. It can grow in saltwater, freshwater, wet soil, or on moist rocks or trees. Some algae can even grow beneath layers of snow and ice. These snow algae contain a chemical that could be likened to antifreeze that protects them from the cold.
There are seven major types of algae and hundreds of thousands of different species. Some are simple, single-celled organisms, like pond scum, others have multiple cells, like seaweeds. Some algae such as kelps are as big as forests. Giant kelps grow at a rate of 18 inches (45 cm) per day. They can be as tall as 175 feet (53 m).