Though many types of algae are organisms that produce their own food through the process of photosynthesis, they are not classified as plants. In fact, algae are classified outside of the plant kingdom, in a group of their own known as the protista kingdom. Unlike plants, algae do not have specialized reproductive cells. Instead, they reproduce by using spores or self-replicating from broken fragments of the original organism. Algae can be one-celled, two-celled or even multi-celled.
There are over 17,000 different types of green algae. Of all the types of algae, green algae are the most closely related to plant life. Like plants, green algae contain cellulose within their cell walls and pigments such as carotenoids and chloropyll. Green algae live in freshwater as well as in the ocean, and some types live on snow, lichens or other types of marine life. Some types of green algae, such as sea lettuce and chlorella, are eaten by humans.
Most red algae are complex seaweeds. Scientists have found about 5,000 different types of red algae, all of which are found in the ocean. Many types of red algae are edible, including nori and laver. Some types of red algae are harvested and used as food thickeners and stabilizers.
Brown algae are almost all complex seaweeds. The most well-known of the brown algae is kelp, which appears much like a large, marine plant and can grow more than 200 feet in height. Other types of brown algae include sargassum and rockweed. There are about 1,500 different types of brown algae.
Like plants and green algae, dinoflagellates are made of cellulose. Dinoflagellates are one-celled algae that swim using a tail-like structure known as a "flagella." Dinoflagellates are known for emitting a bluish color in the water when disturbed. This bright blue light is known as "bioluminescence." Some types of dinoflagellates are poisonous to humans and other animals and can cause "red tides" and shellfish contamination.