They also reproduce by binary fission. The term algae embraces all autotrophic protists. Most algae live in water, but some are terrestrial. Most algae live near the surface of the water producing 30 to 50 percent of the earth's oxygen. Algae are classified on the basis of conservative characteristics such as the type of cell wall, flagella, photosynthetic pigments, and the form in which food is stored.
Many dinoflagellates are colorless and live as heterotrophs and parasites.
Some produce nerve poisons toxic to vertebrates. Many euglenoids contain two flagella and contain a hard pellicle made of protein just under the plasma membrane. Many contain a red eye spot, which is thought to be used as a photoreceptor. Euglenoids reproduce asexually by dividing lengthwise into two. They do not reproduce sexually.