It refers to an aquatic, photosynthetic way of life, not an evolutionary kinship. 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. Half the species are photosynthetic containing chlorophylls a and c and various carotenoids, and store their food in the form of oils and starch.
Many dinoflagellates are colorless and live as heterotrophs and parasites.
Some produce nerve poisons toxic to vertebrates. Most members of this group live in fresh water, being especially abundant in polluted habitats. Many euglenoids contain two flagella and contain a hard pellicle made of protein just under the plasma membrane. Their chloroplasts contain chlorophyll a and b and carotenoids. Many contain a red eyespot , which is thought to be used as a photoreceptor.