Plant-like protists, which are also called green algae, resemble plants in having a cell wall, chlorophyll and the ability to carry out photosynthesis. They are a diverse group of four phyla, but all live in water and have similar mechanisms for movement and reproduction.
Green algae are unicellular protists with cell walls and photosynthetic organelles called chloroplasts. Their ability to extract energy from sunlight makes them primarily autotrophic, like plants, but in the dark they can switch over to heterotrophism and begin eating organic material near them. Algae typically have an eyespot, which they use to detect the presence or absence of light, and a true flagellum, which is used to actively propel them through the water. They differ from plants by making use of accessory pigments to absorb light, which gives them a wider range of light frequencies to use for energy than the monochromatic approach of green plants. The accessory pigments also give algal populations a range of colors; a bloom of algae can be green, brown or red. Plant-like protists constitute the base of aquatic food chains and, because of their broad range and high populations, they are responsible for over half of the photosynthesis that takes place on Earth.