Plantlike protists, which are commonly known as algae, are estremely diverse. Like plants, algae are autotrophs. Most are able to use the sun's energy to make their own food. Algae that live on the surface of ponds, lakes, and oceans are and impportant food source for other organisms. In addition, much of the oxygen in Earth's atmosphere is made of these algae. Some algae can be unicellular while others are multicellular.