Algae is not a plant, it is a protista; however, it shares many qualities with plants. Like plants, algae can perform photosynthesis.
As much as 75 percent of Earth's oxygen is produced by algae and cyanobacteria. It is the basis of the aquatic food chain, since all life in the water either feeds on algae or on something that eats algae. A few types of algae feed off other living things instead of using sunlight for photosynthesis. These other algae produce toxic substances that cause sores on fish, and the algae then eats the blood and fluids that seep from the sores.