The category of Protists includes many widely ranging microbes, including slime molds, protozoa and primitive algae. When you think of algae, you probably think of seaweed or the green, slimy stuff that forms on the walls of untreated, dirty swimming pools. Here we'll focus on the microscopic algae. Algae are found in bodies of fresh and salt water across the globe. They can also grow on rocks and trees and in soil when enough moisture is available.
Most algae are able to make energy from sunlight, like plants do.
They produce a large amount of the oxygen we breathe. However, at some stages of their lives, some algae get their nutrients from other living things. Some dinoflagellates make their own energy from sunlight, like plants. But others like Pfiesteria produce toxic substances that stun passing fish and cause bleeding sores. The Piesteria then feed on the fish blood and fluids.