The roles of protists include producing and using energy, recycling living systems into constituent nutrients, acting as biological control agents, synthesizing oxygen, and engaging in symbiotic relationships. Protists have commercial roles in the pharmacological, agricultural and biomedical research industries.
Protists belong to the Eukarya domain, having membrane-enclosed nuclei and organelles. Single-celled, they sometimes form colonies that behave like multicellular organisms. Formerly classified as plants or animals, protists encompass photosynthetic and non-photosynthetic organisms. Protists include algae, protozoa and the lower fungi. They exist as free-living or symbiotic organisms in vast numbers. Photosynthetic algae provide up to half of the world's net oxygen.
Protists participate in symbiotic commensalism, acting as hosts to disperse their commensal species. They are parasitic, invading an animal's circulatory or digestive system. Circulatory parasites cause malaria and amoebic dysentery in humans; disabling and fatal diseases. Protists infect domesticated and game animals, making them unsafe or inedible as food sources. Dinoflagellates release a toxin during an algal bloom, killing hundreds of tons of marine life.
Protists have cultural impacts on humanity. Lower fungi caused a potato blight in 19th century Ireland, leading to famine and mass exodus. Another strain disrupted the French wine industry.
The ciliate Tetrahymena is a free-living protist that is easily cultured, making it an ideal candidate for cancer research.