A mutualism is a symbiosis where two or more species gain mutual benefit from their interactions, and suffer negative impacts when the mutualistic interactions are prevented from occurring. Mutualism is a form of symbiosis where the interactions are frequently obligatory, with neither species being capable of surviving without the other. A well-known example of mutualism is the relationship between certain species of algae or blue-green bacteria and fungi that results in organisms called lichens. The fungal member of the relationship provides a spatial habitat for the algae, which in turn provide energy from photosynthesis to the fungus. Mutualistic interactions are thought to be the origin of the many cell organelles like mitochondria and chloroplasts, which may have resulted from the acquisition of free-living phytoplankton and other single-celled organisms by host species. Both the incorporated cell and the host soon evolved so that neither could exist without the other.