A type of mutualism in which the species involved are in close proximity and interdependent with one another in a way that one cannot survive without the other.
A sample of this type is the interaction between fungus and alga forming lichen. In nature, the fungus in lichen provides the alga with water and minerals while the alga uses the minerals and water to make food for the fungus and itself (by photosynthesis). When the alga and fungus of some lichens were cultured separately under laboratory conditions, both of them cannot grow and reproduce without the symbiotic partner.
Word origin: L mūtu(us): mutual, reciprocal + Gk –ismos, –isma: often directly, often through.
Related forms: mutualist (noun), mutualistic (adjective).
Compare: facultative mutualism.
See also: symbiosis.