An association of two species, in which both species are benefited, is called mutualism.
Examples of mutualism
Rhizobium in root nodules of legumes: Rhizobium bacteria live in the root nodules of leguminous plants. The bacteria fix atmospheric nitrogen and make it available to the plants. In exchange for this, bacteria get water, minerals, organic food and protection to the plant.
Lichens: Lichen consists of an alga and fungus living together in a mutualistic relationship. The alga produces food which is used by both the partners. The fungus protects the alga, absorbs water and minerals and attaches the alga to the substratum.
Mycorrhiza: It is an association of soil fungus with the roots of higher plants like conifers. The fungus absorbs water and minerals from the soil and passes them into the plant. The carbohydrates synthesized by the plants is absorbed by the fungus.
Trichonympha and termites: Termites are not capable of digesting cellulose. Trichonympha is a flagellate protozoan living in the gut of termites. It digests cellulose for termites. Termites in turn provide food and shelter to Trichonympha.