In biology, mutualism is an interaction between two species in which both species derive benefit. Mutualism can involve brief or long-term interactions (pollination versus lichen symbiosis between fungus and alga) and may or may not be obligatory for one or both partners. For example, bacteria known as rhizobia can reproduce either in the soil or in (usually) mutualistic symbiosis with legume plants. Mycorrhizal fungi, on the other hand, can be totally dependent on their plant hosts. Microbes often band together for mutual benefit in biofilms to break down solid food sources as in rusticles.
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