A lichen looks like a single organism, but it is actually a symbiotic relationship between different organisms. It is composed of a fungal partner (mycobiont) and one or more photosynthetic partners (photobiont). The photosynthetic partner is generally green algae or cyanobacteria. There are about 20,000 species of lichen on the Earth.
It is debated whether the relationship in a lichen is mutualistic or part of a controlled parasitism. On one hand, the fungus and the photobiont seem to be in a mutualistic relationship because when they are combined, they have the ability to deal with ecological conditions that neither part would be able to handle on its own. It also seems that neither partner is damaged by the other. Upon taking a closer look at a lichen, some might say that the photobiont is a captive of the mycobiont, not a partner. The fungal partner "enslaves" the photobiont to feed from the photobiont’s photosynthesis.

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