Lichens

A lichen is not a single organism at all, but a remarkably close and stable symbiotic association between two very different partners: an alga and a fungus. In this partnership, the algal component is called the phycobiont and the fungal component is called the mycobiont.

Each partner contributes something the other lacks, and the association benefits both. The phycobiont, being photosynthetic, prepares food for the fungus, while the mycobiont absorbs water and minerals from the substrate and provides a protective, moisture-retaining shelter for the algal cells to live in. Because each partner is completely dependent on the other for essentials it cannot itself provide, the two are so intimately interwoven in the lichen body that they behave, structurally and functionally, almost as a single organism. Lichens grow very slowly and typically live for a long time; because they are extremely sensitive to air pollution (particularly sulphur dioxide), the presence or absence of lichens in an area is often used by ecologists as a convenient, low-cost indicator of air quality. Lichens are also of great ecological importance as pioneer colonisers on bare rock surfaces, where the fungal partner slowly breaks down rock to release minerals, gradually helping to form soil in places where no soil previously existed. …