Lichens

Lichens, of which more than twenty thousand species have been named, are complex associations between certain fungi and certain algae. The lichen itself is not an organism, rather it is the morphological and biochemical product of the association. Neither a fungus nor an alga alone can produce a lichen.

The intimate relationship between this two living components of a lichen was once erroneously thought to represent mutualism. In mutualistic relationship, both participants benefit. With lichens, however it appears the fungus actually parasitizes the algae. This is one of the conclusions drawn from experiments in which the two components of lichens were separated and grown apart.

In nature, lichen fungi may encounter and grow around several kinds of algae. Some types of algae the fungi may kill, other types it may reject. Lichen algae are autotrophic, meaning they make their own food through photosynthesis. Lichen fungi are heterotrophic, meaning they depend upon the algae within the lichen to supply their food. Up to ninety percent of the food made by the green algal cells is transferred to the fungus contributes to the association is not well understood.
Lichens are hardy. They grow in many habitats and are often pioneers in hostile environments where few other organism can flourish. They have been known to grow endolithically, having been discovered thriving inside of rocks in Antartica. Lichens help reduce erosion by stabilizing soil. Several kinds of insects glue lichens to their exoskeletons for camouflage. Many species of birds use lichens as building materials for nests. Humans have used lichens for dyes and antibiotics.