Lichens, of which more than twenty

Lichens, of which more than twenty thousand
species have been named, are complex associations
between 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 lichen.

The intimate relationship between these two
living components of lichen was once erroneously
though to represent mutualism. In mutual relationships,
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 saveral 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 than food made by the green alga cell is
transferred to the fungus. What, if anything, the fungus
contributes to the association is not well understood.

Lichens are hardly. The grow in many habitats
and are often pioneers in hostile environments
where few other organisms can flourish.

They have been known to grow
endolithically, having been discovered thriving inside
of rocks in Antartica. Lichen help reduce erosion by
stabilizing soil. Saveral kinds of insects glue lichens to
their exoskeletons for camouflage.

Many species of birds use lichens as building
materials for nests. Human have used lichens for
dyes and antibiotics.

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