A number of fungi form such close associations with microscopic
algae that they are regarded as single entities, called lichens.
Biologists still can't agree whether both partners benefit from
the union, or if the fungus is parasitizing the alga. The fungus
seems to contribute the lion's share to the partnership, but
can only survive if the alga, which produces nutrients by photosynthesis,
is present. Yet many of the algal partners can live independently.
The debate continuesÖ.
Some biologists even believe that Knotted Wrack (Ascophyllum
nodosum), a common seaweed of rocky seashores, is really
a lichen. It is always associated with a fungus.
Lichens can survive conditions that would kill most organisms,
and they are found from the Arctic to the Saharan Desert. One
reason they are so hardy is their ability to extract water and
nutrients from the air. There is a downside: they're very susceptible
to dirty air, and the absence of certain species is a good indicator
of atmospheric pollution.
About one fifth of all known fungi form lichenssome 20,000
species in all. Almost all are Ascomycota. We won't be looking
at lichens any further here. They deserve a website of their