Lichens
 

A lichen is a symbiotic association of millions of photosynthetic microorganisms, usually unicellular or filamentous green algae, held in a mesh of fungal hyphae, which is typically an ascomycete.    In excess of 25,000 species of lichen have been identified to date.  The union of fungal and algal partners is so complete that lichens are given genus and species names as if they were one organism (Campbell, 627).

 

Generally speaking each partner endows the other with things it could not obtain on its own.  Alga provides food for the fungus while cyanobacteria fix nitrogen and provide organic nitrogen.  The fungus provides a suitable environment for algae growth.   Fungal hyphae retain water and minerals, allow gas exchange, and protect the algae.  Additional protection is afforded to the alga as various fungal pigments help shade the algae cells from intense sunlight.  Furthermore, in certain species of lichen the fungus contains toxic compounds that prevent the lichen from being eaten by predators thereby benefiting both species (Campbell, 627).

 

Lichens are important components in the process of succession.  As pioneers on newly cleared rock and soil surfaces (burned forests and volcanic flows) lichens physical penetrate the outer crystals of rock and release acids that help break down the rock and establish soil-trapping lichens (Campbell, 628).

 

Relatively speaking lichens are rather tough organisms with many able to tolerate severe cold and survive desiccation.  Even though some lichens are thousands of years old, many are severely affected by sulfur dioxide and other aerial poisons. A decrease in sensitive lichens accompanied by an increase in hardier species may be an early warning sign of the deterioration in air quality of a particular area (Campbell, 628). 

 

Works Cited

 

Campbell, Neil A. Biology. 4th ed. Menlo Park: The Benjamin/Cummings Publishing

            Company, 1996.