I be lichen it!

Different species of lichen growing on fence boards.

One of the strangest relationships on the planet Earth exists between an alga and a fungus.  Together they exhibit obligate mutualism - a form of symbiosis where each partner of the relationship has to be together to survive.

There are some 15,000 to 20,000 species of lichen across the planet.  The word species is a bit odd in this circumstance as there are actually 2 distinct species making up the single entity.  For the most part, the fungus is a unicellular variety belonging to the Ascomycetes, the same group that yeast belongs to.  The photosynthetic component of the equation often is a form of green algae, although it may also be a member of the cyanobacteria.

Both forms benefit from the presence of the other.  The fungus receives sugars from photosynthesis done by the algal component.  The fungal component provides moisture, other nutrients, and protection for the algae.  

Although lichens are found throughout the planet, there are certain environments where they do not exist.  Hot areas without protection from the sun do not support lichen growth, but there are lichens present in Death Valley where there is ample shade.  They are also present in Antarctica, where they can photosynthesize at temperatures as low as -20° C.

Lichens are, however, very sensitive to pollution, especially sulphur dioxide, and do not thrive around industrial or heavily populated areas.  The presence of lichens is often a strong indicator of the quality of air - clean air when they are abundant, foul air when they are scarce.  As the quality of air improves from one area to another, the frequency of lichens increases.

Thanks for reading.

Eric Svendsen     www.ericspix.com