Lichens are composite organisms comprising of an alga and fungus or a cyanbacterium and fungus in a symbiotic relationship in most cases. The lichen's alga and cyanobacterium perform photosynthesis with the fungus component deriving some of these photosynthetic products for it's growth. The fungus provides mineral nutrients extracted from the air, water and substrate to which is is attached along with water storage and housing. The fungal component also plays a role in the lichen adhering to it's substrate.
Lichens can live in the harshest of environments for example, withstanding high UV radiation levels at South Pole to mountain tops. Salt spray is also not a problem for some species that live on rocks close to the sea. Lichens are long lived and their celluar ultrastructure has been studied to determine the effects of the excess UV radiation on their photosynthetic machinery.
There are approximately 18,000 species known to date. Despite looking totally dehydrated and often dead, lichens quickly assimilate water in the form of rain or fog. These clips also show that numerous organisms such as mites and other small arthropods use the lichens as both a food source and shelter. One of the species shown is Xanthoria parientina.



UPDATE!!!!
The effects of lichens growing on vascular plants is often generalised as that they do not have a negative affect on growth and overall well-being of the plant. This long standing "public opinion" may in fact not be correct.

Here is a relatively recent journal article in a reputable journal. See what you think?

The title is: Lichen Plant Interactions
The Journal is
The Journal of Plant Interactions


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