What are Lichen?

What are lichen? 
They are symbiotic organisms that are composed of a fungus and a photosynthesizing partner - could be a green algae (85% of lichen), or a cyanobacterium (formerly known as blue-green algae)(10% of lichen), or a combination of both (5% of lichens).
Fungi + Algae = Lichen 
Fungi + Cyanobacterium = Lichen
Fungi + Algae + Cyanobacterium = Lichen
The symbiotic relationship between fungi and algae and/or cyanobacterium is unique in the way that it's not just beneficial to both partners but both lose their own identity and a new different, dual organism is formed!

There are three different growth forms of lichen
Crustose - Crust-like, tightly attached to that which it's growing on. Has upper surface but no lower surface.
Foliose - Flat and leaf-like. Can be loosely or tightly attached to surface. Has upper and lower surface. (Images above and below are of this form).
Fruticose - Upright and shrubby, but sometimes hang down. Most of these lichen do not have a distinct upper and lower surface, but have an outer surface. 

Where do lichen grow?
Lichen can be found on every continent - including areas in the Arctic and Antarctic and can be found on mountain tops.
Lichen are the dominant organism on around 8% of the entire world's land surface... That's HUGE!!!

Lichen and their role in nature.
In Arctic regions they are used as a winter food for grazing animals. In more temperate regions they are used by birds as nesting materials, insects and small invertebrate will use it as shelter. Lichen can also aid in the breakdown of rock into soil. The lichen with the Cyanobacteria can even fix nitrogen and add fertilizer to the ecosystem.

Lichen as air quality indicators.
There are some lichen that are very sensitive to air pollution (sulfur dioxide). These lichen have been widely used as indicators of air quality to study air pollution patterns around cities and industrial areas. 
Thankfully, in many developed countries, air quality has improved to the extent that lichen are no longer affected and are returning to areas that had been so polluted the lichen could not grow.