Lichens are plant-like organisms that are rather unusual in that they are an amalgam of two (or occasionally three) organisms : a fungus and algae (or cyanobacterium). They are symbiotic systems, where the partners of the association work together for mutual benefit.  The fungus makes up the bulk of the lichen's form (known as the thallus), it is a complex network of fungal hyphae that surround the algal cells.  The algae (green algae or cyanobacteria) are essential to the association as they can photosynthesise, fixing carbon dioxide and providing both partners with organic carbon compounds (often in the form of sugar alcohols).

Some lichen species are brightly coloured. The colour may vary from a golden yellow to a deep red. The pigments responsible for these colours belong to the anthraquinones.  However, these insoluble, phenolic pigments can have toxic effects. To avoid harm by these pigments, the lichen exports* the pigment from the fungal component of the symbiosis.

The pigment then accumulates / crystallises on the surface of the lichen. The layer of pigment crystals reflects harmful radiation (in the form of UV light) and also blue light, whilst still allowing enough light to pass through for photosynthesis by the algae / cyanobacteria. Exposure to UV light can damage DNA, inducing mutations.  The pigmentl layer is effectively a ‘sunscreen’ for the lichen.

* Recent work at Imperial College and RBG, Kew has identified the genes responsible for pigment production, and the transport of the pigment out of the fungal tissue.

In the past, certain lichen pigments were often used to dye clothing materials.    Parmelia saxatilis, also known as grey crottle, was used to dye wool for Harris Tweed.  This lichen is often found growing on tree trunks and gives a red / brown colour to the material.

Lichen and moss growing together  (thanks to Art for photos)

Woodlands TV has produced two short videos on the biology of lichens :-