Lichen

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, UK. Grey, leafy lichen upper half of trunk; yellowy-green lichen on middle, lower half and up the extreme right side; and a shrubby lichen]]
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**Lichens** are a symbiosis of at least two quite different organisms. The partnership always involves a fungus, which lives with one or more partners which can do photosynthesis. The photobiont partner may be a green alga and/or a cyanobacterium.

There are more than 2,000 species in Britain and Ireland alone, many of which are found in temperate rainforests.

## Classification 
Lichens pose a problem for biological classification, because the three types of organism concerned come from three different kingdoms. After long debate, lichen are now classified as fungi, under the genus and species of the host fungus.p19 There are about 200 different kinds of lichens in Antarctica. In the Horlick Mountains, at 86 degrees south, there are six different kinds of lichen. In the Himalayas, they grow at altitudes up to 18,000 feet (~5500m).p216

The European Space Agency discovered that lichen can survive unprotected in space. Two species of lichen were sealed in a capsule and launched on a Russian Soyuz rocket. Once in orbit the capsules were opened. Two species of lichen were exposed to the vacuum of space, to cosmic radiation and huge swings of temperature. After 15 days the lichens were retrieved and were found to be in full health: no damage was found.

### Rain forests 
There are a huge number of lichen species in rainforests, far outnumbering flowering plants. The lichen are mostly epiphytes, living on trees. A single site in Costa Rica yielded about 300 species of lichen on leaves, in the understory of the rain forest; a single fallen *Elaeocarpus* tree in New Guinea held 173 species; the evergreen laurel *Ocotea atirrensis* was found with 50–80 lichen species on a *single leaf*. Their success is as a pair (or trio). Sometimes the partners can exist as separate organisms: certainly their close relatives can. Probably most of the algae and all of the cyanobacteria can survive on their own, though in more restricted habitats. The details vary according to the specific species or strains involved. The man who first realised the nature of lichen, Simon Schwedener, thought of the partnership as one of controlled parasitism. Obviously, the photobiont provides photosynthetic products: carbohydrates as sugar alcohols (green algae) or glucose (cyanobacteria). The bacteria also convert atmospheric nitrogen (N2) to ammonium ions (NH4+) which the fungus can use in amino acids for proteins. The proposal has been received with some skepticism.

## Lichenometry 
**Lichenometry** is a method of dating exposed rock by using the growth of lichen. It uses an estimate of how long the lichen's radius or width grows over time.

## Other uses 
 growing on quartz]]

Many lichen are sensitive to changes around them. For this reason, they are used by scientists to show changes in the environment, such as air pollution, ozone depletion, and metal contamination. Lichens have also been used to make dyes, perfumes, and herbal medicines.

Some lichen produce natural antibiotics that kill bacteria. Humans have used these compounds as natural antibiotics. Extracts from many *Usnea* species were used to treat wounds in Russia in the mid-twentieth century.

The map lichen *Rhizocarpon geographicum* grows extremely slowly, and can reach an age of over 1000 years. It may be used for dating: the diameter of the largest lichen of a species on a rock surface shows the time that the rock has been exposed.

Lichen are the main winter food for reindeer. The deer can get at shrubby lichen under the snow. In summer they have much more choice of food. Some lichen are eaten by humans. A kind of lichen, which is called *Iwatake* in Japanese and *Seogi* in Korean, is collected from cliffs, and used in various Korean and Japanese foods. Lichen have a high level of acid. Care must be taken, as there are at least two kinds of lichen that are toxic.

The bacterial or algal partners produce pigments that absorb sunlight in photosynthesis. There are reports dating back almost 2000 years of purple and red colours being extracted from lichens. Lichens of the family Roccellaceae are commonly called 'orchella weed' . They have been important in history, because they could supply orcein, a dye.  *Rocella tinctoria* is the source of the pH indicator litmus. Orcein is got by boiling the lichen, and litmus is produced by adding ammonia and air. Orcein is also used as a food dye, with the E number E121.

Today, dyes based on lichens have largely been replaced by synthetic ones.

## References 
## More reading 
*Ahmadjian V. 1993. *The lichen symbiosis*. Wiley, New York. .
*Gilbert, Oliver 2000. *Lichens*. New Naturalist series #86, HarperCollins, London.