Humans and Lichens

 

Food

 

Lichens are eaten by many different cultures across the world. Although

some lichens are only eaten in times of famine, others are a staple food or

even a delicacy. Two obstacles are often encountered when eating lichens:

lichen polysaccharides are generally indigestible to humans, and lichens

usually contain mildly toxic secondary compounds that should be remov-

ed before eating. Very few lichens are poisonous, but those high in vul-

pinic acid or usnic acid are toxic. Most poisonous lichens are yellow.

 

In the past Iceland moss (Cetraria islandica) was an important human

food in northern Europe, and was cooked as a bread, porridge, pudding,

soup, or salad. Wila (Bryoria fremontii) was an important food in parts

of North America, where it was usually pitcooked. Northern peoples in

North America and Siberia traditionally eat the partially digested reindeer

lichen (Cladina spp.) after they remove it from the rumen of caribou or

reindeer that have been killed. Rock tripe (Umbilicaria spp. and Lasalia

spp.) is a lichen that has frequently been used as an emergency food in

North America, and one species, Umbilicaria esculenta, is used in a var-

iety of traditional Korean and Japanese foods.

 

Lichenometry: Lichenometry is a technique used to determine the age

of exposed rock surfaces based on the size of lichen thalli. Introduced

by Beschel in the 1950s, the technique has found many applications. it

is used in archaeology, palaeontology, and geomorphology. It uses the

presumed regular but slow rate of lichen growth to determine the age of

exposed rock. Measuring the diameter (or other size measurement) of the

largest lichen of a species on a rock surface indicates the length of time

since the rock surface was first exposed. Lichen can be preserved on old

rock faces for up to 10,000 years, providing the maximum age limit of the technique, though it is most accurate (within 10% error) when applied to

surfaces that have been exposed for less than 1,000 years. Lichenometry

is especially useful for dating surfaces less than 500 years old, as radio-

carbon dating techniques are less accurate over this period.[107] The lich-

ens most commonly used for lichenometry are those of the Genera Rhizo-

carpon (e.g. the species Rhizocarpon geographicum) and Xanthoria.

 

Biodegradation

 

Lichens have been shown to degrade polyester resins, as can be seen in archaeological sites in the Roman city of Baelo Claudia, Spain. Lichens

can accumulate several environmental pollutants such as lead, copper,

and radionuclides.

 

Dyes

 

Many lichens produce secondary compounds, including pigments that re-

duce harmful amounts of sunlight and powerful toxins that reduce herb-

ivory or kill bacteria. These compounds are very useful for lichen identi-

fication, and have had economic importance as dyes such as cudbear or

primitive antibiotics.

 

The pH indicator (indicated acidic or basic) in the litmus test is a dye ex-

tracted from the lichen Roccella tinctoria by boiling.

 

In the Highlands of Scotland, traditional dyes for Harris tweed and other traditional cloths were made from lichens including the orange Xanthoria

parietina and the grey foliaceous Parmelia saxatilis common on rocks

known as "crottle".

 

There are reports dating almost 2000 years old of lichens being used to

make purple and red dyes. Of great historical and commercial significance

are lichens belonging to the family Roccellaceae, commonly called orchella

weed or orchil. Orcein and other lichen dyes have largely been replaced by synthetic versions.

 

Antibiotic metabolites

 

Lichens produce metabolites proven useful in the medical community.

Most metabolites produced by lichens are structurally and functionally

similar to broad-spectrum antibiotics while few are associated respectively

to antiseptic similarities. These organic acids are the metabolic byproducts

of Crassulacean acid metabolism, the means of photosynthesis by lichens.

 

Usnic acid is the most commonly studied metabolite produced by lichens

and has been associated with the suppression of tuberculosis. It has also

proven bactericidal against Escherichia coli and Staphylococcus aureus

and is considered an antimicrobial agent. It is still unclear if the antimicro-

bial processes derived from lichens are strictly due to their metabolites or

their symbiotic relationship with the fungi that grows on it.

 

Aesthetic appeal

 

Colonies of lichens may be spectacular in appearance, dominating the sur-

face of the visual landscape as part of the aesthetic appeal to paying visitors

of Yosemite National Park and Sequoia National Park. Orange and yellow

lichens add to the ambience of desert trees, rock faces, tundras, and rocky seashores. Intricate webs of lichens hanging from tree branches add a

mysterious aspect to forests. Fruticose lichens are used in model railroad-

ing and other modeling hobbies as a material for making miniature trees

and shrubs.

 

 

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