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What Is A Lichen??


- An Overview on the Biology of Lichens -
by Frank Bungartz

Liking Lichens | What is a Lichen? | Naming Lichens
Why study Lichens? | Further Reading



LIKING LICHENS

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Striking about lichens is the enormous variety of colors and shapes. Lichens are nature's artwork. They grow as inconspicuous crusts, broad lobes and filigreeing beards. They can be orange, deep, pale or bright yellow. They may be green with a yellow tinge, deep green or olive. Some are gray, pale or dark brown, others are mauve, ivory and even black. As colorful blotches they cover almost all of natures' canvas: They grow on the bark of a tree, on dead wood, on bare rock or barren soil. Why do they grow in such a variety of peculiar shapes? What causes all the splendid colors? How can they live virtually without any soil?

We find lichens in the tropical rainforest and the arctic cold deserts. They grow in temperate forests and in the coniferous forest of the Taiga. Thick carpets of lichens cover the arctic tundra and thin crusts embrace the desert soil. How can they survive and thrive in such diverse habitats?


WHAT IS A LICHEN?

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This question is quite difficult to answer. Lichens have long been mistaken as plants. Indeed their "ecological niches" are quite similar to the ecology of plants. Nevertheless they were mostly ignored by botanists and often regarded as odd or unusual organisms. Linnaus, the great Swedish botanist and the father of taxonomy did not hold them in high regard. He referred to them as "rustici pauperimi", the poor peasants of the plant kingdom.

Lichens are indeed peculiar. They appear as an entire organism but are actually composed of two very different partners. One partner is a fungus, also called the mycobiont. Modern classification regards the fungi as an entirely separate kingdom quite unlike plants and probably more closely related to animals. But lichens are not just fungi. The other partner in this relationship is called the photobiont. Photobionts may be green algae or they may be blue-green bacteria. The blue-green bacteria have traditionally also been called "algae". Ecologically they are indeed similar to other algae. They need light to photosynthesize. Blue-Greens are, however, true bacteria with a prokaryotic cell structure. Therefore they are more accurately referred to as cyanobacteria.

The lichen fungus usually provides structure and appearance of the whole lichen. Fungi are organisms which grow as fine filamentous threads called hyphae. In a lichen these hyphae are densely woven around photobiont cells to form the lichen thallus. The thallus is thus the "body" of a lichen composed of a mycobiont with a population of photobiont cells.

Living together in such a tight relationship is commonly referred to as symbiosis. The nature of this close relationship is still not quite resolved although lichens are commonly referred to as the standard example of a symbiosis.

In some lichen symbioses the lichen fungus will grow with green algae as well as cyanobacteria. The symbiosis is then referred to as a tripartite (= three partners) relationship. Only few examples are known of truly tripartite lichens form morphologically and anatomically different thalli either with cyanobacteria or green algae. More often green algae are the main photobiont and cyanobacteria are restricted to specific areas of the lichen thallus. These areas are called cephalodia and probably act as "nitrogen-fixation factories". Nevertheless most lichen fungi only associate with either green algae or cyanobacteria.

Commonly three different growth forms of lichens may be distinguished: crustose, foliose and fruticose. Crustose lichens grow on the rock as a tightly attached crust. They cannot be removed from this substrate without damage. Foliose lichens have an upper and a lower surface. They are flat like the leaf of a tree. Their lower surface can grow tightly on the substrate but they can usually be pried loose without much damage. Fruticose lichens my grow upright like a miniature shrub or they may be pendulous like a the hair of a beard. Various growth forms do not quite fit into this scheme and examples of transitions between different growths forms can easily be found. Lichen morphology and anatomy is quite complex and a variety of terms has been used to describe this variety.


HOW TO NAME A LICHEN

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Taxonomy is the science of naming organisms. In a biological classification system organisms should be named according to their evolutionary history. Very closely related organisms belong to the same species. Several species belong to the same genus, genera make up a family and families belong to orders, classes and finally kingdoms.

How can we classify a lichen? There are at least two very different organisms involved in the lichen symbiosis. Obviously fungi, green algae and cyanobacteria do not share the same evolution. Within the symbiosis all partners may have been suscepted to the same evolutionary forces. Partners within a symbiosis may thus have co-evolved as soon as the entered the symbiotic relationship. Nevertheless the different partners do not share most of their evolution. Which evolutionary history should therefore be referred to by the scientific name of the lichen?

Taxonomy lichens simply interprets lichens as a special form of a fungus which has found a way to exploit algal cells or cyanobacteria. Lichen names therefore apply to the fungus alone as well as to the lichen symbiosis, the way this fungus is associated with algae or cyanobacteria. This concept may be difficult to understand but there are several reasons why the taxonomy of lichens is largely the taxonomy of lichenized fungi. The mycobiont is an obligate symbiont. Under natural conditions lichen fungi have not been found free-living. The photobiont cells are facultative symbionts which an frequently be found independent of the lichen symbiosis. A huge diversity of lichen fungi can be distinguished whereas only very few species of photobionts have been found in a lichen symbiosis.


WHY STUDY LICHENS?

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The splendid colors and shapes, the peculiar aspects of the symbiosis, the diverse ecology - if all these reasons won't suffice, there are certainly more...

Unfortunately we still know very little about the ecological role of lichens and only in few ecosystems lichens are clearly the dominant organisms. In the Tundras lichen carpets form the most of the vegetation and in this ecosystems lichens therefore provide most of the primary biomass production. Even in systems where they are less dominant, their impact on nutrient cycling may be pronounced. Temperate rainforests and tropical cloud forests are virtually covered in lichens. Even in Mediterranean climates such as Southern California lichens can virtually cover entire trees. Studies suggest that their nutrient input to these ecosystems is quite considerable Lichens survive and actually thrive in the most extreme habitats such as cold or hot deserts. Among bacteria, algae and microfungi they are therefore often regarded as the primary colonizers of otherwise barren habitats. On surfaces which would otherwise remain uncolonized lichens may have quite an important impact on weathering and thus soil formation. On monuments they are often believed to have a deteriorating effect and studies how to preserve antique statues or ancient buildings try to understand the impact of lichens growing on these substrates.

Lichens are famous bio-indicators. They absorb most of their nutrients and water from the atmosphere and are thus highly susceptible to air pollution. Traditionally floristic surveys have been used to map various air pollution zones. More refined methods today combine these surveys with physiological measurements under standardized conditions.

Many lichen species are highly adapted to old-growth forests and thus have been used as ancient forest indicators. In the forests of the American Pacific Northwest lichen species have been correlated with undisturbed habitats of the endangered Spotted Owl. In Britain a highly specialized flora of rare woodland species can only be found in ancient Royal Hunting Forests where traditional forest management regimes have been employed over centuries.

Thallus measurements of crustose lichens can be used to obtain lichen growth rates. With lichenometric techniques the age of glacial retreats, land slides and even earthquakes can thus be specified. On the remote Easter Islands in the Pacific the age of stone sculptures has been dated using lichen growth. We now have a fairly good estimate when these islands were last occupied by humans.

The nutritional value of lichen thalli is low. However, in Britain and France grinded thalli of Lasallia pustulata have historically been used to stretch flour during times of scarcity. The biblical "Manna" may be identical with the vagrant desert lichen Sphaerothallia esculenta and even today another lichen, Umbilicaria esculenta, is used to prepare a Japanese delicacy. Native Americans made bread from Alectoria jubata.

Few lichens are poisonous. Letharia vulpina was used in Europe to kill wolves. Other lichen substances often cause allergies. In medieval times several lichen species were used as medical plants. According to the doctrine of signatures it was believed that the lichen Lungwort (Lobaria pulmonaria) will cure bronchitis and tuberculosis. Several lichen substances have indeed antibiotic characteristics and lichen substances like usnic acid are investigated for there cancerostatic characteristics. The lichen Icelandic Moss (Cetraria islandica) is sold in Europe as remedy for sore throats and bronchitis.

Many lichen substances can be used as natural dyes. The wool for Harris Tweed (from the outer Hebrides, Scotland) and Scottish Kilts was traditionally dyed with lichens. In North America Native Americans used Letharia vulpina and other lichen species to dye clothes and rugs. The pH indicator lithmus is derived from Rocella species. In medieval times this coastal lichen was harvested to produce the expensive "royal" purple called "Orseille". Only the kings and cardinals were allowed to wear clothes dyed with Orseille.

In the Mediterranean tons of Evernia prunstri and Pseudevernia furfuracea are harvested annually, pressed and sold as raw material for "mousse d'arbres", a mossy scent still used in expensive perfumes.


FURTHER READING

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NASH III, T.H. (ed.) (1996): Lichen Biology. Cambridge University Press, Cambridge, 303 pp.

PURVIS, O. W. (2000): Lichens. Smithsonian Institution Press, Washington, 112 pp.


 
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