Lichen

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For other uses, see Lichen (disambiguation).
Lichen-covered tree: Grey, leafy (foliose) Parmotrema perlatum on upper half of trunk; yellowy-green Flavoparmelia caperata on middle and lower half and running up the extreme right side; and the fruticose Ramalina farinacea

A lichen is an organism that arises from green algae or blue-green algae (cyanobacteria) living among filaments of a fungus, in a symbiotic (mutually helpful) relationship.[1][2][3] The properties of the whole combination are very different from the sum of the properties of the parts living by themselves. The combination has a very different form (morphology), physiology, and biochemistry. The fungus benefits from the algae or cyanobacteria because they produce food by photosynthesis. The algae or cyanobacteria benefit by being protected from the environment by the filaments of the fungus, which also gather nutrients from the environment and (usually) provide an anchor to it. Lichens may grow like crusts on a surface (crustose), or be bushy (fruticose), leafy (foliose), scaly (squamulose), or have other growth forms.[4] A macrolichen is a lichen that is either foliose or fruticose. A microlichen is everything else.[1] Here, "macro" and "micro" do not refer to size, but to the growth form.[1] Sometimes the growth form of lichens superficially looks like moss, and the common name may contain the word "moss" (e.g., "Reindeer moss"), but lichens are not related to mosses.

Lichens occur in a very wide range of conditions. They are abundant growing on bark, leaves, and hanging from branches "living on thin air" (epiphytes) in rain forests and in temperate woodland. They grow on bare rock, walls, gravestones, roofs, exposed soil surfaces, and in the soil as part of a biological soil crust. They can survive in some of the most extreme environments on Earth: arctic tundra, hot dry deserts, rocky coasts, and toxic slag heaps. They can even live inside solid rock, growing between the grains. Some lichens (vagrant lichens) don't grow on anything, living out their lives blowing about the environment.[1] They are pioneer species, among the first living things on bare rock or areas denuded of life by a disaster.[1] They break rock down in the first step to becoming soil.[3] In some regions, they make up a significant part of the biomass.[citation needed] It is estimated that 6% of Earth's land surface is covered by lichen.[5]

Lichens may be long-lived, with some considered to be among the oldest living things.[3][6] Lichens often have a regular but very slow growth rate, often grow less than a millimeter per year.[3] Since they are often the first to grow in an area after a disaster such as a landslide, measuring the total growth can be used to estimate the date of the disaster. Many lichens are vulnerable to environmental disturbance, which is used in assessing air pollution,[7][8][9] ozone depletion, and metal contamination. Lichens have been used in making dyes, as fixatives in perfumes,[10] and in traditional medicines. Few lichen species are eaten by insects or larger animals.[citation needed] Lichens are parasitized by other fungi (lichenicolous fungi), which are a different species from the fungus living inside the lichen.[11]

Lichens are said to be different "species", but what is meant by "species" is different from what is meant for plants, animals, and fungi, for which "species" implies a common ancestral lineage. Lichens are really combinations of species from two or three different biological kingdoms, so there is no common lineage. By convention, lichens are classified according to the species of their fungus component, not by the species of the algae and/or cyanobacteria growing in them. Lichenized fungus may refer to the entire lichen, or to the fungus growing in it. There are about 20,000 known species of lichenized fungus by this classification system.

Classification[edit]

There are about 20,000 known lichen species.[12] But what is meant by "species" is different from what is meant by biological species in plants, animals, or fungi, where being the same species implies that there is a common ancestral lineage.[12] Because lichens are combinations of members of two or even three different biological kingdoms, these components must have a different ancestral lineage from each other. By convention, lichens are still called "species" anyway, and are classified according to the species of their fungus, not the species of the algae or cyanobacteria. Lichens are given the same scientific name (binomial name) as the fungus in them, which may cause some confusion. The algae bears its own scientific name, which has no relationship to the name of the the lichen or fungi.[13]

Depending on context, lichenized fungus may refer to the entire lichen, or to the fungus when it is in the lichen, which can be grown in culture in isolation from the algae or cyanobacteria. Some algae and cyanobacteria[citation needed] are found naturally living outside of the lichen, but in these cases, have very different properties than those of the lichen.

There is still debate as to the appropriateness of giving the same binomial name to the fungus and to the lichen that combines that fungus with an algae or cyanobacterium. This is especially the case when combining the same fungus with different algae or cyanobacteria produces dramatically different lichen organisms, which would be considered different species by any measure other than the DNA of the fungal component, and which have their own independent evolutionary history.[1][14] If the whole lichen produced by the same fungus growing in association with different algae or cyanobacteria, were to be classified as different "species", the number of "lichen species" would be greater.

Overview[edit]

The fungal partner is called the mycobiont. The photosynthetic partner, algae or cyanobacteria, is called the photobiont. The body of a lichens that does not contain reproductive parts of the fungus is called the thallus. The thallus is different from those of either the fungus or alga growing separately. The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations. In many species the fungus penetrates the algal cell wall, forming penetration pegs or haustoria similar to those produced by pathogenic fungi.[15][16] Lichens are capable of surviving extremely low levels of water content (poikilohydric).[17] However, the re-configuration of membranes following a period of dehydration requires several minutes at least.

The algal or cyanobacterial cells are photosynthetic, and as in plants they reduce atmospheric carbon dioxide into organic carbon sugars to feed both symbionts. Both partners gain water and mineral nutrients mainly from the atmosphere, through rain and dust. The fungal partner protects the alga by retaining water, serving as a larger capture area for mineral nutrients and, in some cases, provides minerals obtained from the substrate. If a cyanobacterium is present, as a primary partner or another symbiont in addition to green alga as in certain tripartite lichens, they can fix atmospheric nitrogen, complementing the activities of the green alga.

Although strains of cyanobacteria found in various cyanolichens are often closely related to one another, they differ from the most closely related free-living strains.[18] The lichen association is a close symbiosis. It extends the ecological range of both partners but is not always obligatory for their growth and reproduction in natural environments, since many of the algal symbionts can live independently. A prominent example is the alga Trentepohlia which forms orange-coloured populations on tree trunks and suitable rock faces. Lichen propagules (diaspores) typically contain cells from both partners, although the fungal components of so-called "fringe species" rely instead on algal cells dispersed by the “core species.”[19]

Lichen associations may be examples of mutualism, commensalism or even parasitism,[citation needed] depending on the species. Cyanobacteria in laboratory settings can grow faster when they are alone rather than when they are part of a lichen.

In tests, lichen survived and showed remarkable results on the adaptation capacity of photosynthetic activity within the simulation time of 34 days under Martian conditions in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center (DLR).[20][21]

Symbionts[edit]

Schematic cross section of lichen, a symbiosis between green algae and a fungus. 1. Thick layers of hyphae, called the cortex 2. Green algae 3. Loosely packed hyphae 4. Anchoring hyphae called rhizines.
"Lichens are fungi that have discovered agriculture" - Trevor Goward[3]

Living as a symbiont in a lichen appears to be a successful way for a fungus to derive essential nutrients, as about 20% of all fungal species have acquired this mode of life. The fungal partner may be an Ascomycete or Basidiomycete.[12] Common algal partners are Trebouxia, Pseudotrebouxia, or Myrmecia. Common cyanobacterium partners include are Nostoc[15] or Scytonema.[12]

The largest number of lichenized fungi occur in the Ascomycota, with about 40% of species forming such an association.[13] Some of these lichenized fungi occur in orders with nonlichenized fungi that live as saprotrophs or plant parasites (for example, the Leotiales, Dothideales, and Pezizales). Other lichen fungi occur in only five orders in which all members are engaged in this habit (Orders Graphidales, Gyalectales, Peltigerales, Pertusariales, and Teloschistales). Lichenized and nonlichenized fungi can even be found in the same genus or species.[citation needed] Overall, about 98% of lichens have an ascomycetous mycobiont. Next to the Ascomycota, the largest number of lichenized fungi occur in the unassigned fungi imperfecti. Comparatively few Basidiomycetes are lichenized, but these include agarics, such as species of Lichenomphalia, clavarioid fungi, such as species of Multiclavula, and corticioid fungi, such as species of Dictyonema.

The autotrophic symbionts occurring in lichens are a wide variety of simple, photosynthetic organisms commonly and traditionally known as algae. These symbionts include both prokaryotic and eukaryotic organisms. Approximately 100 species of photosynthetic partners from 40 genera and five distinct classes (prokaryotic: Cyanophyceae; eukaryotic: Trebouxiophyceae, Phaeophyceae, Chlorophyceae) have been found to associate with the lichen-forming fungi.[22] The prokaryotes belong to the Cyanobacteria, whose representatives are often called bluegreen algae. The bluegreen algae occur as symbionts in about 8% of the known lichens. The most commonly occurring genus is Nostoc.[23] The majority of the lichens contain eukaryotic autotrophs belonging to the Chlorophyta (green algae) or to the Xanthophyta (yellow-green algae). About 90% of all known lichens have a green alga as a symbiont, and among these, Trebouxia is the most common genus, occurring in about 40% of all lichens. The second most commonly represented green alga genus is Trentepohlia. Overall, about 100 species are known to occur as autotrophs in lichens. All the algae are probably able to exist independently in nature as well as in the lichen.[23]

A particular fungus species and algal species are not necessarily always associated together in a lichen. One fungus, for example, can form lichens with a variety of different algae. The thalli produced by a given fungal symbiont with its differing partners will be similar, and the secondary metabolites identical, indicating that the fungus has the dominant role in determining the morphology of the lichen. Further, the same algal species can occur in association with different fungal partners. Lichens are known in which there is one fungus associated with two or even three algal species. Rarely, the reverse can occur, and two or more fungal species can interact to form the same lichen.[23]

Both the lichen and the fungus partner bear the same scientific name, and the lichens are being integrated into the classification schemes for fungi. The alga bears its own scientific name, which bears no relationship to that of the lichen or fungi.[13]

Fungus component[edit]

Depending on context, the entire lichen, or just the fungus that is part of the lichen. Both the lichen and the fungus that is a part of the lichen are currently (2014) given the same species name, which creates an ambiguity. An example of when "lichenized fungus" refers to just the fungus is when the fungus is grown in culture without a phycobiont. An example where "lichenized fungus" refers to the entire lichen is in a list of classified lichens.

Some fungi can only be found living on lichens (obligate parasites), but are not considered part of the lichen. These are referred to as lichenolous fungi.

Photosynthetic component[edit]

The photosynthetic component of a lichen is called the photobiont or phycobiont.[24] Sometimes the photobiont is a green algae (chlorophyta), sometimes a blue-green aglae (cyanobacteria, not really an algae), and sometimes both. The layer of tissue containing the cells of the photobiont is called the "photobiontic layer".[24]

"Clorococcoid" means a green algae (Chlorophyta) that has single cells that are globose, which is common in lichens.[25] This was once classified in the order Chlorococcales, which you may find stated in older literature, but new DNA data shows many independent lines of evolution exist among this formerly large taxonomic group. Chlorococcales is now a relatively small order and may no longer include any lichen photobionts. Trebuxia, once included here, is now considered to be in a separate class, Trebouxiophyceae. "trebouxioid" refers to members of this class or algae resembling them.

"Trebouxioid" means a clorococcoid green algae photobiont is in the genus Trebouxia, or resembles a member of that genus, and is therefore presumably a member of the class Trebouxiophyceae.[24]

Cyanolichens[edit]

A cyanolichen is a lichen with a cyanobacteria as its main photosynthetic component (photobiont).[25] Many cyanolichens are small and black, and have limestone as the substrate. Another cyanolichen group, the jelly lichens ( e.g., from the genera Collema or Leptogium) are large and foliose (e.g., species of Peltigera, Lobaria, and Degelia. These lichen species are grey-blue, especially when dampend or wet. Many of these characterize the Lobarion communities of higher rainfall areas in western Britain, e.g., in the Celtic Rainforest.

Morphology and structure[edit]

Caperat lichen Flavoparmelia caperata (Parmelia caperata) on a branch of a tree
Xanthoria parietina Common orange lichen
Crustose lichens on a wall

Some lichens have the aspect of leaves (foliose lichens); others cover the substrate like a crust (crustose lichens) (illustration, right), others such as the genus Ramalina adopt shrubby forms (fruticose lichens), and there are gelatinous lichens such as the genus Collema.[26]

Although the form of a lichen is determined by the genetic material of the fungal partner, association with a photobiont is required for the development of that form. When grown in the laboratory in the absence of its photobiont, a lichen fungus develops as an undifferentiated mass of hyphae. If combined with its photobiont under appropriate conditions, its characteristic form emerges, in the process called morphogenesis.[27] In a few remarkable cases, a single lichen fungus can develop into two very different lichen forms when associating with either a green algal or a cyanobacterial symbiont. Quite naturally, these alternative forms were at first considered to be different species, until they were found growing in a conjoined manner.

There is evidence to suggest that the lichen symbiosis is parasitic or commensalistic, rather than mutualistic.[28] The photosynthetic partner can exist in nature independently of the fungal partner, but not vice versa. Furthermore, photobiont cells are routinely destroyed in the course of nutrient exchange. The association is able to continue because reproduction of the photobiont cells matches the rate at which they are destroyed.[28]

Under magnification, a section through a typical foliose lichen thallus reveals four layers of interlaced fungal filaments. The uppermost layer is formed by densely agglutinated fungal hyphae building a protective outer layer called the cortex, which can reach several hundred μm in thickness.[29] This cortex may be further topped by an epicortex 0.6-1μm thick in some Parmeliaceae, which may be with or without pores, and is secreted by cells—it is not itself cellular.[29] In lichens that include both green algal and cyanobacterial symbionts, the cyanobacteria may be held on the upper or lower surface in small pustules called cephalodia. Beneath the upper cortex is an algal layer composed of algal cells embedded in rather densely interwoven fungal hyphae. Each cell or group of cells of the photobiont is usually individually wrapped by hyphae, and in some cases penetrated by an haustorium. Beneath this algal layer is a third layer of loosely interwoven fungal hyphae without algal cells. This layer is called the medulla. Beneath the medulla, the bottom surface resembles the upper surface and is called the lower cortex, again consisting of densely packed fungal hyphae. The lower cortex often bears rootlike fungal structures known as rhizines, which serve to attach the thallus to the substrate on which it grows. Lichens also sometimes contain structures made from fungal metabolites, for example crustose lichens sometimes have a polysaccharide layer in the cortex. Although each lichen thallus generally appears homogeneous, some evidence seems to suggest that the fungal component may consist of more than one genetic individual of that species. This seems to also be true of the photobiont species involved.

The rhizene is the "root" that anchors the lichen to the substrate in a foliose lichen.[1]

A podetium (plural podetia) is a lichenized stem-like structure of an apothecium rising from the primary body of the thallus.[24] Since it is part of the reproductive tissue, it is not considered part of the thallus.[24] Since it is part of the reproductive tissue, it is not considered part of the thallus.[24] The podetium may be branched, and sometimes cup-like.Since it is part of the reproductive tissue, it is not considered part of the thallus.[24] They usually bear the pycnidia or apothecia or both.[24]

Growth form, thallus types[edit]

The Fruticose thallus of Usnea filipendula forms hanging tufts
The foliose thallus of Parmotrema tinctorum is leafy.
The crustose thallus of Caloplaca marina covers this rock like paint.
The crustose placodioid form of Caloplaca thallincola radiates outward and the edges peel up from the substrate, but lack a cortex on the underside, unlike foliose lichens.

Lichens may or may not have a surface layer of tissue that is like a "skin", called the cortex. The cortext is an outer layer of the thallus tissue that covers the medulla. When part of a lichen has a cortex it is called corticate.[25] If it lacks a cortext it is called ecorticate.[30]

Growth forms are categorized by the form of the thallus, the part of the body of a lichen that is not involved in sexual reproduction, or the "vegetative" part. This includes the fungus (mycobiont) and the algal cells (photobiont) they surround, but does not include any part with sexual fruit bodies of the mycobiont. There are a wide range of forms and structures. As with non-floral parts of plants, classifications based on morphology may say more about convergent evolution in a common environment than about having common ancestry, and there are gray areas and borderline cases. A species usually has the same overall growth form wherever it is found, but not always.[1] Categories of growth forms are those 1. having multiply branched tufts, 2. having leaf-like structures, 3. forming thin flat crusts, or flat crusts peeling up like leaves at the outer edges, 4. having tiny plate-like structures that sometimes overlap like scales, 5. having masses of stringy filament-like structures, 6. having the appearance of teased wool, 7. forming crusts so thin that they are better described as powdery layers of tiny granules, or 8. having barely any visible or distinguishable thallus at all:

  • 1. fruticose (branched, 3-dimiensional), e.g., Cladonia evansii, C. subtenuis, and Usnea australis - The thallus is extended up, or hangs down, in a branched or tufted structure.
  • 2. foliose (leafy, 2-dimensional planes distributed in 3-D), e.g., Hypogymnia physodes - The thallus generally forms flat, leaf-like lobes that are not firmly bonded to the substrate, with the underside having its own cortex.
  • 3. Crustose lichens (paint-like, flat, 2-dimensional), e.g., Caloplaca marina - The thallus lies flat and tightly bound at all points to the substrate, like it is painted on.
  • 3a. Crustose placodioid (flattened and radiating out from a center, possibly raising up to being leaf-like at the outer edges) - The thallus structure radiates out, with the outer edges peeling up from the substrate at the tips. These may peel up so much as to appear like foliose lichens, but unlike foliose lichens, they lack a cortex under the edges that lift from the substrate.
  • 4. squamulose (scale-like or platelet-like), e.g., Normandina pulchella - The thallus is composed of mostly flat laying, overlapping, scale-like plates ("squamules"), which may become leaf-like in form where they are not attached to the substrate, but which do not have a lower cortex on the underside. Some lichenologists consider squamulous lichens to be a special kind of crustose lichen.[1]
  • 5. Filamentous (hairlike), e.g., Ephebe lanata, Cystocoleus ebeneus - The thallus forms mostly nonbranching hair-like structures that may look overall like matted hair.
  • 6. Byssoid (whispy), e.g., Coenogonium implexum - Whispy like teased cotton or teased wool.[1]
  • 7. leprose (powdery), e.g., Lepraria incana - The thallus is a layer so thin that it is almost like a powder stuck to the substrate.
  • 8. Structureless - Lichens may not have a recognizable thallus structure, including gelatinous lichens where the cyanobacteria produce a polysaccharide that absorbs and retains water.

Another way to classify lichens is by the presence or absence of a cortex. Fruticose lichens have one cortex encircling the branches, even flattened, leaf-like forms; foliose lichens have a separate upper cortex and lower cortex; crustose, placodioid and squamulose lichens have an upper cortex but do not have a lower cortex; and leprose and gelatinous lichens lack any cortex.

History[edit]

Simon Schwendener proposed the dual theory of lichens in 1867.
"Lichenes" fancifully drawn by Ernst Haeckel to emphasize his ideas of symmetry in his Artforms of Nature, 1904

Although lichens had been recognized as organisms for quite some time, it was not until 1867, when Swiss botanist Simon Schwendener proposed his dual theory of lichens, that the true nature of the lichen association began to emerge.[31] Schwendener's hypothesis, which at the time lacked experimental evidence, arose from his extensive analysis of the anatomy and development in lichens, algae, and fungi using a light microscope. Many of the leading lichenologists at the time, such as James Crombie and Nylander, rejected Schwendener's hypothesis because the common consensus was that all living organisms were autonomous.[31]

Other prominent biologists, such as Heinrich Anton de Bary, Albert Bernhard Frank, Melchior Treub and Hermann Hellriegel were not so quick to reject Schwendener's ideas and the concept soon spread into other areas of study, such as microbial, plant, animal and human pathogens.[31][32] When the complex relationships between pathogenic microorganisms and their hosts were finally identified, Schwendener's hypothesis began to gain popularity. Further experimental proof of the dual nature of lichens was obtained when Eugen Thomas published his results in 1939 on the first successful re-synthesis experiment.[31]

Reproduction and dispersal[edit]

Thalli and apothecia on a foliose lichen
Xanthoparmelia sp.

Many lichens reproduce asexually, either by a piece breaking off and growing on its own (vegetative reproduction) or through the dispersal of diaspores containing a few algal cells surrounded by fungal cells.[1] Soredia (singular soredium) are small groups of algal cells surrounded by fungal filaments that form in structures called soralia, from which the soredia can be dispersed by wind.[1] Another form of diaspore is found in isidia, branched, spiny, elongated, outgrowths from the thallus that break off for mechanical dispersal.[1] Fruticose lichens in particular can easily fragment.[citation needed] Because of the relative lack of differentiation in the thallus, the line between diaspore formation and vegetative reproduction is often blurred. Many lichens break up into fragments when they dry, dispersing themselves by wind action, to resume growth when moisture returns.[33][34]

Only the fungal partner in a lichen reproduces sexually.[1]

Many lichen fungi reproduce sexually like other fungi, producing spores formed by meiosis and fusion of gametes. Following dispersal, such fungal spores must meet with a compatible algal partner before a functional lichen can form. This may be a common form of reproduction in basidiolichens, which form fruiting bodies resembling their nonlichenized relatives. Among the ascolichens, spores are produced in spore-producing bodies, the three most common spore body types being raised discs called apothecia (singular apothecium), bottle-like cups with a small hole at the top called perithecia (singular perithecium), and pycnidia, shaped like perithecia but without asci (an ascum is the structure that contains and releases the sexual spores in fungi of the Ascomycota).[35] Many lichens have apothecia that are visible to the naked eye.[1]

Most lichens produce abundant sexual structures; many species appear to disperse only by sexual spores.[36] For example, the crustose lichens Graphis scripta and Ochrolechia parella produce no symbiotic vegetative propagules. Instead, the lichen-forming fungi of these species reproduce sexually by self-fertilization (i.e. they are homothallic). This breeding system may enable successful reproduction in harsh environments.[36]

Growth and longevity[edit]

Lichenometry[edit]

Main article: 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,[37] 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.[38][39]:9 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[citation needed] 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.[40] Lichenometry is especially useful for dating surfaces less than 500 years old, as radiocarbon dating techniques are less accurate over this period.[41] The lichens most commonly used for lichenometry are those of the genera Rhizocarpon (e.g. the species Rhizocarpon geographicum) and Xanthoria.

Ecology[edit]

Lichens must compete with plants for access to sunlight, but because of their small size and slow growth, they thrive in places where higher plants have difficulty growing. Lichens are often the first to settle in places lacking soil, constituting the sole vegetation in some extreme environments such as those found at high mountain elevations and at high latitudes.[42] Some survive in the tough conditions of deserts, and others on frozen soil of the Arctic regions.[43]

A major ecophysiological advantage of lichens is that they are poikilohydric (poikilo- variable, hydric- relating to water), meaning that though they have little control over the status of their hydration, they can tolerate irregular and extended periods of severe desiccation. Like some mosses, liverworts, ferns, and a few "resurrection plants", upon desiccation, lichens enter a metabolic suspension or stasis (known as cryptobiosis) in which the cells of the lichen symbionts are dehydrated to a degree that halts most biochemical activity. In this cryptobiotic state, lichens can survive wider extremes of temperature, radiation and drought in the harsh environments they often inhabit.

Lichens do not have roots and do not need to tap continuous reservoirs of water like most higher plants, thus they can grow in locations impossible for most plants, such as bare rock, sterile soil or sand, and various artificial structures such as walls, roofs and monuments. Many lichens also grow as epiphytes (epi- on the surface, phyte- plant) on plants, particularly on the trunks and branches of trees. When growing on plants, lichens are not parasites; they do not consume any part of the plant nor poison it. Some ground-dwelling lichens, such as members of the subgenus Cladina (reindeer lichens), however, produce allelopathic chemicals which leach into the soil and inhibit the germination of plant seeds and growth of young plants. Stability (that is, longevity) of their substrate is a major factor of lichen habitats. Most lichens grow on stable rock surfaces or the bark of old trees, but many others grow on soil and sand. In these latter cases, lichens are often an important part of soil stabilization; indeed, in some desert ecosystems, vascular (higher) plant seeds cannot become established except in places where lichen crusts stabilize the sand and help retain water.

Pine forest with Cladonia lichen ground-cover

The European Space Agency has discovered that lichens can survive unprotected in space. In an experiment led by Leopoldo Sancho from the Complutense University of Madrid, two species of lichen—Rhizocarpon geographicum and Xanthoria elegans—were sealed in a capsule and launched on a Russian Soyuz rocket on 31 May 2005. Once in orbit the capsules were opened and the lichens were directly exposed to the vacuum of space with its widely fluctuating temperatures and cosmic radiation. After 15 days the lichens were brought back to earth and were found to be in full health with no discernible damage from their time in orbit.[44][45]

When growing on mineral surfaces, some lichens slowly decompose their substrate by chemically degrading and physically disrupting the minerals, contributing to the process of weathering by which rocks are gradually turned into soil. While this contribution to weathering is usually benign, it can cause problems for artificial stone structures. For example, there is an ongoing lichen growth problem on Mount Rushmore National Memorial that requires the employment of mountain-climbing conservators to clean the monument.

Lichens may be eaten by some animals, such as reindeer, living in arctic regions. The larvae of a number of Lepidoptera species feed exclusively on lichens. These include Common Footman and Marbled Beauty. However, lichens are very low in protein and high in carbohydrates, making them unsuitable for some animals. Lichens are also used by the Northern Flying Squirrel for nesting, food, and a water source during winter.

Lichens and soils[edit]

Lichens may be important in contributing nitrogen to soils in some deserts through being eaten, along with their rock substrate, by snails, which then defecate, putting the nitrogen into the soils.[46] Lichens help bind and stabilize soil sand in dunes.[1] In deserts and semi-arid areas, lichens are part of extensive, living biological soil crusts, essential for maintaining the soil structure.[1]

Substrates and habitats[edit]

Substrates[edit]

A lichen that grows on rock is called a saxicolous lichen.[24] Crustose lichens that grow immersed inside rock, with only their fruiting bodies exposed to the air, are called endolithic lichens.[47]

A lichen that grows on bark is called a corticolous lichen.[25] A lichen that grows on wood from which the bark has been stripped is called a lignicolous lichen.[30] Lichens that grow immersed inside plant tissues are called endophloidic lichens or endophloidal lichens.[47]

Umbillicate lichens are attached to the substrate at only one point.[47]

A vagrant lichen is not attached to a substrate at all, and lives its life being blown around by the wind.

Habitats[edit]

Air pollution[edit]

Some lichens, like the foliose Lobaria pulmonaria, are sensitive to air pollution.

If lichens are exposed to air pollutants at all times, without any deciduous parts, they are unable to avoid the accumulation of pollutants. Also lacking stomata and a cuticle, lichens may absorb aerosols and gases over the entire thallus surface from which they may readily diffuse to the photobiont layer.[48] Because lichens do not possess roots, their primary source of most elements is the air, and therefore elemental levels in lichens often reflect the accumulated composition of ambient air. The processes by which atmospheric deposition occurs include fog and dew, gaseous absorption, and dry deposition.[49] Consequently, many environmental studies with lichens emphasize their feasibility as effective biomonitors of atmospheric quality.[48][50][51][52][53]

Not all lichens are equally sensitive to air pollutants, so different lichen species show different levels of sensitivity to specific atmospheric pollutants.[54] The sensitivity of a lichen to air pollution is directly related to the energy needs of the mycobiont, so that the stronger the dependency of the mycobiont on the photobiont, the more sensitive the lichen is to air pollution.[55] Upon exposure to air pollution, the photobiont may use metabolic energy for repair of cellular structures that would otherwise be used for maintenance of photosynthetic activity, therefore leaving less metabolic energy available for the mycobiont. The alteration of the balance between the photobiont and mycobiont can lead to the breakdown of the symbiotic association. Therefore, lichen decline may result not only from the accumulation of toxic substances, but also from altered nutrient supplies that favor one symbiont over the other.[48]

This interaction between lichens and air pollution has been used as a means of monitoring air quality since 1859, with more systematic methods developed by W. Nylander in 1866.[1]

Evolution and paleontology[edit]

The evolution of lichens and the phylum Ascomycota is complex and not well understood, but because there are fifteen different classes of Ascomycetes, scientists generally believe that different lichens have evolved independently from one another through analogous evolution. Lichenized fungi have continued to evolve, developing differently from those that do not form lichens.[citation needed]

Lichenization is an ancient nutritional strategy for fungi. The extreme habitats that lichens inhabit are not ordinarily conducive to producing fossils.[56] The oldest fossil lichens in which both symbiotic partners have been recovered date to the Early Devonian Rhynie chert, about 400 million years old.[57] The slightly older fossil Spongiophyton has also been interpreted as a lichen on morphological[58] and isotopic[59] grounds, although the isotopic basis is decidedly shaky.[60] It has been suggested—although not yet proven—that the even older fossil Nematothallus was a lichen.[61]

It has also been claimed that Ediacaran fossils were lichens,[62] though this claim is controversial.[63] Additional evidence has been marshalled for a lichen interpretation of Dickinsonia.[61] Lichen-like fossils consisting of coccoid cells and thin filaments, preserved in marine phosphorite of the Doushantuo Formation in southern China. These fossils are thought to be 551 to 635 million years old (belonging to the Neoproterozoic era).[64] Discovery of these fossils suggest that fungi developed symbiotic partnerships with photoautotrophs long before the evolution of vascular plants. Winfrenatia, an early zygomycetous lichen symbiosis that may have involved controlled parasitism, is an impression found in Scotland, belonging to the early Devonian times.[65] There are also several examples of fossilized lichens embedded in amber. The fossilized Anzia is found in pieces of amber in northern Europe and dates back approximately 40 million years.[66] Fossilized Lobaria comes from Trinity County in northern California, USA and dates back to the early to middle Miocene.[67]

In 1995, Gargas and colleagues proposed that there were at least five independent origins of lichenization; three in the basidiomycetes and at least two in the Ascomycetes.[68] However, Lutzoni et al. (2000) indicate that lichenization probably evolved earlier and was followed by multiple independent losses. Some non-lichen-forming fungi may have secondarily lost the ability to form a lichen association. As a result, lichenization has been viewed as a highly successful nutritional strategy.[69][70]

Lichens were a component of the early terrestrial ecosystems, and the estimated age of the oldest terrestrial lichen fossil is 400 Ma.[71][72] Recent (2009) studies suggest that the ancestral ecological state of the Ascomycota was saprobism, and that independent lichenization events have occurred multiple times.[73]

Taxonomy and classification[edit]

Lichens are named based on the fungal component, which plays the primary role in determining the lichen's form. The fungus typically comprises the majority of a lichen's bulk, though in filamentous and gelatinous lichens this is not always the case. The lichen fungus is typically a member of the Ascomycota—rarely a member of the Basidiomycota, and then termed basidiolichens to differentiate them from the more common ascolichens. Formerly, some lichen taxonomists placed lichens in their own division, the Mycophycophyta, but this practice is no longer accepted because the components belong to separate lineages. Neither the ascolichens nor the basidiolichens form monophyletic lineages in their respective fungal phyla, but they do form several major solely or primarily lichen-forming groups within each phylum.[74] Even more unusual than basidiolichens is the fungus Geosiphon pyriforme, a member of the Glomeromycota that is unique in that it encloses a cyanobacterial symbiont inside its cells. Geosiphon is not usually considered to be a lichen, and its peculiar symbiosis was not recognized for many years. The genus is more closely allied to endomycorrhizal genera.

The following table lists the orders and families of fungi that include lichen-forming species.

Human use[edit]

Iwatake (Umbilicaria esculenta) gathering at Kumano in Kishū, by Hiroshige II
Main article: Ethnolichenology

Food[edit]

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 removed before eating. Very few lichens are poisonous, but those high in vulpinic acid or usnic acid are toxic.[75] 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 variety of traditional Korean and Japanese foods.

Biodegradation[edit]

Lichens have been shown to degrade polyester resins, as can be seen in archaeological sites in the Roman city of Baelo Claudia Spain.[76] Lichens can accumulate several environmental pollutants such as lead, copper, and radionuclides.[77]

Antibiotic Properties[edit]

Lichens produce metabolites proven useful in the medical community.[78] Most metabolites produced by lichens are structurally and functionally similar to broad-spectrum antibiotics while few are associated respectively to antiseptic similarities.[79] These organic acids are the metabolic byproducts of Crassulacean acid metabolism, the means of photosynthesis by lichens.[80]

Usnic acid is the most commonly studied metabolite produced by lichens and has been associated with the suppression of tuberculosis.[81] It has also proven bactericidal against Escherichia coli and Staphylococcus aureus and is considered an antimicrobial agent. It is still unclear if the antimicrobial processes derived from lichens are strictly due to their metabolites or their symbiotic relationship with the fungi that grows on it.[82]

Other uses[edit]

Many lichens produce secondary compounds, including pigments that reduce harmful amounts of sunlight and powerful toxins that reduce herbivory or kill bacteria. These compounds are very useful for lichen identification, and have had economic importance as dyes such as cudbear or primitive antibiotics.

There are reports dating almost 2000 years old of lichens being used to make purple and red dyes.[83] 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. The pH indicator litmus is a dye extracted from the lichen Roccella tinctoria by boiling. In the Highlands of Scotland, traditional dyes for Harris tweed[1] 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".[84]

Extracts from many Usnea species were used to treat wounds in Russia in the mid-twentieth century.[citation needed] Historically in Europe, Lobaria pulmonaria was collected in large quantities as "Lungwort" and, due to its lung-like appearance, was sold as a cure for lung diseases. Similarly Peltigera leucophlebia was used as a supposed cure for thrush, due to the resemblance of its cephalodia to the appearance of the disease.[15]

The Talmud (Sanhedrin 19b) translates the Hebrew word "vayilafeth" in Ruth 3:8 as referring to Ruth entwining herself around Boaz like lichen.

The substance olivetol is found to be naturally present in certain species[which?] of lichens. This is a property they share with the cannabis plant, which internally produces the related substance olivetolic acid (before using it to biosynthesise tetrahydrocannabinol (THC)).[85]

Lichen is used in model railroading[86] and other modeling hobbies as a material for making miniature trees and shrubs.

An article published in PLoS ONE indicates that a serine protease isolated from certain species of lichen degrades prions.[87][88]

Identification methods[edit]

"Pd" refers to the outcome of the Pd test or is used as an abbrenviation for the chemical used in the test, para-phenylenediamine.[24] "Pd" refers to the outcome of the Pd test or is used as an abbrenviation for the chemical used in the test, para-phenylenediamine.[24] If putting a drop on a lichen turns an area bright yellow to orange, this helps identify it as belonging to either the genus Cladonia or Lecanora.[24]

Gallery[edit]

See also[edit]

References[edit]

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Further reading[edit]

  • Ahmadjian V. (1993). The Lichen Symbiosis. New York: John Wiley & Sons. ISBN 0-471-57885-1. 
  • Brodo, I.M., S.D. Sharnoff, and S. Sharnoff, 2001. Lichens of North America. Yale University Press, New Haven.
  • Gilbert, O. 2004. The Lichen Hunters. The Book Guild Ltd. England.
  • Haugan, Reidar; Timdal, Einar (1992). "Squamarina scopulorum (Lecanoraceae), a new lichen species from Norway". Nordic Journal of Botany 12 (3): 357–360. doi:10.1111/j.1756-1051.1992.tb01314.x. 
  • Hawksworth, D.L. and Seaward, M.R.D. 1977. Lichenology in the British Isles 1568 - 1975. The Richmond Publishing Co. Ltd., 1977.
  • Kershaw, K.A. Physiological Ecology of Lichens, 1985. Cambridge University Press Cambridge.
  • Kirk PM, Cannon PF, Minter DW, Stalpers JA. (2008). Dictionary of the Fungi. (10th ed.). Wallingford: CABI. ISBN 978-0-85199-826-8. 
  • Knowles M.C. (1929). "Lichens of Ireland". Proceedings of the Royal Irish Academy 38: 1–32. 
  • Purvis, O.W., Coppins, B.J., Hawksworth, D.L., James, P.W. and Moore, D.M. (Editors) 1992. The Lichen Flora of Great Britain and Ireland. Natural History Museum, London.
  • Sanders W.B. (2001). "Lichens: interface between mycology and plant morphology". BioScience 51 (12): 1025–1035. doi:10.1641/0006-3568(2001)051[1025:LTIBMA]2.0.CO;2. 
  • Seaward M.R.D. (1984). "Census Catalogue of Irish Lichens". Glasra 8: 1–32. 
  • Whelan, P. 2011. Lichens of Ireland. The Collins Press, Cork, Ireland.

External links[edit]