Lichen
/A lichen is a composite organism that emerges from algae[->0] or cyanobacteria[->1] (or both) living among filaments of a fungus[->2] in a mutually beneficial (symbiotic[->3]) relationship.[1][2][3] The whole combined life form has properties that are very different to properties of its component organisms. Lichens come in many colors, sizes, and forms. The properties are sometimes plant-like, but lichens are not plants. Lichens may grow like a tiny, leafless, branching shrub (fruticose[->4]), like it has leaves (foliose[->5]), like a crust of paint on a surface (crustose[->6]),[4] or have other growth forms.[5] A macrolichen is a lichen that is either bush-like or leafy. A microlichen is everything else.[1] Here, "macro" and "micro" do not refer to size, but to the growth form.[1] Common names for lichens[->7] may contain the word "moss" (e.g., "Reindeer moss[->8]", "Iceland moss[->9]"), and lichens may superficially look like and grow with mosses, but lichens are not related to mosses or any plant.[3]:3 Lichens don't have roots that absorb water and nutrients like in plants.[6]:2 Instead they produce their own food from sunlight, air, water, and minerals in their environment.[7] They are not parasites[->10] on the plants they may grow on, but only use them as a substrate to grow on or in.
Lichens occur from sea level[->11] to high alpine[->12] elevations, in a very wide range of environmental conditions, and can grow on almost any surface.[7] Lichens are abundant growing on bark, leaves, mosses, on other lichens,[6] and hanging from branches "living on thin air" (epiphytes[->13]) in rain forests[->14] and in temperate woodland[->15]. They grow on bare rock, walls, gravestones, roofs, exposed soil surfaces, and in the soil as part of a biological soil crust[->16]. They can survive in some of the most extreme environments on Earth: arctic tundra[->17], hot dry deserts[->18], rocky coasts, and toxic slag heaps[->19]. They can even live inside solid rock, growing between the grains. Some lichens don't grow on anything[->20], living out their lives blowing about the environment.[1] It is estimated that 6% of Earth's land surface is covered by lichen.[8] Colonies of lichens may be spectacular in appearance, dominating much of the surface of the visual landscape in forests and natural places, such as the vertical "paint"[->21] covering the vast rock faces of Yosemite National Park[->22].[9]:2
The fungus benefits from the symbiotic relation because algae or cyanobacteria produce food by photosynthesis[->23]. The algae or cyanobacteria benefit by being protected from the environment by the filaments of the fungus, which also gather moisture and nutrients from the environment, and (usually) provide an anchor to it. Lichenized fungus may refer to the entire lichen, or to the fungus growing in it. The lichen combination of fungus with algae and/or cyanobacteria has a very different form (morphology[->24]), physiology[->25], and biochemistry[->26] than the parts growing by themselves. Lichens are said to be "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[->27]. Lichens are really combinations of species from two or three different biological kingdoms[->28], so there is no common lineage. By convention, lichens have the same scientific name[->29] as the fungus in them, and are not classified according to the species[->30] of the algae and/or cyanobacteria growing in them. The algae or cyanobacteria has its own, unique, scientific name (binomial name[->31]). There are about 20,000 known species of lichens.[10] Some lichens[->32] have lost the ability to reproduce sexually, yet continue to speciate[->33].[11][6] Recent perspectives on lichens include that they are relatively self-contained miniature ecosystems in and of themselves, possibly with more microorganisms[->34] living with the fungi, algae, and/or cyanobacteria, performing other functions as partners in a system that evolves as an even more complex composite organism (holobiont[->35]).[12][13][14][15]
Lichens may be long-lived[->36], with some considered to be among the oldest living things.[3][16] They are among the first living things to grow on fresh rock exposed after an event such as a landslide. The long life-span and slow and regular growth rate of some lichens can be used to date the event (lichenometry[->37]). Many lichens are very sensitive to environmental disturbances and can be used in cheaply[7] assessing air pollution[->38],[17][18][19] ozone[->39] depletion, and metal contamination. Lichens have been used in making dyes[->40], perfumes[->41],[20] and in traditional medicines[->42]. Few lichen species are eaten by insects[7] or larger animals.[citation needed[->43]]

Pronunciation[edit]
In American English, "lichen" rhymes with "hikin' " (/[->44]ˈ[->45]l[->46]aɪ[->47]k[->48]ən[->49]/[->50]). In British English, both this pronunciation and one rhyming with "kitchen" /[->51]ˈ[->52]l[->53]ɪ[->54]tʃ[->55]ən[->56]/[->57]) are used.[21][22][23][24]
Growth forms[edit]
Main article: Lichen growth forms[->58]
Lichens grow in a wide range of shapes and forms (morphologies[->59]). The shape of a lichen is usually determined by the organization of filaments of the fungus.[25] The nonreproductive tissues, or vegetative body parts, is called the thallus. Lichens are grouped by thallus types, since the thallus is usually the most visually prominent part of the lichen. Thallus growth forms typically correspond to a few basic internal structure types. Common names for lichens[->60] often come from a growth form or color that is typical of a lichen genus[->61].
Common lichen growth forms /wiki/File:Letharia_vulpina_JHollinger_crop.jpg
This lichen[->62] grows like a multiply branched tuft or leafless mini-shrub, so has a fruticose[->63] growth form.
/wiki/File:Parmotrema_tinctorum_umenokigoke01.jpg/wiki/File:Parmotrema_tinctorum_umenokigoke01.jpg
This lichen[->64] has leaf-like structures, so is foliose[->65]. i/File:Caloplaca_marina.JPG/wiki/File:Caloplaca_marina.JPG
This lichen[->66] grows like an orange crust coating the rock, so is crustose[->67] .
/wiki/File:Caloplaca_thallincola.jpg/wiki/File:Caloplaca_thallincola.jpg
This lichen grows like a crust, and in a pattern that radiates outward from the center, so has a crustose placodioid[->68] growth form.
/wiki/File:Chrysothrix_chlorina.jpg/wiki/File:Chrysothrix_chlorina.jpg
This lichen grows like powder dusted on the rock so is a leprose lichen[->69]. wiki/File:Blistered_Jelly_Lichen_(983550966).jpg/wiki/File:Blistered_Jelly_Lichen_(983550966).jpg
This lichen[->70] is gelatinous[->71], without internal structure for its parts.
Common groupings of lichen thallus growth forms are: 1. fruticose[->72][26][27] - growing up like a tuft or multiply branched leafless mini-shrub, or hanging down in strands or tassles, 3-dimensional with a nearly round cross section for its branching parts (terete[->73]), 2. foliose[->74][26][27] - growing in 2-dimensional, flat, leaf-like lobes that lift up from the surface, 3. crustose[->75][4][26][27] - crust-like, adhering tightly to a surface (substrate[->76]) like a thick coat of paint, 4. leprose[->77][28] - powdery, 5. gelatinous[->78] - jelly like, 6. filamentous[->79] - stringy or like matted hair, 7. Byssoid[->80])- whispy, like teased wool[->81] , or 8. structureless . There are variations in growth types in a single lichen species, grey areas between the growth type descriptions, and overlapping between growth types, so some authors might describe lichens using different growth type descriptions.
When a crustose lichen gets old, the center may start to crack up like old-dried paint, old-broken asphalt paving, or like the polygonal "islands" of cracked-up mud in a dried lakebed. This is called being rimose[->82] or areolate[->83], and the "island" pieces separated by the cracks are called areolas[->84].[26] The areolas appear separated, but are (or were)[citation needed[->85]] connected by an underlying "prothallus" or "hypothallus".[28] When a crustose lichen grows from a center and appears to radiate out, it is called crustose placodioid[->86]. When the edges of the areolas lift up from the substrate, it is called squamulose[->87].[29]:159 Some people who study lichens (lichenologists[->88]) group squamulose lichens separately from crustose lichens.[citation needed[->89]]
These growth form groups are not precisely defined. Foliose lichens may sometimes branch and appear to be fruticose. Fruticose lichens may have flattened branching parts and appear leafy. Squamulous lichens may appear where the edges lift up. Gelatinous lichens may appear leafy when dry.[29]:159 Means of telling them apart in these cases are in the sections below.
Structures involved in reproduction often appears as discs, bumps, or squiggly lines on the surface of the thallus.[6]:4 The thallus is not always the part of the lichen that is most visually noticeable. Some lichens can grow inside solid rock between the grains (endolithic lichens[->90]), with only the sexual fruiting part visible growing outside the rock.[26] These may be dramatic in color or appearance.[26] Forms of these sexual parts are not in the above growth form categories.[26] The most visually noticeable reproductive parts are often circular, raised, plate-like or disc-like outgrowths, with crinkly edges, and are described in sections below.
Color[edit]
Lichens come in many colors.[6]:4 Coloration is usually determined by the photosynthetic component.[25] Special pigments, such as yellow usnic acid[->91], give lichens a variety of colors, including reds, oranges, yellows, and browns, especially in exposed, dry habitats.[30] In the absence of special pigments, lichens are usually bright green to olive gray when wet, gray or grayish-green to brown when dry.[30] This is because moisture causes the surface skin (cortex[->92]) to become more transparent, exposing the green photobiont layer.[30] Different colored lichens covering large areas of exposed rock surfaces, or lichens covering or hanging from bark can be a spectacular display when the patches of diverse colors "come to life" or "glow" in brilliant displays following rain.
Different colored lichens may inhabit different adjacent sections of a rock face, depending on the angle of exposure to light.[30]
Color is used in identification.[31]:4 Color changes depending on when a lichen is wet or dry.[31] Color descriptions when used for identification are based on when the lichen is dry.[31] Dry lichens with a cyanobacteria as the photosynthetic partner tend to be dark grey, brown, or black.[31]
The underside of the leaf-like lobes of foliose lichens is a different color from the top side (dorsiventral[->93]), often brown or black, sometimes white. A fruticose lichen may have flattened "branches", appearing similar to a foiliose lichen, but the underside of a leaf-like structure on a fruticose lichen is the same color as the top side. The leaf-like lobes of a foliose lichen may branch, giving the appearance of a fruticose lichen, but the underside will be a different color from the top side.[28]
The sheen on some jelly-like gelatinous lichens is from mucusy[->94] excretions.[25]
Internal structure and growth forms[/wiki/File:Lichen_Cross_Section_Diagram.svg/wiki/File:Lichen_Cross_Section_Diagram.svg
/wiki/File:Lichen_Cross_Section_Diagram.svgSchematic cross section of foliose lichen[->95]: 1. The cortex is the outer layer of tightly woven fungus filaments (hyphae[->96]) 2. This photobiont layer has photosynthesizing green algae[->97] 3. Loosely packed hyphae in the medulla 4. A tightly woven lower cortex, with anchoring hyphae called rhizines where the fungus attaches to the substrate.
A lichen is made up of a simple photosynthesizing organism, usually green algae[->98] or cyanobacteria[->99], surrounded by filaments of a fungus. Generally,most of a lichen’s bulk is made of interwoven fungal filaments,[32] although in filamentous[citation needed[->100]] and gelatinous lichens[25] this is not the case. The fungus is called a mycobiont. The photosynthesizing organism is called a photobiont. Algal photobionts are called phycobionts.[33] Cyanobacteria photobionts are called cyanobionts.[33]
The part of a lichen that is not involved in reproduction, the "body" or “vegetative tissue" of a lichen, is called the thallus. The thallus form is very different from any form where the fungus or alga are growing separately. The thallus is made up of filaments of the fungus called hyphae[->101]. The filaments grow by branching then rejoining to create a mesh, which is called being "(anastomose[->102])". The mesh of fungal filaments may be dense or loose.
Generally, the fungal mesh surrounds the algal or cyanobacterial[->103] cells, often enclosing them within complex fungal tissues that is unique to lichen associations. The thallus may or may not have a protective "skin" of densely packed fungal filaments, which is called a cortex. Fruticose lichens have one cortex layer wrapping around the "branches". Foliose lichens have an upper cortex on the top side of the "leaf", and a separate lower cortex on the bottom side. Crustose and squamulose lichens have only an upper cortex, with the "inside" of the lichen in direct contact with the surface they grow on (the substrate). Even if the edges peel up from the substrate and appear flat and leaf-like, they lack a lower cortex, unlike foliose lichens. Filimentous,[citation needed[->104]] byssoid,[citation needed[->105]] leprose,[28] gelatinous,[citation needed[->106]] filimentous,[citation needed[->107]] and other lichens[citation needed[->108]] do not have a cortex, which is called being ecorticate[->109].[34]
Fruticose, foliose, crustose, and squamulose lichens generally have up to three different types of tissue, differentiated[->110] by having different densities of fungal filaments.[32] The top layer, where the lichen contacts the environment, is called a cortex.[32] The cortex is made of densely tightly woven,[citation needed[->111]] packed, and glued together (agglutinated[->112]) fungual filaments.[32] The dense packing makes the cortex act like a protective “skin”, keeping other organisms out, and reducing the intensity of sunlight on the layers below.[32] The cortex layer can be up to several hundred micrometers (μm) in thickness (less than a millimeter).[35] The cortex may be further topped by an epicortex of secretions, not cells, 0.6-1μm thick in some lichens[->113].[35] This secretion layer may or may not have pores.[35]
Below the cortex layer is a layer called the photobiontic layer or symbiont layer.[27][32] The symbiont layer has less densely packed fungal filaments, with the photosynthetic partner embedded in them.[32] The less dense packing allows air circulation during photosynthesis, similar to the anatomy of a leaf.[32] Each cell or group of cells of the photobiont is usually individually wrapped by hyphae, and in some cases penetrated by an haustorium[->114].[25] In crustose lichens and foliose lichens, algae in the photobiontic layer is diffuse among the fungal filaments, decreasing in gradation into the layer below. In fruticose lichens, the photobiontic layer is sharply distinct from the layer below.[25]
The layer beneath the symbiont layer called is called the medulla. The medulla is less densely packed with fungal filaments than the layers above. In foliose lichens, there is (usually[->115])[29]:159 another densely packed layer of fungal filaments called the lower cortex.[28][32] Root-like fungal structures called rhizines (usually[->116])[29]:159 grow from the lower cortex to attach or anchor the lichen to the substrate.[1][28] Fruticose lichens have a single cortex wrapping all the way around the "stems" and "branches".[29] The medulla is the lowest layer, and may form a cottony white inner core for the branchlike thallus, or it may be hollow.[29]:159 Crustose and squamulose lichens lack a lower cortex, and the medulla is in direct contact with the substrate (biology)[->117] that the lichen grows on.
In crustose areolate lichens, the edges of the areolas peel up from the substrate and appear leafy. In squamulose lichens the part of the lichen thallus that is not attached to the substrate may also appear leafy. But these leafy parts lack a lower cortex, which distinguishes crustose and squamulose lichens from foliose lichens.[32] Conversely, foliose lichens may appear flattened against the substrate like a crustose lichen, but most of the leaf-like lobes can be lifted up from the substrate because it is separated from it by a tightly packed lower cortex.[28]
Gelatinous,[29]:159 byssoid,[citation needed[->118]] and leprose[citation needed[->119]] lichens lack a cortex (are ecorticate[->120]), and generally have only undifferentiated tissue, similar to only having a symbiont layer.[citation needed[->121]]
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[->122].
Pruinia[->123] is a whitish coating on top of an upper surface.[36] An epinecral layer[->124] is "a layer of horny dead fungal hyphae with indistinct lumina[->125] in or near the cortex above the algal layer".[36]
Physiology[edit]
Symbiotic relation[edit]
Main article: Symbiosis in lichens[->126]
"Lichens are fungi that have discovered agriculture" - Trevor Goward[37]
A lichen is a composite organism that emerges from an algae[->127] or cyanobacteria[->128] living among the filaments (hyphae[->129]) of a fungus[->130] in a mutually beneficial (symbiotic[->131]) relationship. The fungus benefits from the algae or cyanobacteria because they produce food by photosynthesis[->132]. The algae or cyanobacteria benefit by being protected from the environment by the filaments of the fungus, which also gather moisture and nutrients from the environment, and (usually) provide an anchor to it. Although some photosynthetic partners in a lichen can survive outside the lichen, the lichen symbiotic association extends the ecological range of both partners, whereby most descriptions of lichen associations describe them as symbiotic. 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[->133].If a cyanobacterium[->134] is present, as a primary partner or another symbiont in addition to green alga as in certain tripartite lichens, they can fix atmospheric nitrogen[->135], complementing the activities of the green alga.
The algal or cyanobacterial cells are photosynthetic[->136], and as in plants they reduce[->137] atmospheric carbon dioxide[->138] into organic carbon sugars to feed both symbionts. Phycobionts (algae) produce sugar alcohols[->139] (ribitol[->140], sorbitol[->141], and erythritol[->142]) which are absorbed by the mycobiont (fungus).[33] Cyanobionts produce glucose[->143].[33] Lichenized fungal cells can make the photobiont "leak" out the products of photosynthesis, where they can then be absorbed by the fungus.[6]:5
The lichen combination of algae and/or cyanobacteria with a fungus has a very different form (morphology), physiology, and biochemistry than the component fungus, algae, or cyanobacteria growing by themselves, naturally or in culture. The body (thallus[->144]) of most lichens is different from those of either the fungus or alga growing separately. When grown in the laboratory in the absence of its photobiont, a lichen fungus develops as a structureless, undifferentiated mass of fungal filaments (hyphae[->145]). If combined with its photobiont under appropriate conditions, its characteristic form associated with the photobiont emerges, in the process called morphogenesis[->146].[38] 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.
Evidence that lichens are examples of successful symbiosis[->147] is the fact that lichens can be found in almost every habitat and geographic area on the planet.[12] Two species in two genera of green algae are found in over 35% of all lichens, but can only rarely be found living on their own outside of a lichen.[39]
In a case where one fungal partner simultaneously had two green algae partners that outperform each other in different climates, this might indicate having more than one photosynthetic partner at the same time might enable the lichen to exist in a wider range of habitats and geographic locations.[12]
Phycobionts can have a net output of sugars with only water vapor.[33] The thallus must be saturated with liquid water for cyanobionts to photosynthesize.[33]
Algae produce sugars that are absorbed by the fungus by diffusion into special fungal hyphae called appressoria[->148] or haustoria[->149] in contact with the wall of the algal cells.[40] The appressoria or haustoria may produce a substance that increases permeability of the algal cell walls, and may penetrate the walls.[40] The algae may lose up to 80% of their sugar production to the fungus.[40][41]
Debate over mutualism vs. commensalism and/or parasitism[edit]
Lichen associations may be examples of mutualism[->150], commensalism[->151] or even parasitism[->152], depending on the species. There is evidence to suggest that the lichen symbiosis is parasitic[->153] or commensalistic[->154], rather than mutualistic[->155].[42] The photosynthetic partner can exist in nature independently of the fungal partner, but not vice versa. Photobiont cells are routinely destroyed in the course of nutrient[->156] exchange. The association is able to continue because reproduction of the photobiont cells matches the rate at which they are destroyed.[42] The fungus[->157] surrounds the algal cells,[7] often enclosing them within complex fungal tissues unique to lichen associations. In many species the fungus penetrates the algal cell wall,[7] forming penetration pegs (haustoria[->158]) similar to those produced by fungi that feed on a host (pathogenic fungi[->159]).[43][44] Cyanobacteria[->160] in laboratory settings can grow faster when they are alone rather than when they are part of a lichen.
Miniature ecosystem and holobiont theory[edit]
Symbiosis in lichens is so well-balanced that lichens have been considered to be relatively self-contained miniature ecosystems in and of themselves.[12][13] It is thought that lichens may be even more complex symbiotic systems that include non-photosynthetic bacterial communities performing other functions as partners in a holobiont[->161].[14][15]
Lichenicolous fungi[edit]
Some fungi can only be found living on lichens, and some only on those lichens (obligate[->162] parasites[->163]). These are referred to as lichenicolous fungi[->164], which are a different species from the fungus living inside the lichen and are not considered to be part of the lichen.[45]
Reaction to water[edit]
Moisture makes the cortex become more transparent.[6]:4 This way the algae can conduct photosynthesis when moisture is available, and is protected at other times. When the cortex is more transparent, the algae shows more clearly and the lichen looks greener.
Metabolites and metabolite structures[edit]
Lichens also sometimes contain structures made from fungal metabolites[->165], for example crustose lichens sometimes have a polysaccharide[->166] layer in the cortex.
Secondary metabolites[->167] are often deposited as crystals in the apoplast[->168].[46]
Substrate preference[edit]
Secondary metabolites are thought to play a role in preference for some substrates over others.[46]
Growth rate[edit]
Lichens often have a regular but very slow growth rate of less than a millimeter per year.[37] Different lichen species have been measured to grow as slowly as .5 mm, and as fast as 1/2 meter per year.[47]
In crustose lichens, the area along the margin is where the most active growth is taking place.[29]:159 Most crustose lichens grow only 1–2 mm in diameter per year.[48]
Life span[edit]
Lichens may be long-lived (longevity[->169]), with some considered to be among the oldest living things.[3][16] Lifespan is difficult to measure because the definition of what constitutes the "same" individual lichen is not precise.[47] lichens grow by vegetatively having a piece break off, which may or may not be considered to be the "same" lichen, and because two lichens can grow into each other and then become the "same" lichen.[47]
Response to environmental stress[edit]
Unlike simple dehydration in plants and animals, lichens may experience a complete loss of body water in dry periods.[7] Lichens are capable of surviving extremely low levels of water[->170] content (poikilohydric[->171]).[49] They quickly absorb water when it becomes available again, becoming soft and fleshy.[7] Re-configuration of membranes following a period of dehydration requires several minutes or more.[citation needed[->172]]
In tests, lichen survived and showed remarkable results on the adaptation capacity[->173] of photosynthetic activity[->174] within the simulation time[->175] of 34 days under Martian conditions[->176] in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center[->177] (DLR).[50][51]
The European Space Agency[->178] 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[->179] and Xanthoria elegans[->180]—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.[52][53]
Reproduction and dispersal[edit]
Vegetative reproduction[edit]
Many lichens reproduce asexually, either by a piece breaking off and growing on its own (vegetative reproduction[->181]) or through the dispersal of diaspores[->182] containing a few algal cells surrounded by fungal cells.[1] Because of the relative lack of differentiation in the thallus, the line between diaspore formation and vegetative reproduction is often blurred. Fruticose lichens can easily[citation needed[->183]] fragment, and new lichens can grow from the fragment (vegetative reproduction[->184]).[citation needed[->185]] Many lichens break up into fragments when they dry, dispersing themselves by wind action, to resume growth when moisture returns.[54][55] 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] Isidia (singular "isidium") are branched, spiny, elongated, outgrowths from the thallus that break off for mechanical dispersal.[1] Lichen propagules (diaspores[->186]) 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.”[56]
/wiki/File:Lichen_reproduction1.jpg/wiki/File:Lichen_reproduction1.jpg
/wiki/File:Lichen_reproduction1.jpgXanthoparmelia sp. with reproductive body parts (disc-like apothecia[->187]) at center, surrounded by thallus[->188] parts.
/wiki/File:Lichen_reproduction.jpg/wiki/File:Lichen_reproduction.jpg
/wiki/File:Lichen_reproduction.jpgThallus[->189] and disc-like apothecia[->190] on a foliose lichen[->191]
/wiki/File:Pilophorus_acicularis_109164.jpg/wiki/File:Pilophorus_acicularis_109164.jpg
/wiki/File:Pilophorus_acicularis_109164.jpgCrust-like thallus with pseudopodetia
Sexual reproduction[edit]
Structures involved in reproduction often appears as discs, bumps, or squiggly lines on the surface of the thallus.[6]:4 Only the fungal partner in a lichen reproduces sexually.[1] Many lichen fungi reproduce sexually like other fungi, producing spores formed by meiosis[->192] 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.
Lichens that are in Basidiomycetes[->193] produce mushroom[->194]-like reproductive structures.
Most lichen fungi belong to Ascomycetes[->195] (ascolichens). Among the ascolichens, spores[->196] are produced in spore-producing structures called ascomata[->197].[6] The most common type of ascomata are the apothecium[->198] (plural - apothecia) and perithecium[->199] (plural perithecia).[6]:14 Apothecia are usually cups or plate-like discs located on the top surface of the lichen thallus. When apothecia are shaped like squiggly line segments instead of like discs, they are called lirellae.[6]:14 Perithecia are shaped like flasks that are immersed in the lichen thallus tissue, which has a small hole for the spores to escape the flask, and appear like black dots on the lichen surface.[6]:14
The three most common spore body types being raised discs called apothecia[->200] (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[->201] is the structure that contains and releases the sexual spores in fungi of the Ascomycota[->202]).[57]
The apothecia has a layer of exposed spore-producing cells called asci[->203] (singular - ascum), and is usually a different color from the thallus tissue.[6]:14 When the apothecium has an outer margin, the margin is called the exciple.[6]:14 When the exciple has a color similar to colored thallus tissue the apothecium or lichen is called lecanorine, meaning similar to members of the genus Lecanora[->204].[6]:14 When the exciple is blackened like carbon it is called lecideine meaning similar to members of the genus Lecidea[->205].[6]:14 When the margin is pale or colorless it is called biatorine.[6]:14
A "podetium" (plural podetia) is a lichenized rm linkstem-like structure of an the fruiting body rising from the thallus, associated with some fungi that produce a fungal apothecium[->206].[27] Since it is part of the reproductive tissue, podetia are not considered part of the main body (thallus), but may be visually prominent.[27] The podetium may be branched, and sometimes cup-like. They usually bear the fungal pycnidia[->207] or apothecia[->208] or both.[27] Many lichens have apothecia[->209] that are visible to the naked eye.[1]
Most lichens produce abundant sexual structures.[58] Many species appear to disperse only by sexual spores.[58] 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[->210]). This breeding system may enable successful reproduction in harsh environments.[58]
Mazaedia (singular - mazaedium) are apothecia shaped like a dressmaker's pin in (pin lichen[->211])s, where the fruiting body is a brown or black mass of loose ascospores enclosed by a cup-shaped exciple, which sits on top of a tiny stalk.[6]:15
Taxonomy and classification[edit]

Lichens are classified by the fungal component. Lichen species are given the same scientific name (binomial name[->212]) as the fungus species in the lichen. 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.[59] There are about 13,500 - 17,000 identified lichen species.[40] Nearly 20% of known fungal species are associated with lichens.[40]
"Lichenized fungus" may refer to the entire lichen, or to just the fungus. This may cause confusion without context. A particular fungus species may form lichens with different algae species, giving rise to what appear to be different lichen species, but which are still classified (as of 2014) as the same lichen species.[60]
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[->213]. Neither the ascolichens nor the basidiolichens form monophyletic[->214] lineages in their respective fungal phyla, but they do form several major solely or primarily lichen-forming groups within each phylum.[61] Even more unusual than basidiolichens is the fungus Geosiphon pyriforme[->215], a member of the Glomeromycota[->216] that is unique in that it encloses a cyanobacterial symbiont inside its cells. Geosiphon[->217] 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[->218] genera.
Lichens independently emerged from fungi associating with algae and cyanobacteria at least twice in history.[citation needed[->219]]
Fungi[edit]
The fungal component of a lichen is called the mycobiont. The mycobiont may be an Ascomycete[->220] or Basidiomycete[->221].[10] The associated lichens are called either ascolichens or basidiolichens, respectively.[citation needed[->222]] Living as a symbiont[->223] in a lichen appears to be a successful way for a fungus to derive essential nutrients since about 20% of all fungal species have acquired this mode of life.[citation needed[->224]]
Thalli produced by a given fungal symbiont with its differing partners may be similar,[citation needed[->225]] and the secondary metabolites identical,[citation needed[->226]] indicating[citation needed[->227]] that the fungus has the dominant role in determining the morphology of the lichen. But the same mycobiont with different photobionts may also be produce very different growth forms.[60] Lichens are known in which there is one fungus associated with two or even three algal species.
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.[citation needed[->228]]
The following table lists the orders[->229] and families[->230] of fungi that include lichen-forming species.
Two or more fungal species can interact to form the same lichen.[62]
Photobionts[edit]
The photosynthetic[->231] partner in a lichen is called a photobiont. The photobiont in lichens come from a wide variety of simple prokaryotic[->232] and eukaryotic[->233] organisms. The majority of the lichens contain eukaryotic photobionts that are green algae (Chlorophyta[->234]) or[citation needed[->235]] yellow-green algae (Xanthophyta[->236]).[citation needed[->237]] The prokaryotes are blue green "algae" (cyanobacteria[->238]). Algal photobionts are called phycobionts.[33] Cyanobacteria photobionts are called cyanobionts.[33] About 90% of all known lichens have phycobionts, and about 10" have cyanobionts.[33] Sometimes the photobiont is a green algae (chlorophyta[->239]) or other eukaryote[->240], sometimes a blue-green "algae" (cyanobacteria[->241], and sometimes both. Approximately 100 species of photosynthetic partners from 40[33] genera and five distinct classes (prokaryotic: Cyanophyceae[->242]; eukaryotic: Trebouxiophyceae[->243], Phaeophyceae[->244], Chlorophyceae[->245]) have been found to associate with the lichen-forming fungi.[63]
Common algal[->246] photobionts are from the genus Trebouxia[->247], Trentepohlia[->248], Pseudotrebouxia, or Myrmecia (algae)[->249]. Trebouxia[->250] is the most common genus of green algae in lichens, occurring in about 40% of all lichens. "Trebouxioid" means either a photobiont that is in the genus Trebouxia[->251], or resembles a member of that genus, and is therefore presumably a member of the class Trebouxiophyceae[->252].[27] The second most commonly represented green alga genus is Trentepohlia[->253].[citation needed[->254]] Overall, about 100 species of eukaryotes[citation needed[->255]] are known to occur as photobionts in lichens. All the algae are probably able to exist independently in nature as well as in the lichen.[62]
A "cyanolichen[->256]" is a lichen with a cyanobacteria[->257] as its main photosynthetic component (photobiont[->258]).[64] The most commonly occurring cyanobacteria genus[->259] is Nostoc[->260].[62] Other[43] common cyanobacterium[->261] photobionts are from Scytonema[->262].[10] Many cyanolichens are small and black, and have limestone[->263] as the substrate.[citation needed[->264]] Another cyanolichen group, the jelly lichens[->265] ( e.g., from the genera Collema[->266] or Leptogium[->267]) are large and foliose[->268] (e.g., species of Peltigera[->269], Lobaria[->270], and Degelia[->271]. 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[->272]. Strains of cyanobacteria[->273] found in various cyanolichens[->274] are often closely related to one another.[65] They differ from the most closely related free-living strains.[65]
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[->275] which forms orange-coloured populations on tree trunks and suitable rock faces. Lichen propagules (diaspores[->276]) 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.”[56]
The same cyanobiont species can occur in association with different fungal species as lichen partners.[66] The same phycobiont species can occur in association with different fungal species as lichen partners.[33] More than one phycobiont may be present in a single thallus.[33]
Although each lichen thallus generally appears homogeneous, some evidence seems to suggest that the photobiont component may consist of more than one genetic individual of that species.[citation needed[->277]] A single lichen may contain several algal genotypes[->278].[67][68] These multiple genotypes may better enable response to adaptation to environmental changes, and enable the lichen to inhabit a wider range of environments.[69]
Ecology and interactions with environment[edit]
Substrates and habitats[edit]
Lichens grow in a wide range of substrates and habitats, including some of the most extreme conditions on earth. They are abundant growing on bark, leaves, and hanging from branches "living on thin air" (epiphytes[->279]) in rain forests[->280] and in temperate woodland[->281]. They grow on bare rock, walls, gravestones, roofs, exposed soil surfaces. They can survive in some of the most extreme environments on Earth: arctic tundra[->282], hot dry deserts[->283], rocky coasts, and toxic slag heaps[->284]. They can even live inside solid rock, growing between the grains, and in the soil as part of a biological soil crust[->285] in arid habitats such as deserts. Some lichens don't grow on anything, living out their lives blowing about the environment.[1]
They are not parasites on the plants they may grow on, but only use them as a substrate to grow on. The fungi of some lichen species may "take over" the algae of other lichen species.[89]
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[->286] 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[->287] that requires the employment of mountain-climbing conservators to clean the monument.
Lichens are not parasites[->288] on the trees they grow on,[90] but only use them as a surface for anchoring themselves.[7] Lichens make their own food from their photosynthetic parts and by absorbing minerals from the environment.[7] Lichens growing on leaves may have the appearance of being parasites on the leaves, but they are not.
/wiki/File:CladonioPinetum.jpg/wiki/File:CladonioPinetum.jpg
/wiki/File:CladonioPinetum.jpgPine forest with Cladonia[->289] lichen ground-cover
In the arctic tundra, lichens, together with mosses[->290] and liverworts[->291], make up the majority of the ground cover[->292], which helps insulate the ground and may provide forage for grazing animals. An example is "Reindeer moss[->293]", which is a lichen, not a moss.[7]
A crustose lichen that grows on rock is called a saxicolous lichen[->294].[27][29]:159 Crustose lichens that grow on the rock are epilithic[->295], and those that grow immersed inside rock, growing between the crystals with only their fruiting bodies exposed to the air, are called endolithic lichens[->296].[26][29]:159[64] A crustose lichen that grows on bark is called a corticolous lichen[->297].[29]:159 A lichen that grows on wood from which the bark has been stripped is called a lignicolous lichen[->298].[34] Lichens that grow immersed inside plant tissues are called endophloidic lichens or endophloidal lichens.[26][29]:159 Lichens that use leaves as substrates, whether the leaf is still on the tree or on the ground, are called epiphyllous[->299] or foliicolous[->300].[33] A terricolous lichen[->301] grows on the soil as a substrate. Many squamulous lichens are terricolous.[29]:159 Umbillicate lichens[->302] are foliose lichens that are attached to the substrate at only one point.[26] A vagrant lichen[->303] is not attached to a substrate at all, and lives its life being blown around by the wind.
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.[91] Lichens help bind and stabilize soil sand in dunes.[1] In deserts and semi-arid areas, lichens[->304] are part of extensive, living biological soil crusts[->305], essential for maintaining the soil structure.[1]
Ecological interactions[edit]
Lichens are pioneer species[->306], among the first living things to grow on bare rock or areas denuded of life by a disaster.[1] Lichens may have to 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[->307] in places lacking soil, constituting the sole vegetation in some extreme environments such as those found at high mountain elevations and at high latitudes.[92] Some survive in the tough conditions of deserts, and others on frozen soil of the Arctic regions.[93]
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[->308]. Like some mosses[->309], liverworts[->310], ferns[->311], and a few "resurrection plants[->312]", upon desiccation, lichens enter a metabolic suspension or stasis (known as cryptobiosis[->313]) 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[->314] (epi- on the surface, phyte- plant) on plants, particularly on the trunks and branches of trees. When growing on plants, lichens are not parasites[->315]; they do not consume any part of the plant nor poison it. Some ground-dwelling lichens, such as members of the subgenus Cladina[->316] (reindeer lichens), however, produce allelopathic[->317] 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[->318] 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[->319] seeds cannot become established except in places where lichen crusts stabilize the sand and help retain water.
Lichens may be eaten by some animals, such as reindeer[->320], living in arctic[->321] regions. The larvae[->322] of a number of Lepidoptera[->323] species feed exclusively on lichens. These include Common Footman[->324] and Marbled Beauty[->325]. 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[->326] for nesting, food, and a water source during winter.
Effects of air pollution[edit]
/wiki/File:Lobaria_pulmonaria_010108c.jpg/wiki/File:Lobaria_pulmonaria_010108c.jpg
/wiki/File:Lobaria_pulmonaria_010108c.jpgSome lichens, like the foliose Lobaria pulmonaria[->327], are sensitive to air pollution.
If lichens are exposed to air pollutants at all times, without any deciduous[->328] parts, they are unable to avoid the accumulation of pollutants. Also lacking stomata[->329] and a cuticle[->330], lichens may absorb aerosols[->331] and gases over the entire thallus surface from which they may readily diffuse[->332] to the photobiont layer.[94] Because lichens do not possess roots, their primary source of most elements[->333] 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[->334] and dew[->335], gaseous absorption, and dry deposition.[95] Consequently, many environmental studies with lichens emphasize their feasibility as effective biomonitors[->336] of atmospheric quality.[94][96][97][98][99]
Not all lichens are equally sensitive to air pollutants[->337], so different lichen species show different levels of sensitivity to specific atmospheric pollutants.[100] 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.[101] 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.[94]
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]
Human use[edit]
Food[edit]
Further information: Ethnolichenology[->338]
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[->339] are generally indigestible to humans, and lichens usually contain mildly toxic secondary compounds[->340] that should be removed before eating. Very few lichens are poisonous, but those high in vulpinic acid[->341] or usnic acid[->342] are toxic.[102] Most poisonous lichens are yellow.
In the past Iceland moss[->343] (Cetraria islandica) was an important human food in northern Europe, and was cooked as a bread, porridge, pudding, soup, or salad. Wila[->344] (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[->345] (Cladina spp.) after they remove it from the rumen[->346] of caribou or reindeer that have been killed. Rock tripe[->347] (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[->348], is used in a variety of traditional Korean and Japanese foods.
Lichenometry[edit]
Main article: Lichenometry[->349]
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,[103] the technique has found many applications. it is used in archaeology[->350], palaeontology[->351], and geomorphology[->352]. It uses the presumed regular but slow rate of lichen growth to determine the age of exposed rock[->353].[9]:9[104] 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[->354]] 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.[105] Lichenometry is especially useful for dating surfaces less than 500 years old, as radiocarbon dating techniques are less accurate over this period.[106] The lichens most commonly used for lichenometry are those of the genera Rhizocarpon[->355] (e.g. the species Rhizocarpon geographicum[->356]) and Xanthoria[->357].
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.[107] Lichens can accumulate several environmental pollutants such as lead, copper, and radionuclides.[108]
As dyes[edit]
Many lichens produce secondary compounds, including pigments that reduce harmful amounts of sunlight and powerful toxins that reduce herbivory[->358] or kill bacteria. These compounds are very useful for lichen identification, and have had economic importance as dyes[->359] such as cudbear[->360] or primitive antibiotics[->361].
The pH indicator[->362] (indicated acidic or basic) in the litmus test[->363] is a dye extracted from the lichen Roccella tinctoria[->364] by boiling.
In the Highlands of Scotland, traditional dyes[->365] for Harris tweed[->366][1] and other traditional cloths were made from lichens including the orange Xanthoria parietina[->367] and the grey foliaceous Parmelia saxatilis[->368] common on rocks known as "Crottle".[109]
There are reports dating almost 2000 years old of lichens being used to make purple and red dyes.[110] Of great historical and commercial significance are lichens belonging to the family Roccellaceae[->369], commonly called orchella weed or orchil. Orcein[->370] and other lichen dyes have largely been replaced by synthetic versions.
Antibiotic metabolites[edit]
Lichens produce metabolites proven useful in the medical community.[111] Most metabolites produced by lichens are structurally and functionally similar to broad-spectrum antibiotics while few are associated respectively to antiseptic similarities.[112] These organic acids are the metabolic byproducts of Crassulacean acid metabolism, the means of photosynthesis by lichens.[113]
Usnic acid is the most commonly studied metabolite produced by lichens and has been associated with the suppression of tuberculosis.[114] It has also proven bactericidal against Escherichia coli[->371] and Staphylococcus aureus[->372] 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.[115]
Aesthetic appeal[edit]
Colonies of lichens may be spectacular in appearance, dominating the surface of the visual landscape as part of the aesthetic appeal to paying visitors of Yosemite National Park[->373] and Sequoia National Park[->374].[9]:2 Orange[->375] and yellow[->376] lichens add to the ambience of desert trees, rock faces, tundras, and rocky seashores. Intricate webs of lichens hanging from tree branches[->377] add a mysterious aspect to forests. Fruticose lichens are used in model railroading[->378][116] and other modeling hobbies as a material for making miniature trees and shrubs.
Traditional medicine[edit]
Further information: Ethnolichenology[->379]
Lichens have been used in nonscientific traditional medicine[->380] practices of many cultures. Historically in Europe, Lobaria pulmonaria[->381] was collected in large quantities as "Lungwort", due to its lung-like appearance (see doctrine of signatures[->382]).[citation needed[->383]] Similarly Peltigera leucophlebia[->384] was used as a supposed cure for thrush[->385], due to the resemblance of its cephalodia to the appearance of the disease.[43]
In literature[edit]
The Talmud[->386] (Sanhedrin 19b) translates the Hebrew word "vayilafeth" in Ruth[->387] 3:8 as referring to Ruth[->388] entwining herself around Boaz[->389] like lichen.[dubious[->390] – discuss[->391]][citation needed[->392]]
Potential application in treating Mad Cow Disease[edit]
An article published in PLoS ONE[->393] indicates that a serine protease[->394] isolated from certain species of lichen degrades prions[->395].[117][118]

What is a Lichen?
A lichen is not a single organism; it is a stable symbiotic association between a fungus and algae and/or cyanobacteria.
Like all fungi, lichen fungi require carbon as a food source; this is provided by their symbiotic algae and/or cyanobacteria, that are photosynthetic.
The lichen symbiosis is thought to be a mutualism, since both the fungi and the photosynthetic partners, called photobionts, benefit.
Which fungi form lichens?
· Many unrelated and very different fungi form lichens, including mushroom-forming fungi, and especially cup-fungi.
· 98% of lichen fungi are cup-fungi, or ascomycetes.
· Fully half of all ascomycetes and one in five of all known fungi form lichens.
· Lichenisation is an ecological strategy, or a common nutritional mode among unrelated fungi.
What are lichen photobionts?
· Lichen photobionts are the green algae or cyanobacteria that provide the simple sugars to their fungal partners.
· 90% of all lichens associate with a green-algal photobiont.
· About 100 species of photobionts are known, and the commonest ones are from four main groups.
· Lichen fungi specialise on particular photobionts. Typically they only associate with a small group of related species, though they may associate flexibly with different photobionts according to their environmental situation.
The Lichen Symbiosis
Lichens are made up of two or more closely interacting organisms, a fungus, and one or more partners, called photobionts. The photobiont may be an alga and/or cyanobacteria, both of which can produce simple sugars by photosynthesis. In contrast, fungi are 'heterotrophic' and require an external source of food. The fungi build the structure of the lichen thallus, within which they provide conditions for a long term, stable association with their photobionts, the basis of the lichen symbiosis. There is also convincing evidence for a consistent presence of non-photosynthetic bacteria within the thalli of all lichens so far examined, although the role of these bacteria is as yet unknown. Interestingly, a role for non-photosynthetic bacteria was suspected for many years, as the relichenization of separately cultured fungi and algae in the lab was facilitated by the presence of bacteria.
There is some debate about the exact nature of the symbiotic association between the lichen-fungi and their photobionts. Are the fungi ‘farming’ the photobionts in a controlled parasitism, or are the photobionts gaining some benefit also? There is good evidence for the lichen symbiosis as a mutualism, in which both partners benefit from the relationship. It is clear that fungi obtain their carbon-source in the form of simple sugars, but the photobionts seem also to be provided with optimal living conditions, in which their populations are often much larger than outside lichens. The photobiont probably also benefits from improved access to mineral nutrients which are provided because of fungal digestion outside their cells. Last, but not least, the interior of lichens is often a place richly infused with complex secondary fungal chemicals found nowhere else in nature, and these compounds are likely to play a role in protection from UV radiation, desiccation, and grazing by herbivores as well.
However, there are also good arguments in favour of the controlled parasitism camp. Up to half of the carbon fixed by algae is immediately converted to fungal sugars which are inaccessible to the alga itself. Secondly, some lichens which can form stable associations with their ‘usual’ host algae, form parasitic-type interactions with non-host algae when grown in the lab. In fact, it is thought that many early stages of developing lichen spores may survive using such a parasitic or saprophytic strategy. Lastly, there are many lineages of lichen fungi that are parasitic on other lichens – the so-called lichenicolous lichens! In some cases, non-lichen fungi have evolved from lichenised forms. These can be specialised opportunistic parasites or saprophytes or even symbionts, competing for nutrients with other fungi in the lichen thallus.
Lichen Fungi
In the early study of lichens, their ‘dual nature’ as fungi with separate internal symbionts was not recognised, and their membership in the fungal kingdom was very much questioned. In fact, a legacy of exclusion from accepted mycological research persisted until the 1970s, despite their obvious affinities with non-lichen fungi. With the advent of molecular biology, the shared history of lichens and non-lichens has been elucidated (and accepted), and we now know that the fungi that form lichens have evolved from many only distantly related lineages across the fungal tree of life, uniting them and their non-lichen relatives in the Kingdom Fungi. Lichen fungi are a heterogeneous group; they are similar only ecologically, in that they share the nutritional strategy of gaining carbon from an internal symbiotic photosynthetic partner, the photobiont. In the study of lichens, the name and classification belongs to the fungal partner, which in most cases is the dominant member of the association, at least in terms of biomass.
Lichen fungi have evolved independently several times within the mushroom-forming fungi and relatives (the basidiomycetes), but much more commonly, from within the cup fungi (the ascomycetes). About 98% of lichens belong to the cup fungi, and it is these lineages that produce almost all of the familiar and colourful crusts, rosette-forming, foliose (leafy) types, and shrubby or fruticose lichens that many people recognise. Probably more than ten distinct major lineages of fungi within the ascomycetes are lichenised. Current estimates suggest that one fifth of all known fungi and half of all ascomycetes are lichenised, with about 28,000 species worldwide. As with most organisms, lichen fungi are most diverse and least studied in the tropics.
For example, the genus Arthonia is comprised of a mix of lichenised and non-lichenised species and includes many which are specialist parasites, only found on one or a few closely-related host lichens. In a single genus, then, we have a case of lichen parasites evolving from lichen fungi! Other non-lichen fungi arose from lichenised ancestors, such as Stictis and Ostropa.
Fungi are classified in part by the type of spore-producing structures they produce, with the cup fungi (ascomycetes) named for the open, cup-shaped structures which often bear the sexual spores of the fungi. Not all ascomycetes have these cup-shaped structures, however, and, easily observed morphological characteristics like fruit type (cup-like apothecia versus flask-shaped perithecia, for example) cannot always be used to assess relationships. Unfortunately, this means that not all fungi sharing a single characteristic are likely to be related. However, some order can be distilled. The bulk of lichen diversity belongs to the class including the well-known genera Lecanora, Cladonia, Parmelia and Peltigera (Lecanoromycetes, or the Lecanora-group), where spores are borne mostly in open or cup-shaped fruits (apothecia). This group of fungi is very old, estimated to have evolved during the Carboniferous period. The very first lichens probably date back to before the origin of land plants, when most of the biodiversity of Earth was in the sea.
Many Arthonia relatives also have open cup type fruits, but their development is quite different, giving a clue that they are not closely related to the Lecanora-group. Instead, they are more closely related to other ascomycetes that have flask-shaped spore-bearing structures (perithecia). Similarly, for still other lichen groups, morphological similarities have been confirmed by molecular evidence to point to their widely disparate origins in the ascomycete tree of life. For examples of these, students would be advised to visit the tropics, where the members of the Arthonia-, Trypethelium- and Pyrenula- groups form conspicuous and sometimes colourful crusts. In Britain, the smooth barked trees of the western districts are good places to see some of our Arthonia and Pyrenula species.
Students of lichenology will probably not be surprised to read that lichen fungi can be difficult to identify, partly due to the paucity of morphological characters to go on, but also due to the repeated and independent evolution of such characters. For example, the fruticose habit has evolved repeatedly within the Lecanora-group, but also within the distantly related Arthonia-group. Unrelated fungi repeatedly evolve similar morphologies to succeed under similar conditions, making morphological identification especially difficult in some groups.
Lichen Photobionts
Fungi are heterotrophic, meaning that, like animals, they require a carbon source to survive. The lichen fungi share a common ecological strategy of hosting an internal population of photosynthetic cells, from which they obtain their carbon source in the form of simple sugars. These photosynthetic cells can either be green algae (Chlorophyta) or cyanobacteria or sometimes both, in which case the cyanobacteria are localised in distinct areas of the thallus. As the photosynthetic partners come from divergent parts of the tree of life (green plants vs bacteria), the term photobiont is used as collective term for any of them. The majority of lichens have green-algal photobionts, with only about 10% containing cyanobacteria.
The role of the photobiont in lichens is clear – to provide carbon in the form of simple sugars. These sugars are used by the fungi to maintain physiological functions, to grow, and reproduce. However, in the case of lichens with both green algae and cyanobacteria, the lichen gets an added nutrient input from the cyanobacteria in the form of fixed nitrogen. Although lichens can probably access inorganic nitrogen from the atmosphere directly, it can be a limiting nutrient, so having an internal source can be an advantage especially in heavily leached environments. Lichen fungi can capture a very high proportion of the carbon fixed by their symbionts, immediately assimilating and transforming it into molecules unsuitable for the photobionts’ use.
Only about 100 species of photobionts are commonly found across all known lichens, representing 4 main genera. The vast majority of photobionts are from the genus Trebouxia, followed by Trentopohlia (both Chlorophyta), Nostoc and Scytonema (both Cyanobacteria). Most green-algal photobionts are unicellular green forms, but small colonial types and filamentous algae occur as well. Within the lichen thallus, most photobionts have a different morphology than they would when grown in isolation, so few photobionts can be reliably identified using traditional microscopic methods. Instead, it is best to rely on culturing studies, and more often, on molecular methods, as many different strains have very similar morphology. The jelly-lichens are one exception, where the chain-of-pearls structure of Nostoc is very clear under the microscope.
Recent studies on green-algal lichens have shown that lichen fungi can associate with different photobionts, and we suspect that these ‘choices’ in association occur according to their ecological needs. For example, the same fungal species will use different photobionts in different ecological settings, even within similar geographic areas. Some individual lichens even contain more than a single photobiont strain, a situation that has been better explored in other symbiotic systems. In corals and other marine reef organisms, the animals can regulate the populations of their varying photobionts to maximize photosynthetic output according to environmental variation. Plants are known to regulate their nutritional symbionts as well, by cutting off raw materials they can’t produce themselves – perhaps lichens will be found to be similarly carefully regulating their symbionts.

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