Lichen

For other uses, see Lichen (disambiguation).

A tree covered with leafy foliose lichens and shrubby fruticose lichens

Common lichen growth forms

Letharia vulpina , wolf lichen, grows like a multiple-branched tuft or leafless mini-shrub, so it has a fruticose growth form.

Flavoparmelia caperata has leaf-like structures, so it is foliose.

Caloplaca marina grows like an orange crust coating the rock, so it is crustose.

Caloplaca thallincola  [ sv ] grows like a crust, and in a pattern that radiates outward from the center, so it has a crustose placodioid growth form.

Chrysothrix chlorina grows like powder dusted on the rock so it is leprose.

Collema nigrescens is gelatinous, without internal structure for its parts.

A lichen ( /ˈlaɪkən/ LY-kən, UK also /ˈlɪtʃən/ LITCH-ən) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species ^[1] in a mutualistic relationship. ^{[2] [3] [4]} Lichens are important actors in nutrient cycling  and act as the producers for which many higher trophic feeders feed off of, such as reindeer, gastropods, nematodes, mites, and springtails. ^{[5] [6] [7] [8]} Lichens have properties different from those of their component organisms. They come in many colors, sizes, and forms and are sometimes plant-like, but are not plants. They may have tiny, leafless branches (fruticose); flat leaf-like structures (foliose); grow crust-like, adhering tightly to a surface (substrate) like a thick coat of paint (crustose); ^[9] have a powder-like appearance (leprose); or other growth forms. ^[10]

A macrolichen is a lichen that is either bush-like or leafy; all other lichens are termed microlichens. ^[2] Here, "macro" and "micro" do not refer to size, but to the growth form. ^[2] Common names for lichens may contain the word moss (e.g., "reindeer moss", "Iceland moss"), and lichens may superficially look like and grow with mosses, but they are not closely related to mosses or any plant. ^{[4] : 3} Lichens do not have roots that absorb water and nutrients as plants do, ^{[11] : 2} but like plants, they produce their own nutrition by photosynthesis. ^[12] When they grow on plants, they do not live as parasites, but instead use the plant's surface as a substrate.

Lichens occur from sea level to high alpine elevations, in many environmental conditions, and can grow on almost any surface. ^[12] They are abundant growing on bark, leaves, mosses, or other lichens ^[11] and hanging from branches "living on thin air" (epiphytes) in rainforests and in temperate woodland. They grow on rock, walls, gravestones, roofs, exposed soil surfaces, rubber, bones, and in the soil as part of biological soil crusts. Various lichens have adapted to 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.

It is estimated that 6–8% of Earth's land surface is covered by lichens. ^[13] There are about 20,000 known species. ^[14] Some lichens have lost the ability to reproduce sexually, yet continue to speciate. ^{[11] [15]} They can be seen as being relatively self-contained miniature ecosystems, where the fungi, algae, or cyanobacteria have the potential to engage with other microorganisms in a functioning system that may evolve as an even more complex composite organism. ^{[16] [17] [18] [19]} Lichens may be long-lived, with some considered to be among the oldest living things. ^{[4] [20]} 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 species can be used to date events (lichenometry).

Etymology and pronunciation

The English word lichen derives from the Greek λειχήνleichēncode: ell promoted to code: el ("tree moss, lichen, lichen-like eruption on skin") via Latin lichen. ^{[21] [22] [23]} The Greek noun, which literally means "licker", derives from the verb λείχεινleicheincode: ell promoted to code: el , "to lick". ^{[24] [25]} In American English, "lichen" is pronounced the same as the verb "liken" ( /ˈlaɪkən/ ). In British English, both this pronunciation and one rhyming with "kitchen" ( /ˈlɪtʃən/ ) are used. ^{[26] [27] [28]}

Anatomy and morphology

Growth forms

Main article: Lichen growth forms

Lichens grow in a wide range of shapes and forms (morphologies). The shape of a lichen is usually determined by the organization of the fungal filaments. ^[29] The nonreproductive tissues, or vegetative body parts, are called the thallus. Lichens are grouped by thallus type, 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 often come from a growth form or color that is typical of a lichen genus.

Common groupings of lichen thallus growth forms are:

1. fruticose ^{[30] [31] [32]} – growing like a tuft or multiple-branched leafless mini-shrub, upright or hanging down, 3-dimensional branches with nearly round cross section (terete) or flattened
2. foliose ^{[30] [31]} – growing in 2-dimensional, flat, leaf-like lobes
3. crustose ^{[9] [30] [31]} – crust-like, adhering tightly to a surface (substrate) like a thick coat of paint
4. squamulose ^[32] – formed of small leaf-like scales crustose below but free at the tips
5. leprose ^[33] – powdery
6. gelatinous – jelly-like
7. filamentous – stringy or like matted hair
8. byssoid – wispy, like teased wool
9. 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 or areolate, and the "island" pieces separated by the cracks are called areolas. ^[30] The areolas appear separated, but are (or were)^{[ citation needed ]} connected by an underlying prothallus or hypothallus. ^[33] When a crustose lichen grows from a center and appears to radiate out, it is called crustose placodioid. When the edges of the areolas lift up from the substrate, it is called squamulose. ^{[34] : 159  [32]}

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. Squamulose lichens may appear where the edges lift up. Gelatinous lichens may appear leafy when dry. ^{[34] : 159} Means of telling them apart in these cases are in the sections below.

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), with only the sexual fruiting part visible growing outside the rock. ^[30] These may be dramatic in color or appearance. ^[30] Forms of these sexual parts are not in the above growth form categories. ^[30] 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

Lichens come in many colors. ^{[11] : 4} Coloration is usually determined by the photosynthetic component. ^[29] Special pigments, such as yellow usnic acid, give lichens a variety of colors, including reds, oranges, yellows, and browns, especially in exposed, dry habitats. ^[35] In the absence of special pigments, lichens are usually bright green to olive gray when wet, gray or grayish-green to brown when dry. ^[35] This is because moisture causes the surface skin (cortex) to become more transparent, exposing the green photobiont layer. ^[35] 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. ^[35] 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" covering the vast rock faces of Yosemite National Park. ^[36]

Color is used in identification. ^{[37] : 4} The color of a lichen changes depending on whether the lichen is wet or dry. ^[37] Color descriptions used for identification are based on the color that shows when the lichen is dry. ^[37] Dry lichens with a cyanobacterium as the photosynthetic partner tend to be dark grey, brown, or black. ^[37]

The underside of the leaf-like lobes of foliose lichens is a different color from the top side (dorsiventral), often brown or black, sometimes white. A fruticose lichen may have flattened "branches", appearing similar to a foliose 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. ^[33]

The sheen on some jelly-like gelatinous lichens is created by mucilaginous secretions. ^[29]

Internal structure

The cyanobacterium Hyella caespitosa with fungal hyphae in the lichen Pyrenocollema halodytes

A lichen consists of a simple photosynthesizing organism, usually a green alga or cyanobacterium, surrounded by filaments of a fungus. Generally, most of a lichen's bulk is made of interwoven fungal filaments, ^[38] but this is reversed in filamentous and gelatinous lichens. ^[29] The fungus is called a mycobiont. The photosynthesizing organism is called a photobiont. Algal photobionts are called phycobionts. ^[39] Cyanobacteria photobionts are called cyanobionts. ^[39]

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 . The filaments grow by branching then rejoining to create a mesh, which is called being "anastomosed". The mesh of fungal filaments may be dense or loose.

Generally, the fungal mesh surrounds the algal or cyanobacterial cells, often enclosing them within complex fungal tissues that are unique to lichen associations. The thallus may or may not have a protective "skin" of densely packed fungal filaments, often containing a second fungal species, ^[1] 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. Filamentous, byssoid, leprose, ^[33] gelatinous, and other lichens do not have a cortex, which is called being ecorticate. ^[40]

Schematic cross section of foliose lichen:
(a) The cortex is the outer layer of tightly woven fungus filaments (hyphae)
(b) This photobiont layer has photosynthesizing green algae
(c) Loosely packed hyphae in the medulla
(d) A tightly woven lower cortex
(e) Anchoring hyphae called rhizines where the fungus attaches to the substrate

Fruticose, foliose, crustose, and squamulose lichens generally have up to three different types of tissue, differentiated by having different densities of fungal filaments. ^[38] The top layer, where the lichen contacts the environment, is called a cortex. ^[38] The cortex is made of densely tightly woven, packed, and glued together (agglutinated) fungal filaments. ^[38] 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. ^[38] The cortex layer can be up to several hundred micrometers (μm) in thickness (less than a millimeter). ^[41] The cortex may be further topped by an epicortex of secretions, not cells, 0.6–1 μm thick in some lichens. ^[41] This secretion layer may or may not have pores. ^[41]

Below the cortex layer is a layer called the photobiontic layer or symbiont layer. ^{[31] [38]} The symbiont layer has less densely packed fungal filaments, with the photosynthetic partner embedded in them. ^[38] The less dense packing allows air circulation during photosynthesis, similar to the anatomy of a leaf. ^[38] Each cell or group of cells of the photobiont is usually individually wrapped by hyphae, and in some cases penetrated by a haustorium. ^[29] In crustose and foliose lichens, algae in the photobiontic layer are 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. ^[29]

The layer beneath the symbiont layer is called the medulla . The medulla is less densely packed with fungal filaments than the layers above. In foliose lichens, there is usually, as in Peltigera , ^{[34] : 159} another densely packed layer of fungal filaments called the lower cortex. ^{[33] [38]} Root-like fungal structures called rhizines (usually) ^{[34] : 159} grow from the lower cortex to attach or anchor the lichen to the substrate. ^{[2] [33]} Fruticose lichens have a single cortex wrapping all the way around the "stems" and "branches". ^[34] The medulla is the lowest layer, and may form a cottony white inner core for the branchlike thallus, or it may be hollow. ^{[34] : 159} Crustose and squamulose lichens lack a lower cortex, and the medulla is in direct contact with the substrate 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. ^[38] 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. ^[33]

Gelatinous, ^{[34] : 159} byssoid, and leprose lichens lack a cortex (are ecorticate), and generally have only undifferentiated tissue, similar to only having a symbiont layer.^{[ citation needed ]}

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 .

Pruinia is a whitish coating on top of an upper surface. ^[42] An epinecral layer is "a layer of horny dead fungal hyphae with indistinct lumina in or near the cortex above the algal layer". ^[42]

In August 2016, it was reported that some macrolichens have more than one species of fungus in their tissues. ^[1]

Physiology

Symbiotic relation

Main article: Symbiosis in lichens

Lichens are fungi that have discovered agriculture

— Trevor Goward ^[43]

A lichen is a composite organism that emerges from algae or cyanobacteria living among the filaments (hyphae) of the fungi in a mutually beneficial symbiotic relationship. The fungi benefit from the carbohydrates produced by the algae or cyanobacteria via photosynthesis. The algae or cyanobacteria benefit by being protected from the environment by the filaments of the fungi, 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. If a cyanobacterium is present, as a primary partner or another symbiont in addition to a green alga as in certain tripartite lichens, they can fix atmospheric nitrogen, complementing the activities of the green alga.

In three different lineages the fungal partner has independently lost the mitochondrial gene atp9, which has key functions in mitochondrial energy production. The loss makes the fungi completely dependent on their symbionts. ^[44]

The algal or cyanobacterial cells are photosynthetic and, as in plants, they reduce atmospheric carbon dioxide into organic carbon sugars to feed both symbionts. Phycobionts (algae) produce sugar alcohols (ribitol, sorbitol, and erythritol), which are absorbed by the mycobiont (fungus). ^[39] Cyanobionts produce glucose. ^[39] Lichenized fungal cells can make the photobiont "leak" out the products of photosynthesis, where they can then be absorbed by the fungus. ^{[11] : 5}

It appears many, probably the majority, of lichen also live in a symbiotic relationship with an order of basidiomycete yeasts called Cyphobasidiales. The absence of this third partner could explain why growing lichen in the laboratory is difficult. The yeast cells are responsible for the formation of the characteristic cortex of the lichen thallus, and could also be important for its shape. ^[45]

The lichen combination of alga or cyanobacterium with a fungus has a very different form (morphology), physiology, and biochemistry than the component fungus, alga, or cyanobacterium growing by itself, naturally or in culture. The body (thallus) 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). If combined with its photobiont under appropriate conditions, its characteristic form associated with the photobiont emerges, in the process called morphogenesis. ^[4] 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.^{[ citation needed ]}

Evidence that lichens are examples of successful symbiosis is the fact that lichens can be found in almost every habitat and geographic area on the planet. ^[16] 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. ^[46]

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. ^[16]

At least one form of lichen, the North American beard-like lichens, are constituted of not two but three symbiotic partners: an ascomycetous fungus, a photosynthetic alga, and, unexpectedly, a basidiomycetous yeast. ^[47]

Phycobionts can have a net output of sugars with only water vapor. ^[39] The thallus must be saturated with liquid water for cyanobionts to photosynthesize. ^[39]

Algae produce sugars that are absorbed by the fungus by diffusion into special fungal hyphae called appressoria or haustoria in contact with the wall of the algal cells. ^[48] The appressoria or haustoria may produce a substance that increases permeability of the algal cell walls, and may penetrate the walls. ^[48] The algae may contribute up to 80% of their sugar production to the fungus. ^[48]

Ecology

Lichen associations may be examples of mutualism or commensalism, but the lichen relationship can be considered parasitic ^[49] under circumstances where 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 exchange. The association continues because reproduction of the photobiont cells matches the rate at which they are destroyed. ^[49] The fungus surrounds the algal cells, ^[12] often enclosing them within complex fungal tissues unique to lichen associations. In many species the fungus penetrates the algal cell wall, ^[12] forming penetration pegs (haustoria) similar to those produced by pathogenic fungi that feed on a host. ^{[32] [50]} Cyanobacteria in laboratory settings can grow faster when they are alone rather than when they are part of a lichen.

Miniature ecosystem and holobiont theory

Symbiosis in lichens is so well-balanced that lichens have been considered to be relatively self-contained miniature ecosystems in and of themselves. ^{[16] [17]} 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. ^{[18] [19]}

Many lichens are very sensitive to environmental disturbances and can be used to cheaply ^[12] assess air pollution, ^{[51] [52] [53]} ozone depletion, and metal contamination. Lichens have been used in making dyes, perfumes, ^[54] and in traditional medicines. A few lichen species are eaten by insects ^[12] or larger animals, such as reindeer. ^[55] Lichens are widely used as environmental indicators or bio-indicators. When air is very badly polluted with sulphur dioxide, there may be no lichens present; only some green algae can tolerate those conditions. If the air is clean, then shrubby, hairy and leafy lichens become abundant. A few lichen species can tolerate fairly high levels of pollution, and are commonly found in urban areas, on pavements, walls and tree bark. The most sensitive lichens are shrubby and leafy, while the most tolerant lichens are all crusty in appearance. Since industrialisation, many of the shrubby and leafy lichens such as Ramalina , Usnea and Lobaria species have very limited ranges, often being confined to the areas which have the cleanest air.

Lichenicolous fungi

Some fungi can only be found living on lichens as obligate parasites. These are referred to as lichenicolous fungi, and are a different species from the fungus living inside the lichen; thus they are not considered to be part of the lichen. ^[56]

Reaction to water

Moisture makes the cortex become more transparent. ^{[11] : 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 show more clearly and the lichen looks greener.

Metabolites, metabolite structures and bioactivity

Lichens can show intense antioxidant activity. ^{[57] [58]} Secondary metabolites are often deposited as crystals in the apoplast. ^[59] Secondary metabolites are thought to play a role in preference for some substrates over others. ^[59]

Sometimes lichens contain structures made from fungal metabolites, for example crustose lichens sometimes have a polysaccharide ^{[ clarification needed ]} layer in the cortex.^{[ citation needed ]}

Growth rate

Lichens often have a regular but very slow growth rate of less than a millimeter per year.

In crustose lichens, the area along the margin is where the most active growth is taking place. ^{[34] : 159} Most crustose lichens grow only 1–2 mm in diameter per year.

Life span

Lichens may be long-lived, with some considered to be among the oldest living organisms. ^{[4] [20]} Lifespan is difficult to measure because what defines the "same" individual lichen is not precise. ^[60] Lichens grow by vegetatively breaking off a piece, which may or may not be defined as the "same" lichen, and two lichens can merge, then becoming the "same" lichen. ^[60] An Arctic species called "map lichen" ( Rhizocarpon geographicum ) has been dated at 8,600 years, apparently the world's oldest living organism. ^[61]

Response to environmental stress

Unlike simple dehydration in plants and animals, lichens may experience a complete loss of body water in dry periods. ^[12] Lichens are capable of surviving extremely low levels of water content (poikilohydric). ^{[62] : 5–6} They quickly absorb water when it becomes available again, becoming soft and fleshy. ^[12]

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). ^{[63] [64]}

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 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 unchanged in their ability to photosynthesize. ^{[65] [66]}

Reproduction and dispersal

Vegetative reproduction

Xanthoparmelia sp. with dark-colored reproductive structures (disc-like apothecia) at center, surrounded by a pale coloured vegetative thallus.

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. ^[2] 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 ]} fragment, and new lichens can grow from the fragment (vegetative reproduction).^{[ citation needed ]} Many lichens break up into fragments when they dry, dispersing themselves by wind action, to resume growth when moisture returns. ^{[67] [68]} 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. ^[2] Isidia (singular: "isidium") are branched, spiny, elongated, outgrowths from the thallus that break off for mechanical dispersal. ^[2] 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". ^[69]

Sexual reproduction

Disc-like apothecia (left) and thallus (right) on a foliose lichen

Structures involved in reproduction often appear as discs, bumps, or squiggly lines on the surface of the thallus. ^{[11] : 4} Though it has been argued that sexual reproduction in photobionts is selected against, there is strong evidence that suggests meiotic activities (sexual reproduction) in Trebouxia. ^{[70] [71]} 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.

Some lichen fungi belong to the phylum Basidiomycota (basidiolichens) and produce mushroom-like reproductive structures resembling those of their nonlichenized relatives.

Most lichen fungi belong to Ascomycetes (ascolichens). Among the ascolichens, spores are produced in spore-producing structures called ascomata . ^[11] The most common types of ascomata are the apothecium (plural: apothecia) and perithecium (plural: perithecia). ^{[11] : 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. ^{[11] : 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. ^{[11] : 14}

The three most common spore body types are raised discs called apothecia (singular: apothecium), bottle-like cups with a small hole at the top called perithecia (singular: perithecium), and pycnidia (singular: pycnidium), shaped like perithecia but without asci (an ascus is the structure that contains and releases the sexual spores in fungi of the Ascomycota). ^[72]

The apothecium has a layer of exposed spore-producing cells called asci (singular: ascus), and is usually a different color from the thallus tissue. ^{[11] : 14} When the apothecium has an outer margin, the margin is called the exciple. ^{[11] : 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 . ^{[11] : 14} When the exciple is blackened like carbon it is called lecideine meaning similar to members of the genus Lecidea . ^{[11] : 14} When the margin is pale or colorless it is called biatorine. ^{[11] : 14}

Crust-like thallus with pseudopodetia

A "podetium" (plural: podetia) is a lichenized stalk-like structure of the fruiting body rising from the thallus, associated with some fungi that produce a fungal apothecium. ^[31] Since it is part of the reproductive tissue, podetia are not considered part of the main body (thallus), but may be visually prominent. ^[31] The podetium may be branched, and sometimes cup-like. They usually bear the fungal pycnidia or apothecia or both. ^[31] Many lichens have apothecia that are visible to the naked eye. ^[2]

Most lichens produce abundant sexual structures. ^[73] Many species appear to disperse only by sexual spores. ^[73] 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. ^[73]

Mazaedia (singular: mazaedium) are apothecia shaped like a dressmaker's pin in pin lichens, 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. ^{[11] : 15}

Taxonomy and classification

Lichens are classified by the fungal component. Lichen species are given the same scientific name (binomial name) 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 fungus. ^[74] There are about 13,500–17,000 identified lichen species. ^[48] Nearly 20% of known fungal species are associated with lichens. ^[48]

"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. ^[75]

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. ^[76] 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. Fungi from Verrucariales also form marine lichens with the brown algae Petroderma maculiforme, ^[77] and have a symbiotic relationship with seaweed (such as rockweed) and Blidingia minima , where the algae are the dominant components. The fungi is thought to help the rockweeds to resist desiccation when exposed to air. ^{[78] [79]} In addition, lichens can also use yellow-green algae ( Heterococcus ) as their symbiotic partner. ^[80]

Lichens independently emerged from fungi associating with algae and cyanobacteria multiple times throughout history. ^[81]

Fungi

The fungal component of a lichen is called the mycobiont. The mycobiont may be an Ascomycete or Basidiomycete. ^[14] The associated lichens are called either ascolichens or basidiolichens, respectively. Living as a symbiont 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. ^[82]

Thalli produced by a given fungal symbiont with its differing partners may be similar,^{[ citation needed ]} and the secondary metabolites identical,^{[ citation needed ]} indicating^{[ citation needed ]} that the fungus has the dominant role in determining the morphology of the lichen. But the same mycobiont with different photobionts may also produce very different growth forms. ^[75] 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 ]}

Two or more fungal species can interact to form the same lichen. ^[83]

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