Lichen

Beginning in 1867, a lichen (

The English word lichen derives from the Greek λειχήν leichēn ("tree moss, lichen, lichen-like eruption on skin") via Latin lichen.^[23][24][25] The Greek noun, which literally means "licker", derives from the verb λείχειν leichein, "to lick".^[26][27] 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.^[28][29][30]

Anatomy and morphology

Growth forms

Lichens grow in a wide range of shapes and forms; this external appearance is known as their morphology. The shape of a lichen is usually determined by the organization of the fungal filaments.^[31] 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^[32][33][34] – 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^[32][33] – growing in 2-dimensional, flat, leaf-like lobes
3. crustose^[10][32][33] – crust-like, adhering tightly to a surface (substrate) like a thick coat of paint
4. squamulose^[34] – formed of small leaf-like scales crustose below but free at the tips
5. leprose^[35]
   – powdery
6. gelatinous
   – jelly-like
7. filamentous
   – stringy or like matted hair
8. teased wool
9. structureless

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.[36]^: 159

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.^[32] These may be dramatic in color or appearance.^[32] Forms of these sexual parts are not in the above growth form categories.^[32] 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.

Different colored lichens may inhabit different adjacent sections of a rock face, depending on the angle of exposure to light.[37] 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.^[38]

Color is used in identification.^[39]: 4 The color of a lichen changes depending on whether the lichen is wet or dry.^[39] Color descriptions used for identification are based on the color that shows when the lichen is dry.^[39] Dry lichens with a cyanobacterium as the photosynthetic partner tend to be dark grey, brown, or black.^[39]

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.^[35]

The sheen on some jelly-like gelatinous lichens is created by

Fruticose, foliose, crustose, and squamulose lichens generally have up to three different types of tissue, differentiated by having different densities of fungal filaments.^[40] The top layer, where the lichen contacts the environment, is called a cortex.^[40] The cortex is made of densely tightly woven, packed, and glued together (agglutinated) fungal filaments.^[40] 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.^[40] The cortex layer can be up to several hundred micrometers (μm) in thickness (less than a millimeter).^[43] The cortex may be further topped by an epicortex of secretions, not cells, 0.6–1 μm thick in some lichens.^[43] This secretion layer may or may not have pores.^[43]

Below the cortex layer is a layer called the photobiontic layer or symbiont layer.^[33][40] The symbiont layer has less densely packed fungal filaments, with the photosynthetic partner embedded in them.^[40] The less dense packing allows air circulation during photosynthesis, similar to the anatomy of a leaf.^[40] Each cell or group of cells of the photobiont is usually individually wrapped by hyphae, and in some cases penetrated by a haustorium.^[31] 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.^[31]

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, as in Peltigera,^[36]: 159 there is usually another densely packed layer of fungal filaments called the lower cortex.^[35][40] Root-like fungal structures called rhizines (usually)^[36]: 159 grow from the lower cortex to attach or anchor the lichen to the substrate.^[2][35] Fruticose lichens have a single cortex wrapping all the way around the "stems" and "branches".^[36] The medulla is the lowest layer, and may form a cottony white inner core for the branchlike thallus, or it may be hollow.^[36]: 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.^[40] 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.^[35]

Gelatinous,

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.

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

Physiology

Symbiotic relation

Lichens are fungi that have discovered agriculture

— Trevor Goward^[45]

A lichen is a composite organism that emerges from

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.[46]

The algal or cyanobacterial cells are

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.^[17] 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.^[48]

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.^[17]

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.^[49]

Phycobionts can have a net output of sugars with only water vapor.^[41] The thallus must be saturated with liquid water for cyanobionts to photosynthesize.^[41]

Algae produce sugars that are absorbed by the fungus by diffusion into special fungal hyphae called

Symbiosis in lichens is so well-balanced that lichens have been considered to be relatively self-contained miniature ecosystems in and of themselves.[17]^[18] 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.^[19][20]

Many lichens are very sensitive to environmental disturbances and can be used to cheaply^[13] assess air pollution,^[53][54][55] ozone depletion, and metal contamination. Lichens have been used in making dyes, perfumes (oakmoss),^[56] and in traditional medicines. A few lichen species are eaten by insects^[13] or larger animals, such as reindeer.^[57] 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

Moisture makes the cortex become more transparent.[12]^: 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.

In crustose lichens, the area along the margin is where the most active growth is taking place.[36]^: 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][21] Lifespan is difficult to measure because what defines the "same" individual lichen is not precise.^[62] 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.^[62] One specimen of Rhizocarpon geographicum on East Baffin Island has an estimated age of 9500 years.^[63][64] Thalli of Rhizocarpon geographicum and Rhizocarpon eupetraeoides/inarense in the central Brooks Range of northern Alaska have been given a maximum possible age of 10,000–11,500 years.^[65][66]

Response to environmental stress

Unlike simple dehydration in plants and animals, lichens may experience a complete loss of body water in dry periods.

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 Rusavskia 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.^[70][71]

Reproduction and dispersal

Vegetative reproduction

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 fragment, and new lichens can grow from the fragment (vegetative reproduction). Many lichens break up into fragments when they dry, dispersing themselves by wind action, to resume growth when moisture returns.^[72][73] 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".^[74]

Sexual reproduction

Structures involved in reproduction often appear as discs, bumps, or squiggly lines on the surface of the thallus.[12]^: 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.^[75][76] 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

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).^[77]

The apothecium has a layer of exposed spore-producing cells called asci (singular: ascus), and is usually a different color from the thallus tissue.^[12]: 14 When the apothecium has an outer margin, the margin is called the exciple.^[12]: 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.^[12]: 14 When the exciple is blackened like carbon it is called lecideine meaning similar to members of the genus Lecidea.^[12]: 14 When the margin is pale or colorless it is called biatorine.^[12]: 14

A "

"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.[83]

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

Lichens independently emerged from fungi associating with algae and cyanobacteria multiple times throughout history.[89]

Fungi

The fungal component of a lichen is called the mycobiont. The mycobiont may be an

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.^[83] 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.^[91]

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