A lichen ( , ) is a composite organism that arises from

algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, bl ...

living among filaments of multiple fungi species in a mutualistic relationship.Introduction to Lichens – An Alliance between Kingdoms
. University of California Museum of Paleontology. 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); have a powder-like appearance ( leprose); or other growth forms. A ''macrolichen'' is a lichen that is either bush-like or leafy; all other lichens are termed ''microlichens''. Here, "macro" and "micro" do not refer to size, but to the growth form. Common names for lichens may contain the word ''

moss Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta (, ) '' sensu stricto''. Bryophyta (''sensu lato'', Schimp. 1879) may also refer to the parent group bryophytes, which comprise liverworts, mosses, and hor ...

'' (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.Brodo, Irwin M. and Duran Sharnoff, Sylvia (2001) ''Lichens of North America''. . Lichens do not have roots that absorb water and nutrients as plants do,Sharnoff, Stephen (2014) ''Field Guide to California Lichens'', Yale University Press. but like plants, they produce their own nutrition by photosynthesis. When they grow on plants, they do not live as parasites, but instead use the plant's surface as a

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (locomotion), the surface over which an organism lo ...

. Lichens occur from sea level to high alpine elevations, in many environmental conditions, and can grow on almost any surface. They are abundant growing on bark,

leaves A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...

, mosses, or other lichens and hanging from branches "living on thin air" (

epiphyte An epiphyte is an organism that grows on the surface of a plant and derives its moisture and nutrients from the air, rain, water (in marine environments) or from debris accumulating around it. The plants on which epiphytes grow are called phoroph ...

s) in

rainforest Rainforests are characterized by a closed and continuous tree canopy, moisture-dependent vegetation, the presence of epiphytes and lianas and the absence of wildfire. Rainforest can be classified as tropical rainforest or temperate rainfores ...

s and in

temperate woodland A woodland () is, in the broad sense, land covered with trees, or in a narrow sense, synonymous with wood (or in the U.S., the ''plurale tantum'' woods), a low-density forest forming open habitats with plenty of sunlight and limited shade (see ...

. 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

desert A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About o ...

s, rocky coasts, and toxic

slag Slag is a by-product of smelting (pyrometallurgical) ores and used metals. Broadly, it can be classified as ferrous (by-products of processing iron and steel), ferroalloy (by-product of ferroalloy production) or non-ferrous/base metals (by-prod ...

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. There are about 20,000 known species. Some lichens have lost the ability to reproduce sexually, yet continue to speciate. 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. Lichens may be long-lived, with some considered to be among the oldest living things. 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 ("tree moss, lichen, lichen-like eruption on skin") via

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power ...

. The Greek noun, which literally means "licker", derives from the verb , "to lick". In American English, "lichen" is pronounced the same as the verb "liken" (). In British English, both this pronunciation and one rhyming with "kitchen" () are used.

Anatomy and morphology

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. 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 Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

. Common groupings of lichen thallus growth forms are: # fruticose – 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 # foliose – growing in 2-dimensional, flat, leaf-like lobes # crustose – crust-like, adhering tightly to a surface (

) like a thick coat of paint # squamulose – formed of small leaf-like scales crustose below but free at the tips # leprose – powdery # gelatinous – jelly-like #

filamentous The word filament, which is descended from Latin ''filum'' meaning " thread", is used in English for a variety of thread-like structures, including: Astronomy * Galaxy filament, the largest known cosmic structures in the universe * Solar filament ...

– stringy or like matted hair #

byssoid Lichens are composite organisms made up of multiple species: a fungal partner, one or more photosynthetic partners, and sometimes a basidiomycete yeast. They are regularly grouped by their external appearance – a characteristic known as their g ...

– wispy, like

teased wool Carding is a mechanical process that disentangles, cleans and intermixes fibres to produce a continuous web or sliver suitable for subsequent processing. This is achieved by passing the fibres between differentially moving surfaces covered wit ...

# 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. The areolas appear separated, but are (or were) connected by an underlying prothallus or hypothallus. 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. 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. Means of telling them apart in these cases are in the sections below. Structures involved in reproduction often appear as discs, bumps, or squiggly lines on the surface of the thallus. 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 lichen An endolithic lichen is a crustose Crustose is a habit of some types of algae and lichens in which the organism grows tightly appressed to a substrate, forming a biological layer. ''Crustose'' adheres very closely to the substrates at all points. ...

s), with only the sexual fruiting part visible growing outside the rock. These may be dramatic in color or appearance. Forms of these sexual parts are not in the above growth form categories.Lichen Vocabulary, Lichens of North America Information, Sylvia and Stephen Sharnoff

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. Coloration is usually determined by the photosynthetic component. Special pigments, such as yellow

usnic acid Usnic acid is a naturally occurring dibenzofuran derivative found in several lichen species with the formula C18H16O7. It was first isolated by German scientist W. Knop in 1844 and first synthesized between 1933-1937 by Curd and Robertson. Usnic a ...

, give lichens a variety of colors, including reds, oranges, yellows, and browns, especially in exposed, dry habitats. In the absence of special pigments, lichens are usually bright green to olive gray when wet, gray or grayish-green to brown when dry. This is because moisture causes the surface skin ( cortex) to become more transparent, exposing the green photobiont layer. 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. 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. Color is used in identification.Michigan Lichens, Julie Jones Medlin, B. Jain Publishers, 1996, , 9780877370390

The color of a lichen changes depending on whether the lichen is wet or dry. Color descriptions used for identification are based on the color that shows when the lichen is dry. Dry lichens with a cyanobacterium as the photosynthetic partner tend to be dark grey, brown, or black. 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. The sheen on some jelly-like gelatinous lichens is created by mucilaginous secretions.

Internal structure

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,Lichens: More on Morphology, University of California Museum of Paleontology

but this is reversed in filamentous and gelatinous lichens. The fungus is called a ''mycobiont''. The photosynthesizing organism is called a ''photobiont''. Algal photobionts are called ''phycobionts''. Cyanobacteria photobionts are called ''cyanobionts''. 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 A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one or ...

''. The filaments grow by branching then rejoining to create a mesh, which is called being "

anastomosed An anastomosis (, plural anastomoses) is a connection or opening between two things (especially cavities or passages) that are normally diverging or branching, such as between blood vessels, leaf veins, or streams. Such a connection may be norm ...

". The mesh of fungal filaments may be dense or loose. Generally, the fungal mesh surrounds the algal or

cyanobacterial Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

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, 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, gelatinous, and other lichens do not have a cortex, which is called being

ecorticate A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship. Fruticose, foliose, crustose, and squamulose lichens generally have up to three different types of tissue, differentiated by having different densities of fungal filaments. The top layer, where the lichen contacts the environment, is called a ''cortex''. The cortex is made of densely tightly woven, packed, and glued together ( agglutinated) fungal filaments. 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. The cortex layer can be up to several hundred micrometers (μm) in thickness (less than a millimeter). The cortex may be further topped by an epicortex of secretions, not cells, 0.6–1 μm thick in some lichens. This secretion layer may or may not have pores. Below the cortex layer is a layer called the ''photobiontic layer'' or ''symbiont layer''. The symbiont layer has less densely packed fungal filaments, with the photosynthetic partner embedded in them. The less dense packing allows air circulation during photosynthesis, similar to the anatomy of a leaf. Each cell or group of cells of the photobiont is usually individually wrapped by hyphae, and in some cases penetrated by a haustorium. 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. 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, as in '' Peltigera'', another densely packed layer of fungal filaments called the lower cortex. Root-like fungal structures called ''rhizines'' (

usually A convention is a set of agreed, stipulated, or generally accepted standards, norms, social norms, or criteria, often taking the form of a custom. In a social context, a convention may retain the character of an "unwritten law" of custom (for e ...

) grow from the lower cortex to attach or anchor the lichen to the substrate. Fruticose lichens have a single cortex wrapping all the way around the "stems" and "branches". The medulla is the lowest layer, and may form a cottony white inner core for the branchlike thallus, or it may be hollow. Crustose and squamulose lichens lack a lower cortex, and the medulla is in direct contact with the

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. 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. Gelatinous, byssoid, and leprose lichens lack a cortex (are

ecorticate A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.cephalodia {{Short description, Morphological structure found in some lichens Cephalodia (singular cephalodium) are small gall-like structures found in some species of lichens that contain cyanobacterial symbionts Symbiosis (from Greek , , "living toget ...

''. '' Pruinia'' is a whitish coating on top of an upper surface. An ''

epinecral layer A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.lumina in or near the cortex above the algal layer". In August 2016, it was reported that some macrolichens have more than one species of fungus in their tissues.

Physiology

Symbiotic relation

A lichen is a composite organism that emerges from

living among the filaments (

) of the fungi in a mutually beneficial

symbiotic Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...

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

. If a cyanobacterium is present, as a primary partner or another symbiont in addition to a green alga as in certain

tripartite Tripartite means composed of or split into three parts, or refers to three parties. Specifically, it may also refer to any of the following: * 3 (number) * Tripartite language * Tripartite motto * Tripartite System in British education * Tripa ...

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. The algal or cyanobacterial cells are

photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in c ...

and, as in plants, they reduce atmospheric

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

into organic carbon sugars to feed both symbionts. Phycobionts (algae) produce sugar alcohols ( ribitol,

sorbitol Sorbitol (), less commonly known as glucitol (), is a sugar alcohol with a sweet taste which the human body metabolizes slowly. It can be obtained by reduction of glucose, which changes the converted aldehyde group (−CHO) to a primary alcohol g ...

, and erythritol), which are absorbed by the mycobiont (fungus). Cyanobionts produce glucose. Lichenized fungal cells can make the photobiont "leak" out the products of photosynthesis, where they can then be absorbed by the fungus. 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. 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 (

). If combined with its photobiont under appropriate conditions, its characteristic form associated with the photobiont emerges, in the process called morphogenesis. 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 Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...

is the fact that lichens can be found in almost every habitat and geographic area on the planet. 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. 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. 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. Phycobionts can have a net output of sugars with only water vapor. The thallus must be saturated with liquid water for cyanobionts to photosynthesize. Algae produce sugars that are absorbed by the fungus by diffusion into special fungal hyphae called

appressoria An appressorium is a specialized cell typical of many fungal plant pathogens that is used to infect host plants. It is a flattened, hyphal "pressing" organ, from which a minute infection peg grows and enters the host, using turgor pressure capable ...

or haustoria in contact with the wall of the algal cells. The appressoria or haustoria may produce a substance that increases permeability of the algal cell walls, and may penetrate the walls. The algae may contribute up to 80% of their sugar production to the fungus.

Ecology

Lichen associations may be examples of mutualism or commensalism, but the lichen relationship can be considered parasitic 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 A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...

exchange. The association continues because reproduction of the photobiont cells matches the rate at which they are destroyed. The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations. In many species the fungus penetrates the algal cell wall, forming penetration pegs ( haustoria) similar to those produced by pathogenic fungi that feed on a host.

Cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, bl ...

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. Honegger, R. (1991) ''Fungal evolution: symbiosis and morphogenesis, Symbiosis as a Source of Evolutionary Innovation'', Margulis, L., and Fester, R. (eds). Cambridge, MA, USA: The MIT Press, pp. 319–340. 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.Barreno, E., Herrera-Campos, M., García-Breijo, F., Gasulla, F., and Reig-Armiñana, J. (2008
"Non photosynthetic bacteria associated to cortical structures on Ramalina and ''Usnea'' thalli from Mexico"
Asilomar, Pacific Grove, CA, USA: Abstracts IAL 6- ABLS Joint Meeting. Many lichens are very sensitive to environmental disturbances and can be used to cheaply assess air pollution, ozone depletion, and metal contamination. Lichens have been used in making

dye A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution an ...

s, perfumes, and in traditional medicines. A few lichen species are eaten by insects or larger animals, such as reindeer. 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 {{wiktionary, obligate As an adjective, obligate means "by necessity" (antonym ''facultative'') and is used mainly in biology in phrases such as: * Obligate aerobe, an organism that cannot survive without oxygen * Obligate anaerobe, an organism that ...

parasites. These are referred to as

lichenicolous fungi A lichenicolous fungus is a parasitic fungus that only lives on lichen as the host. A lichenicolous fungus is not the same as the fungus that is the component of the lichen, which is known as a lichenized fungus. They are most commonly specific ...

, and are a different species from the fungus living inside the lichen; thus they are not considered to be part of the lichen.

Reaction to water

Moisture makes the cortex become more transparent. 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. Secondary metabolites are often deposited as crystals in the apoplast. Secondary metabolites are thought to play a role in preference for some substrates over others. Sometimes lichens contain structures made from fungal

metabolite In biochemistry, a metabolite is an intermediate or end product of metabolism. The term is usually used for small molecules. Metabolites have various functions, including fuel, structure, signaling, stimulatory and inhibitory effects on enzymes, c ...

s, for example crustose lichens sometimes have a

polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...

layer in the cortex.

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. 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. Lifespan is difficult to measure because what defines the "same" individual lichen is not precise. 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. An

Arctic The Arctic ( or ) is a polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada ( Yukon, Northwest Territories, Nunavut), Danish Realm ( Greenland), Finland, Iceland ...

species called "map lichen" ('' Rhizocarpon geographicum'') has been dated at 8,600 years, apparently the world's oldest living organism.

Response to environmental stress

Unlike simple dehydration in plants and animals, lichens may experience a ''complete'' loss of body water in dry periods. Lichens are capable of surviving extremely low levels of water content ( poikilohydric). They quickly absorb water when it becomes available again, becoming soft and fleshy. 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). The

European Space Agency , owners = , headquarters = Paris, Île-de-France, France , coordinates = , spaceport = Guiana Space Centre , seal = File:ESA emblem seal.png , seal_size = 130px , image = Views in the Main Control Room (1205 ...

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.

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. 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 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. ''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. ''Isidia'' (singular: "isidium") are branched, spiny, elongated, outgrowths from the thallus that break off for mechanical dispersal. Lichen propagules (

diaspores In botany, a diaspore is a plant dispersal unit consisting of a seed or spore plus any additional tissues that assist dispersal. In some seed plants, the diaspore is a seed and fruit together, or a seed and elaiosome. In a few seed plants, the di ...

) 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".

Sexual reproduction

Structures involved in reproduction often appear as discs, bumps, or squiggly lines on the surface of the thallus. 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''. 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 Basidiomycota () is one of two large divisions that, together with the Ascomycota, constitute the subkingdom Dikarya (often referred to as the "higher fungi") within the kingdom Fungi. Members are known as basidiomycetes. More specifically, Basi ...

(''basidiolichens'') and produce mushroom-like reproductive structures resembling those of their nonlichenized relatives. Most lichen fungi belong to

Ascomycetes Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defi ...

(''ascolichens''). Among the ascolichens,

spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...

s are produced in spore-producing structures called '' ascomata''. The most common types of ascomata are the '' apothecium'' (plural: apothecia) and '' perithecium'' (plural: perithecia). 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''. 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. 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 An ascus (; ) is the sexual spore-bearing cell produced in ascomycete fungi. Each ascus usually contains eight ascospores (or octad), produced by meiosis followed, in most species, by a mitotic cell division. However, asci in some genera or s ...

is the structure that contains and releases the sexual spores in fungi of the

Ascomycota Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The def ...

). The apothecium has a layer of exposed spore-producing cells called ''

asci ASCI or Asci may refer to: * Advertising Standards Council of India * Asci, the plural of ascus, in fungal anatomy * Accelerated Strategic Computing Initiative * American Society for Clinical Investigation * Argus Sour Crude Index * Association of ...

'' (singular: ascus), and is usually a different color from the thallus tissue. When the apothecium has an outer margin, the margin is called the ''exciple''. 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''. When the exciple is blackened like carbon it is called ''lecideine'' meaning similar to members of the genus '' Lecidea''. When the margin is pale or colorless it is called ''biatorine''. Pilophorus acicularis 109164

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. Since it is part of the reproductive tissue, podetia are not considered part of the main body (thallus), but may be visually prominent. The podetium may be branched, and sometimes cup-like. They usually bear the fungal pycnidia or apothecia or both. Many lichens have apothecia that are visible to the naked eye. Most lichens produce abundant sexual structures. Many species appear to disperse only by sexual spores. 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. ''Mazaedia'' (singular: mazaedium) are apothecia shaped like a dressmaker's pin in

pin lichen Lichens are wikt:composite, composite organisms made up of multiple species: a fungus, fungal partner, one or more photosynthetic partners, and sometimes a yeast, basidiomycete yeast. They are regularly grouped by their external appearance – a c ...

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.

Taxonomy and classification

Lichens are classified by the fungal component. Lichen species are given the same scientific name (

binomial name In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bot ...

) 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. There are about 13,500–17,000 identified lichen species. Nearly 20% of known fungal species are associated with lichens. "''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. 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 In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

lineages in their respective fungal phyla, but they do form several major solely or primarily lichen-forming groups within each phylum. Even more unusual than basidiolichens is the fungus '' Geosiphon pyriforme'', a member of the

Glomeromycota Glomeromycota (often referred to as glomeromycetes, as they include only one class, Glomeromycetes) are one of eight currently recognized divisions within the kingdom Fungi, with approximately 230 described species. Members of the Glomeromycot ...

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'', and have a symbiotic relationship with

seaweed Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of '' Rhodophyta'' (red), ''Phaeophyta'' (brown) and ''Chlorophyta'' (green) macroalgae. Seaweed species such as ...

(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. In addition, lichens can also use yellow-green algae ('' Heterococcus'') as their symbiotic partner. Lichens independently emerged from fungi associating with algae and cyanobacteria multiple times throughout history.

Fungi

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

Ascomycete Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defi ...

or Basidiomycete. 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. Thalli produced by a given fungal symbiont with its differing partners may be similar, and the secondary metabolites identical, indicating 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. 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. Two or more fungal species can interact to form the same lichen. The following table lists the

orders Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

and families of fungi that include lichen-forming species.

Photobionts

The

partner in a lichen is called a ''photobiont''. The photobionts in lichens come from a variety of simple prokaryotic and eukaryotic organisms. In the majority of lichens the photobiont is a green alga ( Chlorophyta) or a cyanobacterium. In some lichens both types are present; in such cases, the alga is typically the primary partner, with the cyanobacteria being located in cryptic pockets. Algal photobionts are called ''phycobionts'', while cyanobacterial photobionts are called ''cyanobionts''. According to one source, about 90% of all known lichens have phycobionts, and about 10% have cyanobionts, while another source states that two thirds of lichens have green algae as phycobiont, and about one third have a cyanobiont. Approximately 100 species of photosynthetic partners from 40 genera and five distinct classes (prokaryotic: Cyanophyceae; eukaryotic: Trebouxiophyceae,

Phaeophyceae Brown algae (singular: alga), comprising the class Phaeophyceae, are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Brown algae are the major seaweeds of the temperate and po ...

Chlorophyceae The Chlorophyceae are one of the classes of green algae, distinguished mainly on the basis of ultrastructural morphology. They are usually green due to the dominance of pigments chlorophyll a and chlorophyll b. The chloroplast may be discoid, p ...

) have been found to associate with the lichen-forming fungi. Common algal photobionts are from the genera '' Trebouxia'', '' Trentepohlia'', '' Pseudotrebouxia'', or ''

Myrmecia Myrmecia can refer to: * ''Myrmecia'' (alga), genus of algae associated with lichens * ''Myrmecia'' (ant), genus of ants called bulldog ants * Myrmecia (skin), a kind of deep wart on the human hands or feet See also * '' Copromorpha myrmecias'' ...

''. ''Trebouxia'' 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'', or resembles a member of that genus, and is therefore presumably a member of the class Trebouxiophyceae. The second most commonly represented green alga genus is '' Trentepohlia''. Overall, about 100 species of eukaryotes 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. A " cyanolichen" is a lichen with a cyanobacterium as its main photosynthetic component (photobiont). Most cyanolichen are also ascolichens, but a few basidiolichen like Dictyonema and Acantholichen have cyanobacteria as their partner. The most commonly occurring cyanobacterium

is '' Nostoc''. Other common cyanobacterium photobionts are from '' Scytonema''. Many cyanolichens are small and black, and have limestone as the substrate. Another cyanolichen group, the

jelly lichen ''Collema'' (jelly lichen) is a genus of lichens in the family Collemataceae. The photobiont is the cyanobacterium genus ''Nostoc''.Dobson, F.S. (2000) Lichens, an illustrated guide to the British and Irish species. 4th edition. Richmond publishi ...

s of the genera '' Collema'' or '' Leptogium'' are gelatinous and live on moist soils. Another group of large and foliose species including '' Peltigera'', '' Lobaria'', and ''

Degelia ''Degelia'' is a genus of lichen-forming fungi in the family Pannariaceae. The genus is named after Swedish lichenologist Gunnar Degelius. The genus was circumscribed by Lars Arvidsson and David John Galloway in 1981. Species *''Degelia calc ...

'' are grey-blue, especially when dampened or wet. Many of these characterize the Lobarion communities of higher rainfall areas in western Britain, e.g., in the

Celtic rain forest Celtic Rainforest is a colloquial term which refers to the temperate rainforest of Ireland and Great Britain. These woodlands are also variously referred to as Atlantic rainforest, Upland Oakwoods, Atlantic Oakwoods or Western Oakwoods. Today, the ...

. Strains of cyanobacteria found in various cyanolichens are often closely related to one another. They differ from the most closely related free-living strains. The lichen association is a close symbiosis. It extends the ecological range of both partners but is not always obligatory for their growth and reproduction in natural environments, since many of the algal symbionts can live independently. A prominent example is the alga '' Trentepohlia'', which forms orange-coloured populations on tree trunks and suitable rock faces. Lichen propagules (

) 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". The same cyanobiont species can occur in association with different fungal species as lichen partners. The same phycobiont species can occur in association with different fungal species as lichen partners. More than one phycobiont may be present in a single thallus. A single lichen may contain several algal

genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...

s. These multiple genotypes may better enable response to adaptation to environmental changes, and enable the lichen to inhabit a wider range of environments.

Controversy over classification method and species names

There are about 20,000 known lichen species. But what is meant by "species" is different from what is meant by biological species in plants, animals, or fungi, where being the same species implies that there is a common

ancestral lineage An evolutionary lineage is a temporal series of populations, organisms, cells, or genes connected by a continuous line of descent from ancestor to descendant.The Oxford English Dictionary defines biological lineage as "a sequence of species each of ...

. Because lichens are combinations of members of two or even three different biological kingdoms, these components ''must'' have a ''different'' ancestral lineage from each other. By convention, lichens are still called "species" anyway, and are classified according to the species of their fungus, not the species of the algae or cyanobacteria. Lichens are given the same scientific name (

) as the fungus in them, which may cause some confusion. The alga bears its own scientific name, which has no relationship to the name of the lichen or fungus. Depending on context, "lichenized fungus" may refer to the entire lichen, or to the fungus when it is in the lichen, which can be grown in culture in isolation from the algae or cyanobacteria. Some algae and cyanobacteria are found naturally living outside of the lichen. The fungal, algal, or cyanobacterial component of a lichen can be grown by itself in culture. When growing by themselves, the fungus, algae, or cyanobacteria have very different properties than those of the lichen. Lichen properties such as growth form, physiology, and biochemistry, are very different from the combination of the properties of the fungus and the algae or cyanobacteria. The same fungus growing in combination with different algae or cyanobacteria, can produce lichens that are very different in most properties, meeting non-DNA criteria for being different "species". Historically, these different combinations were classified as different species. When the fungus is identified as being the same using modern DNA methods, these apparently different species get reclassified as the ''same'' species under the current (2014) convention for classification by fungal component. This has led to debate about this classification convention. These apparently different "species" have their own independent evolutionary history. There is also debate as to the appropriateness of giving the same binomial name to the fungus, and to the lichen that combines that fungus with an alga or cyanobacterium (

synecdoche Synecdoche ( ) is a type of metonymy: it is a figure of speech in which a term for a part of something is used to refer to the whole (''pars pro toto''), or vice versa (''totum pro parte''). The term comes from Greek . Examples in common Engl ...

). This is especially the case when combining the same fungus with different algae or cyanobacteria produces dramatically different lichen organisms, which would be considered different species by any measure other than the DNA of the fungal component. If the whole lichen produced by the same fungus growing in association with different algae or cyanobacteria, were to be classified as different "species", the number of "lichen species" would be greater.

Diversity

The largest number of lichenized fungi occur in the

, with about 40% of species forming such an association. Some of these lichenized fungi occur in orders with nonlichenized fungi that live as saprotrophs or plant parasites (for example, the Leotiales, Dothideales, and

Pezizales The Pezizales are an order of the subphylum Pezizomycotina within the phylum Ascomycota. The order contains 16 families, 199 genera, and 1683 species. It contains a number of species of economic importance, such as morels, the black and white tr ...

). Other lichen fungi occur in only five

in which all members are engaged in this habit (Orders Graphidales,

Gyalectales ''Gyalectales'' is an order of lichen-forming fungi in the class Lecanoromycetes. It contains 5 families, 15 genera and about 550 species. Taxonomy The Gyalectales were introduced in a 1974 publication by Aino Henssen and Martin Jahns, but not ...

, Peltigerales,

Pertusariales The Pertusariales are an order of fungi in the class Lecanoromycetes. It contains the following families: Agyriaceae, Coccotremataceae, Icmadophilaceae, Megasporaceae, Microcaliciaceae, Ochrolechiaceae, Pertusariaceae, Varicellariaceae, and ...

, and Teloschistales). Overall, about 98% of lichens have an ascomycetous mycobiont. Next to the Ascomycota, the largest number of lichenized fungi occur in the unassigned fungi imperfecti, a catch-all category for fungi whose sexual form of reproduction has never been observed. Comparatively few basidiomycetes are lichenized, but these include agarics, such as species of '' Lichenomphalia'', clavarioid fungi, such as species of '' Multiclavula'', and corticioid fungi, such as species of '' Dictyonema''.

Identification methods

Lichen identification uses growth form, microscopy and reactions to chemical tests. The outcome of the "Pd test" is called "Pd", which is also used as an abbreviation for the chemical used in the test,

para-phenylenediamine ''p''-Phenylenediamine (PPD) is an organic compound with the formula C6H4(NH2)2. This derivative of aniline is a white solid, but samples can darken due to air oxidation. It is mainly used as a component of engineering polymers and composites lik ...

. If putting a drop on a lichen turns an area bright yellow to orange, this helps identify it as belonging to either the genus '' Cladonia'' or '' Lecanora''.

Evolution and paleontology

The fossil record for lichens is poor."Lichens: Fossil Record"
, University of California Museum of Paleontology. The extreme habitats that lichens dominate, such as tundra, mountains, and deserts, are not ordinarily conducive to producing fossils. There are fossilized lichens embedded in amber. The fossilized ''Anzia'' is found in pieces of amber in northern Europe and dates back approximately 40 million years. Lichen fragments are also found in fossil leaf beds, such as '' Lobaria'' from Trinity County in northern California, USA, dating back to the early to middle Miocene. The oldest fossil lichen in which both symbiotic partners have been recovered is '' Winfrenatia'', an early zygomycetous (

n) lichen symbiosis that may have involved controlled parasitism, is permineralized in the Rhynie Chert of Scotland, dating from early Early Devonian, about 400 million years ago. The slightly older fossil ''

Spongiophyton ''Spongiophyton'' was a thallose fossil of the early to mid-Devonian, which is notoriously difficult to classify. ''Spongiophyton'' displayed dichotomous branching, and a flattened/elliptical cross section with a thick (20–80 μm) upper ...

'' has also been interpreted as a lichen on morphological and isotopic grounds, although the isotopic basis is decidedly shaky. It has been demonstrated that

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

fossils ''

Nematothallus ''Nematothallus'' is a form genus comprising cuticle-like fossils. Some of its constituents likely represent red algae, whereas others resemble lichens. History of research ''Nematothallus'' was first described by Lang in 1937, who envision ...

'' and '' Prototaxites'' were lichenized. Thus lichenized

and

were a component of Early Silurian-

terrestrial ecosystems. Newer research suggests that lichen evolved after the evolution of land plants. The ancestral ecological state of both

and

was probably saprobism, and independent lichenization events may have occurred multiple times. In 1995, Gargas and colleagues proposed that there were at least five independent origins of lichenization; three in the basidiomycetes and at least two in the Ascomycetes. Lutzoni et al. (2001) suggest lichenization probably evolved earlier and was followed by multiple independent losses. Some non-lichen-forming fungi may have secondarily lost the ability to form a lichen association. As a result, lichenization has been viewed as a highly successful nutritional strategy. Lichenized

may extend well back into the Precambrian. Lichen-like fossils consisting of coccoid cells (

?) and thin filaments (mucoromycotinan

?) are permineralized in marine

phosphorite Phosphorite, phosphate rock or rock phosphate is a non-detrital sedimentary rock that contains high amounts of phosphate minerals. The phosphate content of phosphorite (or grade of phosphate rock) varies greatly, from 4% to 20% phosphorus pentoxi ...

of the Doushantuo Formation in southern China. These fossils are thought to be 551 to 635 million years old or

Ediacaran The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and th ...

. Ediacaran acritarchs also have many similarities with

n vesicles and spores. It has also been claimed that Ediacaran fossils including ''

Dickinsonia ''Dickinsonia'' is an extinct genus of basal animal that lived during the late Ediacaran period in what is now Australia, China, Russia and Ukraine. The individual ''Dickinsonia'' typically resembles a bilaterally symmetrical ribbed oval. Its a ...

'', were lichens, although this claim is controversial. Endosymbiotic

comparable with living '' Geosiphon'' may extend back into the

Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...

in the form of 1500 million year old '' Horodyskia'' and 2200 million year old '' Diskagma''. Discovery of these fossils suggest that fungi developed symbiotic partnerships with photoautotrophs long before the evolution of vascular plants, though the Ediacaran lichen hypothesis is largely rejected due to an inappropriate definition of lichens based on taphonomy and substrate ecology. However, a 2019 study by the same scientist who rejected the Ediacaran lichen hypothesis, Nelsen, used new time-calibrated phylogenies to conclude that there is no evidence of lichen before the existence of vascular plants. Lecanoromycetes, one of the most common classes of lichen-forming fungi, diverged from its ancestor, which may have also been lichen forming, around 258 million years ago, during the late Paleozoic period. However, the closely related clade Euritiomycetes appears to have become lichen-forming only 52 million years ago, during the early Cenozoic period.

Ecology and interactions with environment

Substrates and habitats

Der Heilige Dominikus am Nordturm des Regensburger Doms

Lichens cover about 7% of the planet's surface and grow on and 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 in rain forests and in

. They grow on bare rock, walls, gravestones, roofs, and exposed soil surfaces. They can survive in some of the most extreme environments on Earth: arctic tundra, hot dry

s, rocky coasts, and toxic slag heaps. They can live inside solid rock, growing between the grains, and in the soil as part of a biological soil crust in arid habitats such as deserts. Some lichens do not grow on anything, living out their lives blowing about the environment. When growing on mineral surfaces, some lichens slowly decompose their substrate by chemically degrading and physically disrupting the minerals, contributing to the process of weathering by which rocks are gradually turned into soil. While this contribution to weathering is usually benign, it can cause problems for artificial stone structures. For example, there is an ongoing lichen growth problem on Mount Rushmore National Memorial that requires the employment of mountain-climbing conservators to clean the monument. Lichens are not parasites on the plants they grow on, but only use them as a substrate. The fungi of some lichen species may "take over" the algae of other lichen species. Lichens make their own food from their photosynthetic parts and by absorbing minerals from the environment. Lichens growing on leaves may have the appearance of being parasites on the leaves, but they are not. Some lichens in ''Diploschistes'' parasitise other lichens. ''Diploschistes muscorum'' starts its development in the tissue of a host ''Cladonia'' species. In the arctic tundra, lichens, together with

mosses Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta (, ) ''sensu stricto''. Bryophyta (''sensu lato'', Schimp. 1879) may also refer to the parent group bryophytes, which comprise liverworts, mosses, and horn ...

and

liverworts The Marchantiophyta () are a division of non-vascular land plants commonly referred to as hepatics or liverworts. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of ge ...

, make up the majority of the ground cover, which helps insulate the ground and may provide forage for grazing animals. An example is " Reindeer moss", which is a lichen, not a moss. There are only two species of known permanently submerged lichens; '' Hydrothyria venosa'' is found in fresh water environments, and ''

Verrucaria serpuloides ''Verrucaria serpuloides'' is a species of saxicolous (rock-dwelling), crustose lichen belonging to the family Verrucariaceae. It is native to the Antarctic Peninsula. It is one of only two permanently submerged species of lichen, the other being ...

'' is found in marine environments. A crustose lichen that grows on rock is called a '' saxicolous lichen''.''Mosses Lichens & Ferns of Northwest North America'', Dale H. Vitt, Janet E. Marsh, Robin B. Bovey, Lone Pine Publishing Company, Crustose lichens that grow on the rock are epilithic, and those that grow immersed inside rock, growing between the crystals with only their fruiting bodies exposed to the air, are called ''

s''. A crustose lichen that grows on bark is called a ''

corticolous lichen A corticolous lichen is a lichen that grows on bark.Alan Silverside's Lichen Glossary (a-f), Alan Silverside/ref> This is contrasted with lignicolous lichen, which grows on wood that has had the bark stripped from it,Alan Silverside's Lichen Glossa ...

''. A lichen that grows on wood from which the bark has been stripped is called a ''

lignicolous lichen A lignocolous lichen is a lichen that grows on wood that has the bark stripped from it.Alan Silverside's Lichen Glossary (g-o), Alan Silverside/ref> This is to be compared to a corticolous lichen that grows on bark,Alan Silverside's Lichen Glossary ...

''. Lichens that grow immersed inside plant tissues are called '' endophloidic lichens'' or '' endophloidal lichens''. Lichens that use leaves as substrates, whether the leaf is still on the tree or on the ground, are called '' epiphyllous'' or ''

foliicolous Foliicolous refers to the growth habit of certain lichens, algae, and fungi that prefer to grow on the leaves of vascular plants Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plant ...

''. A ''

terricolous lichen A terricolous lichen is a lichen that grows on the soil as a substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is at ...

'' grows on the soil as a substrate. Many squamulous lichens are terricolous. ''

Umbilicate lichen An umbilicate lichen is a lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship. A ''

vagrant lichen A vagrant lichen is a lichen that is either not attached to a substrate, or can become unattached then blow around, yet continue to grow and flourish. Some authors reserve the expression "vagrant lichen" for those lichens that never attach, that is, ...

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

Lichens and soils

In addition to distinct physical mechanisms by which lichens break down raw stone, recent studies indicate lichens attack stone chemically, entering newly chelated minerals into the ecology.

The lichen exudates, which have powerful chelating capacity, the widespread occurrence of mineral neoformation, particularly metal oxalates, together with the characteristics of weathered substrates, all confirm the significance of lichens as chemical weathering agents.

Over time, this activity creates new fertile soil from lifeless stone. 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. Lichens help bind and stabilize soil sand in dunes. In deserts and semi-arid areas, lichens are part of extensive, living biological soil crusts, essential for maintaining the soil structure. Lichens have a long fossil record in soils dating back 2.2 billion years.

Ecological interactions

Lichens are pioneer species, among the first living things to grow on bare rock or areas denuded of life by a disaster. 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 in places lacking soil, constituting the sole vegetation in some extreme environments such as those found at high mountain elevations and at high latitudes. Some survive in the tough conditions of deserts, and others on frozen soil of the Arctic regions. 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 Desiccation () is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. ...

. Like some

es,

, ferns and a few

resurrection plant A resurrection plant is any poikilohydric plant that can survive extreme dehydration, even over months or years. Examples include: * ''Anastatica hierochuntica'', also known as the Rose of Jericho, a plant species native to deserts of North Africa ...

s, upon desiccation, lichens enter a metabolic suspension or stasis (known as

cryptobiosis Cryptobiosis or anabiosis is a metabolic state of life entered by an organism in response to adverse environmental conditions such as desiccation, freezing, and oxygen deficiency. In the cryptobiotic state, all measurable metabolic processes stop ...

) 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. Moss_lichen

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

s (''epi''- on the surface, ''phyte''- plant) on plants, particularly on the trunks and branches of trees. When growing on plants, lichens are not parasites; they do not consume any part of the plant nor poison it. Lichens produce allelopathy, allelopathic chemicals that inhibit the growth of mosses. Some ground-dwelling lichens, such as members of the subgenus ''Cladoniaceae, Cladina'' (reindeer lichens), produce allelopathic chemicals that leach into the soil and inhibit the germination of seeds, spruce and other plants. Stability (that is, longevity) of their

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 plant, vascular (higher) plant 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, living in arctic regions. The larvae of a number of Lepidoptera species feed exclusively on lichens. These include common footman and marbled beauty. They are very low in protein and high in carbohydrates, making them unsuitable for some animals. The Northern flying squirrel uses it for nesting, food and winter water.

Effects of air pollution

If lichens are exposed to air pollutants at all times, without any deciduous parts, they are unable to avoid the accumulation of pollutants. Also lacking stomata and a plant cuticle, cuticle, lichens may absorb aerosols and gases over the entire thallus surface from which they may readily diffusion, diffuse to the photobiont layer. Because lichens do not possess roots, their primary source of most chemical element, elements is the air, and therefore elemental levels in lichens often reflect the accumulated composition of ambient air. The processes by which atmospheric deposition occurs include fog and dew, gaseous absorption, and dry deposition. Consequently, many environmental studies with lichens emphasize their feasibility as effective bioindicator, biomonitors of atmospheric quality. Not all lichens are equally sensitive to air pollution, air pollutants, so different lichen species show different levels of sensitivity to specific atmospheric pollutants. 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. Upon exposure to air pollution, the photobiont may use metabolic energy for repair of its cellular structures that would otherwise be used for maintenance of its 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. 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 William Nylander (botanist), William Nylander in 1866.

Human use

Hiroshige II - Kishu kumano iwatake tori - Shokoku meisho hyakkei

Food

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

s are generally indigestible to humans, and lichens usually contain mildly toxic secondary compounds that should be removed before eating. Very few lichens are poisonous, but those high in vulpinic acid or

are toxic. Most poisonous lichens are yellow. In the past, Iceland moss (''Cetraria islandica'') was an important source of food for humans in northern Europe, and was cooked as a bread, porridge, pudding, soup, or salad. ''Bryoria fremontii'' (edible horsehair lichen) was an important food in parts of North America, where it was usually Pit barbecue, pitcooked. Northern peoples in North America and Siberia traditionally eat the partially digested Cladonia rangiferina, reindeer lichen (''Cladina'' spp.) after they remove it from the rumen of caribou or reindeer that have been killed. Rock tripe (''Umbilicaria'' spp. and ''Lasalia'' spp.) is a lichen that has frequently been used as an emergency food in North America, and one species, ''Umbilicaria esculenta'', (''iwatake'' in Japanese) is used in a variety of traditional Korean and Japanese foods.

Lichenometry

Lichenometry is a technique used to determine the age of exposed rock surfaces based on the size of lichen thalli. Introduced by Beschel in the 1950s, the technique has found many applications. it is used in archaeology, palaeontology, and geomorphology. It uses the presumed regular but slow rate of lichen growth to determine the Surface exposure dating, age of exposed rock. 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 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. Lichenometry is especially useful for dating surfaces less than 500 years old, as radiocarbon dating techniques are less accurate over this period. The lichens most commonly used for lichenometry are those of the genera ''Rhizocarpon'' (e.g. the species '' Rhizocarpon geographicum'', map lichen) and ''Xanthoria''.

Biodegradation

Lichens have been shown to degrade polyester resins, as can be seen in archaeological sites in the Roman city of Baelo Claudia in Spain. Lichens can accumulate several environmental pollutants such as lead, copper, and radionuclides. Some species of lichen, such as ''Parmelia sulcata'' (called a hammered shield lichen, among other names) and ''Lobaria pulmonaria'' (lung lichen), and many in the Cladonia, ''Cladonia'' genus, have been shown to produce serine proteases capable of the degradation of pathogenic forms of prion protein (PrP), which may be useful in treating contaminated environmental reservoirs.

Dyes

Many lichens produce secondary compounds, including Biological pigment, pigments that reduce harmful amounts of sunlight and powerful toxins that deter herbivores or kill bacteria. These compounds are very useful for lichen identification, and have had economic importance as

s such as cudbear or primitive antibiotics. A pH indicator (which can indicate acidic or basic substances) called litmus is a dye extracted from the lichen ''Roccella tinctoria'' ("dyer's weed") by boiling. It gives its name to the well-known Litmus test (chemistry), litmus test. Traditional dyes of the Scottish Highlands for Harris tweed and other traditional cloths were made from lichens, including the orange ''Xanthoria parietina'' ("common orange lichen") and the grey foliaceous ''Parmelia saxatilis'' common on rocks and known colloquially as "crottle". There are reports dating almost 2,000 years old of lichens being used to make purple and red dyes. Of great historical and commercial significance are lichens belonging to the family ''Roccellaceae'', commonly called orchella weed or orchil. Orcein and other lichen dyes have largely been replaced by Dye#Natural and synthetic dyes, synthetic versions.

Traditional medicine and research

Historically, in traditional medicine of Europe, ''Lobaria pulmonaria'' was collected in large quantities as "lungwort", due to its lung-like appearance (the "doctrine of signatures" suggesting that herbs can treat body parts that they physically resemble). Similarly, ''Peltigera leucophlebia'' ("ruffled freckled pelt") was used as a supposed cure for candidiasis, thrush, due to the resemblance of its cephalodia to the appearance of the disease. Lichens produce

s being researched for their potential therapeutic or diagnostic value. Some metabolites produced by lichens are structurally and functionally similar to broad-spectrum antibiotics while few are associated respectively to antiseptic similarities.Morton, E.; Winters, J. and Smith, L. (2010)
"An Analysis of Antiseptic and Antibiotic Properties of Variously Treated Mosses and Lichens"
. University of Michigan Biological Station Usnic acid is the most commonly studied metabolite produced by lichens. It is also under research as a Bactericide, bactericidal agent against ''Escherichia coli'' and ''Staphylococcus aureus''.

Aesthetic appeal

Colonies of lichens may be spectacular in appearance, dominating the surface of the visual landscape as part of the aesthetic appeal to visitors of Yosemite National Park, Sequoia National Park, and the Bay of Fires. Caloplaca, Orange and Xanthoria, yellow lichens add to the ambience of desert trees, tundras, and rocky seashores. Intricate webs of lichens Usnea, hanging from tree branches add a mysterious aspect to forests. Fruticose lichens are used in model railroading and other modeling hobbies as a material for making miniature trees and shrubs.

In literature

In early Midrashic literature, the Hebrew word "''vayilafeth''" in Book of Ruth, Ruth 3:8 is explained as referring to Ruth (biblical figure), Ruth entwining herself around Boaz like lichen. The 10th century Arab physician, Al-Tamimi, the physician, Al-Tamimi, mentions lichens dissolved in vinegar and rose water being used in his day for the treatment of skin diseases and rashes. The plot of John Wyndham's science fiction novel ''Trouble with Lichen'' revolves around an anti-aging chemical extracted from a lichen.

History

Although lichens had been recognized as organisms for quite some time, it was not until 1867, when Swiss botanist Simon Schwendener proposed his dual theory of lichens, that lichens are a combination of fungi with algae or cyanobacteria, whereby the true nature of the lichen association began to emerge. Schwendener's hypothesis, which at the time lacked experimental evidence, arose from his extensive analysis of the anatomy and development in lichens, algae, and fungi using a light microscopy, light microscope. Many of the leading lichenologists at the time, such as James Mascall Morrison Crombie, James Crombie and William Nylander (botanist), Nylander, rejected Schwendener's hypothesis because the common consensus was that all living organisms were autonomous. Other prominent biologists, such as Heinrich Anton de Bary, Albert Bernhard Frank, Beatrix Potter, Melchior Treub and Hermann Hellriegel were not so quick to reject Schwendener's ideas and the concept soon spread into other areas of study, such as microbial, plant, animal and human pathogens. When the complex relationships between pathogenic microorganisms and their hosts were finally identified, Schwendener's hypothesis began to gain popularity. Further experimental proof of the dual nature of lichens was obtained when Eugen Thomas published his results in 1939 on the first successful re-synthesis experiment. In the 2010s, a new facet of the fungi-algae partnership was discovered. Toby Spribille and colleagues found that many types of lichen that were long thought to be ascomycota, ascomycete-algae pairs were actually ascomycete-basidiomycota, basidiomycete-algae trios.

Gallery

Lobaria pulmonaria 01.JPG, ''Lobaria pulmonaria'', tree lungwort, lung lichen, lung moss; Upper Bavaria, Germany Cladonia macilenta 9497.JPG, ''Cladonia macilenta'' var. ''bacillaris'' 'Lipstick Cladonia' Usnea australis.jpg, '' Usnea australis'', a fruticose form, growing on a tree branch Kananakislichen.jpg, ''Hypogymnia'' cf. ''tubulosa'' with ''Bryoria'' sp. and ''Tuckermannopsis'' sp. in the Canadian Rockies Fruticose lichen branches blackpine lake.jpg, ''Letharia'' sp. with ''Bryoria'' sp. on pine branches near Blackpine Lake, Washington State Lettucelichen.jpg, ''Lobaria oregana'', commonly called 'Lettuce lichen', in the Hoh Rainforest, Washington State Epiphytic lichen.jpg, A lichen growing on a rock in a Brazilian cloud forest Lichen squamulose.jpg, ''Xanthoparmelia'' cf. ''lavicola,'' a foliose lichen, on basalt. Rhizocarpon geographicum01.jpg, Map lichen ('' Rhizocarpon geographicum'') on rock LogLichen.jpg, ''Physcia millegrana'' (a foliose lichen), with an unlichenized polypore fungus (bottom right), on a fallen log. caribou moss.jpg, Reindeer moss (''Cladonia rangiferina'') Lichenlimestone.JPG, Crustose lichens on limestone in Alta Murgia-Southern Italy Plants flowers ice rocks lichens 209.jpg, ''Cladonia'' cf. ''cristatella,'' a lichen commonly referred to as 'British Soldiers'. Notice the red tips. N2 Lichen.jpg, A crusty crustose lichen on a wall Lichen on Lilac bush.JPG, Lichen on a lilac bush Plants flowers ice rocks lichens 230.jpg, Foliose lichens on rock growing outward and dying in the center. These lichens are at least several decades old. Reddish-colored lichen on volcanic rock.jpg, ''Xanthoria'' sp. lichen on volcanic rock in Craters of the Moon National Monument (Idaho, USA) Lichen on the riverside.jpg, ''Lecanora'' cf. ''muralis'' lichen on the banks of the Bega canal in Timișoara, Romania Lichen growing on concrete dust (2010 04 08).jpg, Microscopic view of lichen growing on a piece of concrete dust.