Crustose lichens are

lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.substrate (soil, rock, tree bark, etc.), making separation from the substrate impossible without destruction. The basic structure of crustose lichens consists of a

cortex Cortex or cortical may refer to: Biology * Cortex (anatomy), the outermost layer of an organ ** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain'' *** Motor cortex, the regions of the cerebral cortex i ...

layer, an algal layer, and a medulla. The upper cortex layer is differentiated and is usually pigmented. The algal layer lies beneath the cortex. The medulla fastens the lichen to the substrate and is made up of

fungal A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...

hypha 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 ...

e. The surface of crustose lichens is characterized by branching cracks that periodically close in response to climatic variations such as alternate wetting and drying regimes.

Subtypes

* Powdery – considered as the simplest subtype due to the absence of an organized thallus. :The thallus appears powdery. :E.g. Genera ''

Lepraria ''Lepraria'' is a genus of leprose crustose lichens that grows on its substrate like patches of granular, caked up, mealy dust grains.A taxonomic revision of the North American species of Lepraria s.l. that produce divaricatic acid, with note ...

'', ''Vezdaea'' * Endolithic – grows inside the rock, usually in interstitial spaces between mineral grains. The :upper cortex is usually developed. :E.g. Genus ''

Lecidea ''Lecidea'' is a genus of crustose lichens with a carbon black ring or outer margin ( exciple) around the fruiting body disc (apothecium), usually (or always) found growing on (saxicolous) or in (endolithic An endolith or endolithic is an or ...

'' * Epilithic – grows on top of the rock without penetrating the rock substrate. :E.g. '' Acarospora fuscata'' * Epiphloeodal – grows only on the surface of plants. :E.g. ''

Lecania ''Lecania'' is a genus of lichenized fungi in the family Ramalinaceae. The genus was circumscribed by Abramo Bartolommeo Massalongo in 1853. ''Lecania'' is widely distributed, especially in temperate regions, and contains about 64 species. ...

naegelii'' * Endophloeodic – grows underneath the cuticle of leaves or stems. :E.g. '' Amandinea punctata'' * Squamulose – has a scale-like appearance resulting from partial separation from substrate. :It is an intermediate form between crustose and foliose. :E.g. Genus ''

Psora ''Psora'' is a genus of lichen-forming fungi in the family Psoraceae. Members of the genus are commonly called fishscale lichens. Lichens in the genus ''Psora'' are known to have a squamulose thallus and anthraquinone Anthraquinone, also call ...

, Catapyrenium, Coriscium'' * Peltate – similar to squamulose, but thallus remains attached near the central area. :E.g. '' Peltula euploca'' * Bullate – has an extremely inflated appearance. :E.g. Genus '' Mobergia'' * Effigurate - has radially arranged marginal lobes that are prolonged. :E.g. Genera '' Acarospora,

Pleopsidium ''Pleopsidium'' is a genus of lichenized fungi in the family Acarosporaceae. The widespread genus, which contains four species, was circumscribed by lichenologist Gustav Wilhelm Körber Gustav Wilhelm Körber (10 January 1817, Hirschberg – ...

'' * Lobate – characterized by a thallus that radially arranged with lobes that are partially raised. :E.g. Genera '' Caloplaca,

Lecanora ''Lecanora'' is a genus of lichen commonly called rim lichens.Field Guide to California Lichens, Stephen Sharnoff, Yale University Press, 2014, Lichens in the genus ''Squamarina'' are also called rim lichens. Members of the genus have roughly ci ...

'' * Suffruticose – clusters of coralloid cushions. :E.g. '' Peltula clavata'' Lepraria incana G34.JPG, ''Lepraria incana'', a powdery lichen Brown Tile and Gold Cobblestone Lichens (5037667763).jpg, ''Lecidea atrobrunnea'', an endolithic lichen Acarospora K. Brodzińskiego.JPG, ''Acarospora fuscata'', an epilithic lichen Amandinea punctata 2 - Lindsey.jpg, ''Amandinea punctata'', an endophloedic lichen Psora decipiens 300406a.jpg, ''Psora decipiens'', a squamulose lichen Peltula euploca Jymm.jpg, ''Peltula euploca'', a peltate lichen Gold Cobblestone Lichen (4752151307).jpg, ''Acarospora socialis'', an effigurate lichen Lecanora.dispersa2.-.lindsey.jpg, ''Lecanora dispersa'', a lobate lichen

Structure

Crustose lichen forms a thin crust adhering closely to the substratum. In some cases, this crust may be thick and lumpy, and may be detached, in part, or submerged below its surface. The

thallus Thallus (plural: thalli), from Latinized Greek (), meaning "a green shoot" or "twig", is the vegetative tissue of some organisms in diverse groups such as algae, fungi, some liverworts, lichens, and the Myxogastria. Many of these organisms wer ...

of a crustose lichen is usually only discernible because of the discolouration of the substrate. Some crustose lichens have thalli consisting of scattered or loosely grouped granules. Crustose lichens differ from the

leprose lichen 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 ...

by having an upper cortex and algal cells that are located directly beneath the cortex. The thallus of a crustose lichen has a patchwork or crazy-paving appearance. The patches, or areolae, can be as large as 1 cm in diameter or very small and raised, giving them the appearance of a wart. The surface of the thallus is generally smooth, however it is sometimes broken up by “rimose” cracks. These cracks are a by-product of thallus surface shrinkage, which is caused by alternate wetting and drying. An underlayer of fungal hyphae, the

hypothallus In true slime molds (myxogastria), lichens, and in species of the family Clavicipitaceae, the hypothallus is the layer on which the fruit body sits, lying in contact with the substrate. The word is derived from the Ancient Greek root ''hypó'' ("u ...

, is present on some species of crustose lichens. A dark rim on the areolae may form in areas where the hypothallus is exposed. This may also be present on the thallus itself. These fungal hyphae are usually what attach the thallus firmly to the substrate.

Growth

In general, lichens do not grow very quickly. Annual growth rates vary among different growth forms. Crustose lichens have the lowest rates of growth. The diameter and area of the thallus exponentially increase, provided that organic substances are distributed uniformly in all parts of the lichens. However, as the thallus increases in size, the circumference also increases, which corresponds to a larger increase in volume. As a result, movement and uniform distribution of organic substances become more difficult.

Growth factors

The growth of crustose lichens is dependent on several factors, including moisture levels, sunlight, and temperature. High rates of precipitation and high moisture levels promote the growth of crustose lichens. Crustose lichens are more prevalent in areas with higher precipitation. A similar trend is observed when aridity is taken into account. Crustose lichens prefer sites of lower aridity.

Sunlight

The amount of sunlight that lichens receive determines the rate at which photosynthesis occurs. Moreover, surface area also influences photosynthetic rates. In high sunlight conditions, foliose lichens with broad lobes are prevalent. In comparison, crustose lichens have less surface area than

foliose Foliose lichen is one of the morphological classes of lichens, which are complex organisms that arise from the symbiotic relationship between fungi and a photosynthetic partner, typically algae. This partnership allows lichen to live in diverse ...

lichens and will tend to have slower photosynthetic rates. Generally, higher levels of sunlight promote growth of the crustose lichens.

Temperature

Extreme temperatures are unfavorable for the growth of crustose lichens. Temperatures below 0 °C can result in cessation of growth and thalli freezing. Annual growth rates for the ''

Rhizocarpon ''Rhizocarpon'' is a genus of crustose, saxicolous (or sometimes lichenicolous), lichens in the family Rhizocarpaceae. The genus is common in arctic-alpine environments, but also occurs throughout temperate, subtropical, and even tropical regio ...

'' subgenus show a correlation with annual and winter mean temperatures, but not with mean summer temperatures. Unfortunately, little faith can be put in these correlations because they use unvalidated measures of unknown accuracy and precision and measurement of growth was done along a single diameter. Since thallus growth along any radius might not match growth along any other radius it is unclear if these correlations are meaningful. Various publications can be consulted to see that there is tremendous within thallus variation in lateral growth (e.g.,). The scientific basis of lichenometric dating and the reliability of lichen growth rate measurements in general have been questioned and critically reviewed in a paper by Osborn et al. (2015). Those criticisms of lichenometric dating have yet to be answered.

Photosynthesis

Photosynthetic rates vary among lichen growth forms due to differences and variations in thalli thicknesses. Irregular thicknesses in crustose lichens result in greater variation in photosynthetic rates relative to more uniformly thick forms such as foliose lichens.

Distribution and habitat

Crustose lichens can be found in a wide range of areas. They can be found, among others, together with

epiphytic 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 ...

algae 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 ...

, living on the surfaces of leaves of tropical evergreen trees and shrubs. They also thrive in carbonate-rich

karst Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant ro ...

areas. In southern

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...

, it has been estimated that 5-30% of rock outcrops in bare karst areas and 30-70% in forest karst areas are covered with crustose lichens. Crustose lichens also flourish in extreme environments. Various species of crustose lichens, including ''Biatora granulosa'' and ''Lecidea uliginosa,'' were found covering recently-burned surfaces caused by a subarctic forest fire in an area near the

Great Slave Lake Great Slave Lake (french: Grand lac des Esclaves), known traditionally as Tıdeè in Tłı̨chǫ Yatıì (Dogrib), Tinde’e in Wıìlıìdeh Yatii / Tetsǫ́t’ıné Yatıé (Dogrib / Chipewyan), Tu Nedhé in Dëne Sųłıné Yatıé (Chi ...

. Crustose lichens also grow in areas of high elevations, such as the western Himalayan region. Concentrations of terricolous crustose lichens were highest in areas of higher elevation, relative to other foliose and

fruticose A fruticose lichen is a form of lichen fungi that is characterized by a coral-like shrubby or bushy growth structure. It is formed from a symbiotic relationship of a photobiont such as green algae or less commonly cyanobacteria and one, two or mor ...

lichens. In areas of high pollution, the majority of lichens are killed and are the first plants to disappear in cities due to their high sensitivity to atmospheric pollutants. Nonetheless, surrounding the central area of cities in which most plants cannot thrive, crustose lichens ''

Physcia ''Physcia'' is a genus of lichenized fungi in the family Physciaceae. The genus name means "inflated" or "sausage-like", referring to the form of the type species. According to a 2008 estimate, the widespread genus contains 73 species. Species ...

'' or ''

Xanthoria ''Xanthoria'' is a genus of lichenized fungi in the family Teloschistaceae. Common names include orange lichen,United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and North ...

Significance

Saxicolous A saxicolous lichen is a lichen that grows on rock. The prefix "sax" from the Latin means "rock" or "stone". Characteristics Saxicolous lichens exhibit very slow growth rates. They may develop on rock substrates for long periods of time, give ...

crustose lichens play an important role in the weathering of rocks. Repeated contraction and expansion of thalli occurs in response to alternate periods of wetting and drying, resulting in the breakdown of rock fragments and removal of mineral grains from the rock surfaces. Crustose lichens also chemically weather rocks through

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

. In a study conducted by Kitagawa and Watanabe (2004), the crustose genus '' Porpidia'' altered minerals, specifically

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...

granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

. Furthermore,

vermiculite Vermiculite is a hydrous phyllosilicate mineral which undergoes significant expansion when heated. Exfoliation occurs when the mineral is heated sufficiently, and commercial furnaces can routinely produce this effect. Vermiculite forms by the wea ...

-like minerals were formed as a result of biotite alteration through hydrolysis. Crustose lichens living in karst areas have substantial influence on

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 transpar ...

flux at the boundary between the lithosphere and atmosphere because they increase the rates of corrosion of carbonate rocks in these areas. Some species of crustose lichens exhibit

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of ...

properties. ''Lepraria chlorina'' contains substantial amounts of

vulpinic acid Vulpinic acid is a natural product first found in and important in the symbiosis underlying the biology of lichens. It is a simple methyl ester derivative of its parent compound, pulvinic acid, and a close relative of pulvinone, both of which ...

, which is a chemical that has

anti-inflammatory Anti-inflammatory is the property of a substance or treatment that reduces inflammation or swelling. Anti-inflammatory drugs, also called anti-inflammatories, make up about half of analgesics. These drugs remedy pain by reducing inflammation as o ...

properties. Crustose lichens may also be used for dating rock surfaces, through a technique called

lichenometry In archaeology, palaeontology, and geomorphology, lichenometry is a geomorphic method of geochronologic dating that uses lichen growth to determine the age of exposed rock, based on a presumed specific rate of increase in radial size over time. ...

. As soon as a rock is exposed to the Earth’s atmosphere, spores of various organisms are driven into the crevices on the surface. The majority of these spores die under the extreme conditions of a rock surface, an area where water evaporates rapidly and daily fluxes in temperatures are quite large. The spores of some crustose lichens, however, can develop on these surfaces. Eventually the crustose spores form small and round thalli and increase in diameter yearly. When lichens are used for dating a rock surface, only the diameters of the largest thalli of one species are measured, as there is an assumption that only they began development when the surface was initially exposed. The age of exposure of a rock surface is then extrapolated from records.

References

{{reflist, refs= {{cite journal, last1=Perez-Llano, first1=G.A., title=Lichens: Their biological and economic significance, journal=Botanical Review, date=1944, volume=10, issue=1, pages=1–65, jstor=4353298, doi=10.1007/bf02861799 Lichenology