The following outline provides an overview of and topical guide to lichens.
Lichen – composite organism made up of multiple species – a fungal partner, one or more photosynthetic partners, which can be either green algae or cyanobacteria, and, in at least 52 genera of lichens, a yeast. [1] In American English, "lichen" is pronounced the same as the verb "liken" ( /ˈlaɪkən/ ). In British English, both this pronunciation and one rhyming with "kitchen" ( /ˈlɪtʃən/ ) are used. [2]
A lichen can be described as all of the following:
Lichen systematics – Although they are composite organisms, lichens have traditionally been classified on the basis of their fungal partner. These span eight different biological classes, 39 orders, 117 families, and around 1,000 genera. [34] [35]
Lichens fall into eight fungal classes and several subclasses: [38]
They are split across nearly 40 orders. Those which cannot be assigned to a particular order are assigned instead to "incertae sedis" within the appropriate class. These orders were listed in Lücking, Hodkinson and Leavitt's 2016 treatise on the classification of lichenized fungi, except where otherwise noted, [38] with orders updated in 2021. [34]
They fall into 117 families. Those which cannot be assigned to a particular family are assigned instead to "incertae sedis" within the appropriate order. These were listed in Lücking, Hodkinson and Leavitt's 2016 treatise on the classification of lichenized fungi, except where otherwise noted; [35] families were updated in 2021. [34]
Extant lichens are found in more than 1000 genera. These were listed in Lücking, Hodkinson and Leavitt's 2016 treatise on the classification of lichenized fungi, except where otherwise noted. [35]
In 2009, taxonomists estimated that the total number of lichen species (including those yet undiscovered) might be as high as 28,000. [100] By 2016, 19,387 species of lichens had been described and widely accepted. [101]
Lichen growth forms – These vary depending on the species:
Lichens can be classified by the substrate on which they grow:
Photobiont – the photosynthetic partner in a lichen. [118]
Lichen product – organic products, known as secondary metabolites, produced by lichens; these provide a variety of protections for the lichen – from microbes, viruses, herbivores, radiation, oxidants and more. [120]
Lichenology – the study of lichens. [122]
The Cladoniaceae are a family of lichen-forming fungi in the order Lecanorales, comprising about 560 species distributed amongst 18 genera. This family is one of the largest among lichen-forming fungi and is globally distributed, from Arctic tundra to tropical rainforests, favouring humid environments while being intolerant of arid conditions. Molecular phylogenetics has significantly advanced the understanding of their complex taxonomic history, revealing intricate evolutionary relationships and leading to a refined classification. Notable members include reindeer moss and cup lichens of the genus Cladonia, which consist of about 500 species and forms a significant part of the diet for large mammals in taiga and tundra ecosystems.
The Lecanoraceae are a family of lichenized fungi in the order Lecanorales. Species of this family have a widespread distribution.
The Teloschistaceae are a large family of mostly lichen-forming fungi belonging to the class Lecanoromycetes in the division Ascomycota. The family has a cosmopolitan distribution, although its members occur predominantly in temperate regions. Most members are lichens that either live on rock or on bark, but about 40 species are lichenicolous – meaning they are non-lichenised fungi that live on other lichens. Many members of the Teloschistaceae are readily identifiable by their vibrant orange to yellow hue, a result of their frequent anthraquinone content. The presence of these anthraquinone pigments, which confer protection from ultraviolet light, enabled this group to expand from shaded forest habitats to harsher environmental conditions of sunny and arid ecosystems during the Late Cretaceous.
Badimia is a genus of foliicolous (leaf-inhabiting) lichens in the family Ramalinaceae.
Psilolechia is a genus of four species of crustose lichens. It is the only member of Psilolechiaceae, a family that was created in 2014 to contain this genus.
Physcia is a genus of lichen-forming fungi in the family Physciaceae. The widely distributed genus contains about 80 species. The genus is cosmopolitan, and has been extensively studied in various regions in the past several decades, with significant biodiversity in South America identified as a central diversity hotspot. Physcia species are foliose, lobate lichens that grow with a loose to close appressed habit. Their upper surface is typically whitish, pale greenish, green-grey, or dark grey in colour. The thallus colour remains relatively unchanged when moistened. Physcia lichens typically grow on bark, on wood, or rock, although they have occasionally been recorded dwelling on man-made structures. They thrive in nutrient-rich environments and are expanding rapidly in urban areas of the United Kingdom previously affected by SO2 pollution.
Josefpoeltia is a genus of lichen-forming fungi in the family Teloschistaceae. It has three species. The genus was circumscribed in 1997 by lichenologists Sergey Kondratyuk and Ingvar Kärnefelt, with J. boliviensis assigned as the type species. The genus name honours lichenologist Josef Poelt, (1924-1995) who was a German-Austrian botanist and was Professor of Systematic Botany at the Free University of Berlin in 1965.
Flavoparmelia is a genus of foliose lichens in the family Parmeliaceae. Because of their appearance, they are commonly known as greenshield lichens. The widely distributed genus contains 32 species. It was circumscribed by American lichenologist Mason Hale in 1986 to contain 17 former Pseudoparmelia species with broad lobes, usnic acid in the cortex, and isolichenan in the cell walls.
Punctelia is a genus of foliose lichens belonging to the large family Parmeliaceae. The genus, which contains about 50 species, was segregated from genus Parmelia in 1982. Characteristics that define Punctelia include the presence of hook-like to thread-like conidia, simple rhizines, and point-like pseudocyphellae. It is this last feature that is alluded to in the vernacular names speckled shield lichens or speckleback lichens.
Oevstedalia is a genus of fungi of uncertain placement in the subphylum Pezizomycotina. This is a monotypic genus, containing the single lichen species Oevstedalia antarctica. The genus was circumscribed by Damien Ertz and Paul Diederich in 2004. Previously classified in the Dothideomycetes, Oevstedalia was moved to Pezizomycotina incertae sedis due to the lack of DNA data available for the genus.
Frigidopyrenia is a fungal genus in the family Xanthopyreniaceae. It is a monotypic genus, containing the single species Frigidopyrenia bryospila, a subarctic crustose lichen. This lichen was originally described by Finnish lichenologist William Nylander in 1864 as Verrucaria bryospila. It was shuffled to several genera in its taxonomic history before Martin Grube circumscribed Frigidopyrenia in 2005 to contain it. As of 2017, no molecular sequence data was available for Frigidopyrenia.
Mawsonia is a fungal genus in the family Lichinaceae. It is a monospecific genus, containing the single species Mawsonia harrissonii, a rare saxicolous, crustose lichen found in Antarctica.
Mycobilimbia is a genus of lichen-forming fungi in the family Ramalinaceae.
André Aptroot is a Dutch mycologist and lichenologist. His primary research focus is on biodiversity, particularly tropical lichens, encompassing systematics, floristic surveys, and taxonomic reviews. A prolific researcher, he has published more than 500 scientific papers and described hundreds of new fungal and lichen species.
Lichens are symbiotic organisms made up of multiple species: a fungus, one or more photobionts and sometimes a yeast. They are regularly grouped by their external appearance – a characteristic known as their growth form. This form, which is based on the appearance of vegetative part of the lichen, varies depending on the species and the environmental conditions it faces. Those who study lichens (lichenologists) have described a dozen of these forms: areolate, byssoid, calicioid, cladoniform, crustose, filamentous, foliose, fruticose, gelatinous, leprose, placoidioid and squamulose. Traditionally, crustose (flat), foliose (leafy) and fruticose (shrubby) are considered to be the three main forms. In addition to these more formalised, traditional growth types, there are a handful of informal types named for their resemblance to the lichens of specific genera. These include alectorioid, catapyrenioid, cetrarioid, hypogymnioid, parmelioid and usneoid.
John Alan (Jack) Elix emeritus professor in chemistry at the Australian National University, is an organic chemist who has contributed in many fields: lichenology, lichen chemotaxonomy, plant physiology and biodiversity and natural product chemistry. He has authored 2282 species names, and 67 genera in the field of mycology. Elix edited the exsiccata series Lichenes Australasici exsiccati.
Serusiauxiella is a genus of leaf-dwelling lichens in the family Strigulaceae. It has three species, all of which are found in wet tropical forests in China. The genus was circumscribed in 2020 by Shu-Hua Jiang, Robert Lücking, and Jiang-Chu Wei, with Serusiauxiella filifera assigned as the type species. The genus name honours Belgian lichenologist Emmanuël Sérusiaux, "in recognition of his important contributions to lichenology, foliicolous lichens and the genus Strigula".
Herpothallon is a genus of crustose lichens in the family Arthoniaceae. It has about 50 species.
Robert Lücking is a German lichenologist, known for his extensive research on foliicolous lichens and his significant contributions to the taxonomy, ecology, and biodiversity of fungi and lichens. He earned his master's and PhD from the University of Ulm, focusing on foliicolous lichens. He has received numerous awards for his work, including the Mason E. Hale Award for his doctoral thesis, the Augustin Pyramus de Candolle prize for his monograph, and the Tuckerman Award twice for his publications in the scientific journal The Bryologist.
Graphidales is an order of lichen-forming fungi in the class Lecanoromycetes. It contains 6 families, about 81 genera and about 2,228 species. Family Graphidaceae are the largest crustose family within Graphidales order comprising more than 2000 species, which are widely distributed in tropical and subtropical regions of the world.
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