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]

What type of thing is a lichen?

A lichen can be described as all of the following:

• Life form – an entity that is alive.

• Composite organism – a symbiotic life form composed of multiple partners from different biological domains, families and kingdoms, and into different phyla, classes and divisions within those domains and kingdoms. In the case of lichens, a fungal partner (the mycobiont) combines with one or more photosynthetic partner(s) (the photobiont) as well as (in some cases) a yeast.

• Eukaryote (domain) – organisms with a cell nucleus within a nuclear envelope; both the mycobiont and any algal partners fall into this domain.^[3]

• Fungi (kingdom) – the fungal partner and any yeast partner fall into this kingdom.^[1]

• Ascomycota (phylum) and/or Basidiomycota (phylum)^[4]

• For the biological classes and families these fungi belong to, see below.

• Chlorophyta (division) – if the photobiont is a green alga, it falls into this taxonomic division.^[5]

• Trebouxiophyceae (class)

• Trebouxiaceae (family)^[6]

• Ulvophyceae (class)

• Trentepohliaceae (family)^[6]

• Prokaryote (domain) – organisms without a cell nucleus; any cyanobacterial partner falls into this domain.^[3]

• Bacteria (domain)^[7]

• Cyanobacteria (phylum)^[7]

Nature of lichens

Morphology

• Lichen anatomy and physiology

• Apoplast – the symbiotic interface zone between the mycobiont and photobiont, outside the cell membranes or walls of both.^[8]
• Haustorium (pl. haustoria) – a root-like structure which allows the fungal partner to extract nutrients from its photosynthetic partner(s).^[9]

• Lichen morphology – a lichen's external appearance and structures are very different than those of its individual partners.^[10]

• Ascocarp – the fruiting body of a lichen, which contains the asci.^[11]

• Ascus (pl. asci) – a sexual, fungal spore-bearing structure, typically sac-like in shape.^[12]
• Ascospore – a fungal spore, the product of meiosis, produced in an ascus.^[13]

• Epispore – a transparent bag-like outer covering on some spores,^[9] which helps to determine spore shape.^[14]

• Cephalodium (pl. cephalodia) – a gall-like structure that contains cyanobacteria^[15]
• Hypha (pl. hyphae) – a long, branching, thread-like structure composed of one or more fungal cells, which typically makes up a large part of lichens; hyphae are densely compacted in the cortex and more loosely interwoven in the medulla.^[16]
• Pycnidium (pl. pycnidia) – a flask-shaped, asexual fruiting body possessed by some lichens.^[17]

• Conidium (pl. conidia) – an asexual fungal spore produced in pycnidia by some lichens.^[18]

• Rhizine – a root-like structure that anchors a lichen to the substrate on which it grows.^[19]
• Soralium (pl. soralia) – a localized region or structure, typically a crack or pore, containing soredium.^[20]

• Soredium (pl. soredia) – asexual reproductive propagules composed of loose clumps of fungal hyphae and photobiont cells, produced in soralia.^[20]

• Thallus (pl. thalli) – the vegetative body of a lichen, made up of both mycobiont and photobiont components.^[21]

• Cortex – the lichen's outer layer(s), made up of tightly woven fungal filaments.^[22]
• Isidium (pl. isidia) – outgrowths of the thallus which contain photobiont cells and provide means of vegetative reproduction for some lichens.^[23]
• Medulla – a loose layer of interwoven fungal hyphae within the thallus.^[24]
• Podetium (pl. podetia) – an upright secondary thallus, which supports the fruiting bodies of Cladonia species.^[25]

Ecology

• Symbiosis in lichens – the relationship between the lichen partners can be complicated; while generally mutualistic, sometimes it is not. Recent research also shows other partners, including bacteria and "accessory" fungi, may be involved.^[27]
• Asexual reproduction in lichens – many lichens reproduce asexually, using one or more of various methods which allow the dispersal of bundles of both fungal hyphae and photobionts.^[28]
• Sexual reproduction in lichens – most lichens reproduce sexually using ascospores, which means they have to acquire their photobiont partners somehow after germinating.^[29]
• Lichens and nitrogen cycling – some lichens (in particular those with cyanobacteria as a photobiont) can fix nitrogen.^[26]
• Lichen biogeography – the study of the current distribution of extant lichens and the reasons for those distributions.^[30]
• Lichen resynthesis – lichens can be artificially "recreated" by combining partners in a lab.^[31]
• Lichens and pedogenesis – lichens contribute to the formation of soil by breaking down rock.^[32]
  • Biological soil crust – lichens are among the common dominant biota in biocrusts, one of the world's largest environmental community types in terms of area covered.^[33]
• Photosynthesis in lichens

Types of lichens

Lichen lists

Lichen taxonomical classifications

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]

• Ascolichen – a lichen whose fungal partner is a member of the Ascomycota, one of the two main fungal divisions.^[36]
• Basidiolichen – a lichen whose fungal partner is a member of the Basidiomycota, the other of the two main fungal divisions; these are far fewer in occurrence than ascolichens.^[37]

Classes

Lichens fall into eight fungal classes and several subclasses:^[38]

Orders

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]

Families

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]

Genera

• List of common names of lichen genera

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]

Species

In 2009, taxonomists estimated that the total number of lichen species (including those yet undiscovered) might be as high as 28,000.^[98] By 2016, 19,387 species of lichens had been described and widely accepted.^[99]

Lichens, by growth form

Lichen growth forms – These vary depending on the species:

• Crustose – paint-like appearance that adheres tightly to the underlying substrate.^[101]
  • Areolate – crustose, but divided into rounded or polygonal pieces by means of cracks.^[102]
  • Leprose – powdery or granular appearance.^[102]
  • Calicioid – crustose growth with small fruiting bodies which resemble sewing pins.^[103]
  • Placodioid – crustose in the centre and lobed at the periphery.^[102]
• Foliose – flattened, leafy appearance.^[102]
• Fruticose – shrubby, bush-like or coral-like appearance.^[102]
  • Byssoid – wispy, with the appearance of teased wool.^[104]
  • Filamentous – thin, threadlike growth, often with a matted appearance.^[105]
• Gelatinous – jelly-like interior, due to presence of cyanobacteria.^[106]
• Squamulose – scaly, sometimes leafy appearance; can resemble a foliose lichen but usually has no outer cortex.^[107]
  • Cladoniform – squamulose, but with fruticose podetia.^[108]

Lichens, by substrate

Lichens can be classified by the substrate on which they grow:

• Bryophilous lichen – on mosses or liverworts.^[109]
  • Hepaticolous lichen – on liverworts.^[110]
  • Muscicolous lichen – on mosses.^[111]
• Corticolous lichen – on bark.^[109]
  • Ramicolous lichen – on twigs.^[111]
• Foliicolous lichen – on plant leaves.^[109]
  • Epiphyllous lichen – on the upper surface of a leaf.^[112]
  • Hypophyllous lichen – on the lower surface of a leaf.^[112]
• Lichenicolous lichen – on other lichens.^[109]
• Lignicolous lichen – on wood stripped of bark.^[109]
• Omnicolous lichen – on a variety of substrates.^[111]
• Plasticolous lichen – on plastic.^[113]
• Saxicolous lichen – on stone.^[109]
  • Endolithic lichen – within stone.^[22]
• Terricolous lichen – on soil.^[111]
• Vagrant lichen – loose, on no substrate.^[114]

Lichens, by region

Africa

• List of lichens of Madagascar
• List of lichens of Namibia

Antarctica

Asia

• List of lichens of Sri Lanka

Australia

• List of lichens of Western Australia

Europe

• List of lichens of Sweden

North America

• List of lichens of Maryland
  • List of lichens of Soldiers Delight – lichens of a nature reserve in Maryland
• List of lichen species of Montana
• Lichens of the Sierra Nevada (U.S.)

Oceania

Pacific

South America

Photobiont

Photobiont – the photosynthetic partner in a lichen.^[116]

• Cyanolichen – a lichen with a cyanobacteria photobiont.^[117]
• List of lichen photobionts

Lichen metabolites

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.^[118]

• List of lichen products

Study of lichens

Lichenology – the study of lichens.^[120]

• Acharius Medal – awarded for lifetime achievement in lichenology.^[121]
• Evolution of lichens – lichenization of fungi has occurred multiple times, and several pathways towards acquiring photobionts have arisen.
  • List of fossil lichens
• Exsiccata (plural exsiccatae) – a published set of preserved specimens, numbered and distributed with printed labels.^[122]
• History of lichenology
• Lichenometry – a process where measuring the growth of a lichen colony over time can be used to estimate the minimum age of the substrate on which it is growing.^[123]
• Spot test (lichen) – chemical tests used to aid in species identification.^[124]

Threats

• Lichenicolous fungus – parasitic fungus that uses lichens as a host.^[125]

• List of lichenicolous fungi

• Lichens as bioindicators – lichens are sensitive to various pollutants and can be thus be used as bioindicators.^[126]

• Lichens and air pollution – many lichens are sensitive to various forms of air pollution.^[127]
• Lichens and climate change – the inability of algae to quickly evolve means that climate change may adversely impact lichens.^[128]

Lichens in culture

• Cultural depictions of lichens
• Trouble with Lichen – science fiction novel by John Wyndham in which lichens play a major role.^[130]
• Edible lichen – some lichens have traditionally been used as food.^[131]
• Ethnolichenology – a branch of ethnobotany that studies human usage of lichens.^[132]

Lichen organizations

• American Bryological and Lichenological Society (ABLS)

• The Bryologist – peer-reviewed journal published by ABLS.

• Australasian Lichen Society

• Australasian Lichenology – official publication of the Australasian Lichen Society.

• British Lichen Society (BLS)

• The Lichenologist – peer-reviewed journal published by the BLS.

• Bryological and Lichenological Association for Central Europe (BLAM)

• Herzogia – peer-reviewed journal published by BLAM.

• Bryological and Lichenological Working Group (Bryologische en Lichenologische Werkgroep, BLWG)

• Buxbaumiella – peer-reviewed journal published by BLWG.

• Dutch Bryological and Lichenological Society

• Lindbergia – peer-reviewed journal co-published by the Dutch Bryological and Lichenological Society and the Nordic Bryological Society.

• Indian Lichenological Society
• International Association for Lichenology (IAL)
• Nordic Bryological Society

Independent lichenological journals

• Asian Journal of Mycology – an international peer-reviewed journal published by Mae Fah Luang University in Thailand.
• Bibliotheca Lichenologica – scientific monographs on lichens and mosses.
• Hattoria – an international, peer-reviewed journal issued by Hattori Botanical Laboratory.
• International Journal of Mycology and Lichenology