Outline of lichens explained

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. 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.

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 from 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.

Fungi (kingdom) – the fungal partner and any yeast partner fall into this kingdom.

Ascomycota (phylum) and/or Basidiomycota (phylum)

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.

Trebouxiophyceae (class)

Trebouxiaceae (family)

Ulvophyceae (class)

Trentepohliaceae (family)

Prokaryote (domain) – organisms without a cell nucleus; any cyanobacterial partner falls into this domain.

Bacteria (domain)

Cyanobacteria (phylum)

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.

Haustorium – a root-like structure which allows the fungal partner to extract nutrients from its photosynthetic partner(s).
Lichen morphology – a lichen's external appearance and structures are very different than those of its individual partners.

Ascocarp – the fruiting body of a lichen, which contains the asci.

Ascus – a sexual, fungal spore-bearing structure, typically sac-like in shape.

Ascospore – a fungal spore, the product of meiosis, produced in an ascus.

Epispore – a transparent bag-like outer covering on some spores, which helps to determine spore shape.

Cephalodium – a gall-like structure that contains cyanobacteria

Hypha – 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.

Pycnidium – a flask-shaped, asexual fruiting body possessed by some lichens.

Conidium – an asexual fungal spore produced in pycnidia by some lichens.

Rhizine – a root-like structure that anchors a lichen to the substrate on which it grows.

Soralium – a localized region or structure, typically a crack or pore, containing soredium.

Soredium – asexual reproductive propagules composed of loose clumps of fungal hyphae and photobiont cells, produced in soralia.

Thallus – the vegetative body of a lichen, made up of both mycobiont and photobiont components.

Cortex – the lichen's outer layer(s), made up of tightly woven fungal filaments.

Isidium – outgrowths of the thallus which contain photobiont cells and provide means of vegetative reproduction for some lichens.

Medulla – a loose layer of interwoven fungal hyphae within the thallus.

Podetium – an upright secondary thallus, which supports the fruiting bodies of Cladonia species.

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.
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.
Sexual reproduction in lichens – most lichens reproduce sexually using ascospores, which means they have to acquire their photobiont partners somehow after germinating.
Lichens and nitrogen cycling – some lichens (in particular those with cyanobacteria as a photobiont) can fix nitrogen.
Lichen biogeography – the study of the current distribution of extant lichens and the reasons for those distributions.
Lichen resynthesis – lichens can be artificially "recreated" by combining partners in a lab.
Lichens and pedogenesis – lichens contribute to the formation of soil by breaking down rock.
- 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.
Photosynthesis in lichens

Types of lichens

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.

Ascolichen – a lichen whose fungal partner is a member of the Ascomycota, one of the two main fungal divisions.
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.

Classes

Lichens fall into eight fungal classes and several subclasses:

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, with orders updated in 2021.

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; families were updated in 2021.

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.

Species

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

Lichens, by growth form

Lichens, by substrate

Lichens, by region

Africa

Asia

Australia

Europe

North America

South America

Photobiont

Lichen metabolites

See main article: Lichen products. 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.