The following outline provides an overview of and topical guide to lichens.
Lichens are composite organisms made up of multiple species. They comprise 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]
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