green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as ...

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

fungus 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 ...

lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.symbiont in a

lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.autotrophic symbionts occurring in lichens are a wide variety of simple, photosynthetic organisms commonly and traditionally known as “algae”. These symbionts include both prokaryotic and

eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

organisms.

Overview of lichens

::''"

Lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.fungi 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 ...

that have discovered agriculture"'' — Trevor Goward A

lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...

),

physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...

, and

biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...

than any of the constituent species growing separately. The algae or cyanobacteria benefit their fungal partner by producing organic carbon compounds through

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

. In return, the fungal partner benefits the algae or cyanobacteria by protecting them from the environment by its filaments, which also gather moisture and nutrients from the environment, and (usually) provide an anchor to it. The majority of the lichens contain eukaryotic autotrophs belonging to the Chlorophyta (green algae) or to the Xanthophyta (

yellow-green algae Yellow-green algae or the Xanthophyceae (xanthophytes) are an important group of heterokont algae. Most live in fresh water, but some are found in marine and soil habitats. They vary from single-celled flagellates to simple colonial and filamento ...

). About 90% of all known lichens have a green alga as a symbiont. Among these, ''

Trebouxia ''Trebouxia'' is a unicellular green alga.Silverside, A. J. (2009). Retrieved from http://www.bioref.lastdragon.org/Chlorophyta/''Trebouxia''.html It is a photosynthetic organism that can exist in almost all habitats found in polar, tropical, and ...

'' is the most common genus, occurring in about 20% of all lichens. The second most commonly represented green alga genus is '' Trentepohlia''. Overall, about 100 species are known to occur as autotrophs in lichens. All the algae and cyanobacteria are believed to be able to survive separately, as well as within the lichen; that is, at present no algae or cyanobacteria are known which can only survive naturally as part of a lichen. Common algal partners are ''

Trebouxia ''Trebouxia'' is a unicellular green alga.Silverside, A. J. (2009). Retrieved from http://www.bioref.lastdragon.org/Chlorophyta/''Trebouxia''.html It is a photosynthetic organism that can exist in almost all habitats found in polar, tropical, and ...

'', '' Pseudotrebouxia'', or ''

Myrmecia Myrmecia can refer to: * ''Myrmecia'' (alga), genus of algae associated with lichens * ''Myrmecia'' (ant), genus of ants called bulldog ants * Myrmecia (skin), a kind of deep wart on the human hands or feet See also * '' Copromorpha myrmecias'' ...

''. The prokaryotes belong to the

Cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

, which are often called by their old name “''bluegreen algae''”. Bluegreen algae occur as symbionts only in about 8% of known lichens. The most commonly occurring genera of symbiotic

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

are ''

Nostoc ''Nostoc'', also known as star jelly, troll’s butter, spit of moon, fallen star, witch's butter (not to be confused with the fungi commonly known as witches' butter), and witch’s jelly, is the most common genus of cyanobacteria found in vari ...

'' and '' Scytonema''.

Nomenclature

Both the lichen and the fungus partner bear the same scientific name, and the lichens are being integrated into the classification schemes for fungi. Depending on context, the taxonomic name can be meant to refer to the entire lichen, or just the fungus that is part of the lichen. The alga or cyanobacterim bears its own scientific name, which bears no relationship to either the name of the lichen or the fungus.

Fungus component

About 20% of all fungal species are able to form lichens. The fungal partner may be an

Ascomycete Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defi ...

or

Basidiomycete Basidiomycota () is one of two large divisions that, together with the Ascomycota, constitute the subkingdom Dikarya (often referred to as the "higher fungi") within the kingdom Fungi. Members are known as basidiomycetes. More specifically, Basi ...

. Overall, about 98% of lichens have an ascomycetous mycobiont. Next to the Ascomycota, the largest number of lichenized fungi occur in the unassigned

fungi imperfecti The fungi imperfecti or imperfect fungi, are fungi which do not fit into the commonly established taxonomic classifications of fungi that are based on biological species concepts or morphological characteristics of sexual structures because thei ...

. Comparatively few

basidiomycetes Basidiomycota () is one of two large divisions that, together with the Ascomycota, constitute the subkingdom Dikarya (often referred to as the "higher fungi") within the kingdom Fungi. Members are known as basidiomycetes. More specifically, Basi ...

are lichenized, but these include

agarics An agaric () is a type of fungus fruiting body characterized by the presence of a pileus (cap) that is clearly differentiated from the stipe (stalk), with lamellae (gills) on the underside of the pileus. In the UK, agarics are called "mushrooms ...

, such as species of '' Lichenomphalia'',

clavarioid fungi The clavarioid fungi are a group of fungi in the ''Basidiomycota'' typically having erect, simple or branched basidiocarps (fruit bodies) that are formed on the ground, on decaying vegetation, or on dead wood. They are colloquially called club fun ...

, such as species of '' Multiclavula'', and corticioid fungi, such as species of ''

Dictyonema ''Dictyonema'' is a genus of mainly tropical basidiolichens in the family Hygrophoraceae. The ''Dictyonema'' symbiosis Most lichens are a symbiosis between an ascomycete fungus and a photosynthetic green alga. However, a small percentage of ...

''. The largest number of lichenized fungi occur in the

Ascomycota Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The def ...

, with about 40% of species forming such an association.Kirk ''et al''., pp. 378–81. Some of these lichenized fungi occur in orders with nonlichenized fungi that live as saprotrophs or plant parasites (for example, the

Leotiales The Leotiales are an order of ascomycete fungi. The order contains 2 families (the Bulgariaceae and the Leotiaceae), 11 genera, and 41 species In biology, a species is the basic unit of classification and a taxonomic rank of an organ ...

,

Dothideales Dothideales are an order of bitunicate fungi consisting mainly of saprobic or plant parasitic species. Description Taxa in this order are characterized by the absence of a hamathecium (defined as hyphae or other tissues between asci) in a loc ...

, and

Pezizales The Pezizales are an order of the subphylum Pezizomycotina within the phylum Ascomycota. The order contains 16 families, 199 genera, and 1683 species. It contains a number of species of economic importance, such as morels, the black and white tr ...

). Other lichen fungi occur in only five

orders Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

in which all members are engaged in this habit (Orders Graphidales,

Gyalectales ''Gyalectales'' is an order of lichen-forming fungi in the class Lecanoromycetes. It contains 5 families, 15 genera and about 550 species. Taxonomy The Gyalectales were introduced in a 1974 publication by Aino Henssen and Martin Jahns, but ...

, Peltigerales,

Pertusariales The Pertusariales are an order of fungi in the class Lecanoromycetes. It contains the following families: Agyriaceae The Agyriaceae are a family of lichenized fungi in the order Pertusariales. It contains two genera: ''Agyrium ''Agyri ...

, and Teloschistales). Lichenized and nonlichenized fungi can even be found in the same genus or species.

Photosynthetic component

The photosynthetic component of a lichen is called the

photobiont A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.phycobiont A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.Cyanophyceae Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blue ...

; eukaryotic: Trebouxiophyceae,

Phaeophyceae Brown algae (singular: alga), comprising the class Phaeophyceae, are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Brown algae are the major seaweeds of the temperate and po ...

,

Chlorophyceae The Chlorophyceae are one of the classes of green algae, distinguished mainly on the basis of ultrastructural morphology. They are usually green due to the dominance of pigments chlorophyll a and chlorophyll b. The chloroplast may be discoid, p ...

) have been found to associate with the lichen-forming fungi. A particular fungus species and algal species are not necessarily always associated together in a lichen. One fungus, for example, can form lichens with a variety of different algae. The

thalli 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 were ...

produced by a given fungal symbiont with its differing partners will be similar, and the secondary metabolites identical, indicating that the fungus has the dominant role in determining the morphology of the lichen. Further, the same algal species can occur in association with different fungal partners. Lichens are known in which there is one fungus associated with two or even three algal species. Rarely, the reverse can occur, and two or more fungal species can interact to form the same lichen.

Green algae

About 90% of all known lichens have a green alga as a symbiont. “ Clorococcoid” means a green alga ( Chlorophyta) that has single cells that are

globose A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...

, which is common in lichens. This was once classified in the order Chlorococcales, which one may find stated in older literature, but new DNA data shows many independent lines of evolution exist among this formerly large taxonomic group. Chlorococcales is now a relatively small order and may no longer include any lichen photobionts. The term “ Trebouxioid” refers to members of the

Trebouxia ''Trebouxia'' is a unicellular green alga.Silverside, A. J. (2009). Retrieved from http://www.bioref.lastdragon.org/Chlorophyta/''Trebouxia''.html It is a photosynthetic organism that can exist in almost all habitats found in polar, tropical, and ...

algae or other algae that resemble them: a clorococcoid green algae photobiont in the genus ''

Trebouxia ''Trebouxia'' is a unicellular green alga.Silverside, A. J. (2009). Retrieved from http://www.bioref.lastdragon.org/Chlorophyta/''Trebouxia''.html It is a photosynthetic organism that can exist in almost all habitats found in polar, tropical, and ...

''. Algae that resemble members of the

Trebouxia ''Trebouxia'' is a unicellular green alga.Silverside, A. J. (2009). Retrieved from http://www.bioref.lastdragon.org/Chlorophyta/''Trebouxia''.html It is a photosynthetic organism that can exist in almost all habitats found in polar, tropical, and ...

are presumed to be in the class Trebouxiophyceae and go by the same descriptive name ('' Trebouxioid'').

Trebouxia ''Trebouxia'' is a unicellular green alga.Silverside, A. J. (2009). Retrieved from http://www.bioref.lastdragon.org/Chlorophyta/''Trebouxia''.html It is a photosynthetic organism that can exist in almost all habitats found in polar, tropical, and ...

was once included here, but is now considered to be in a separate class Trebouxiophyceae.

Cyanolichens

Although the photobionts are almost always green algae ( chlorophyta), sometimes the lichen contains a blue-green alga instead (

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

, not really an alga), and sometimes both types of photobionts are found in the same lichen. A cyanolichen is a lichen with a cyanobacterium as its main photosynthetic component (

photobiont A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

as the substrate. Another cyanolichen group, the jelly lichens ( e.g., from the genera '' Collema'' or ''

Leptogium ''Leptogium'' is a genus of lichen-forming fungi in the family Collemataceae. It has about 110 species. Species *'' Leptogium acadiense'' *'' Leptogium adnatum'' – South America *''Leptogium andegavense'' *'' Leptogium antarcticum'' *''L ...

'') are large and foliose (e.g., species of ''

Peltigera ''Peltigera'' is a genus of approximately 100 species of foliose lichens in the family Peltigeraceae. Commonly known as the dog or pelt lichens, species of ''Peltigera'' are often terricolous (growing on soil), but can also occur on moss, trees, ...

'', ''

Lobaria ''Lobaria'' is a genus of foliose lichens, formerly classified in the family Lobariaceae, but now placed in the Peltigeraceae. They are commonly known as "lung wort" or "lungmoss" as their physical shape somewhat resembles a lung, and their ecol ...

'', and ''

Degelia ''Degelia'' is a genus of lichen-forming fungi in the family Pannariaceae. The genus is named after Swedish lichenologist Gunnar Degelius. The genus was circumscribed by Lars Arvidsson and David John Galloway in 1981. Species *''Degelia calc ...

''. These lichen species are grey-blue, especially when dampened or wet. Many of these characterize the Lobarion communities of higher rainfall areas in western Britain, e.g., in the

Celtic Rainforest Celtic Rainforest is a colloquial term which refers to the temperate rainforest of Ireland and Great Britain. These woodlands are also variously referred to as Atlantic rainforest, Upland Oakwoods, Atlantic Oakwoods or Western Oakwoods. Today, the ...

.

Parasitic fungi

Some fungi can only be found living on lichens as

obligate parasite An obligate parasite or holoparasite is a parasitic organism that cannot complete its life-cycle without exploiting a suitable host. If an obligate parasite cannot obtain a host it will fail to reproduce. This is opposed to a facultative parasite, ...

s; They are not considered part of the lichen. These are referred to as “ lichenolous fungi”.

References

{{Reflist Lichenology