Lichen anatomy and physiology

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Lichen anatomy and physiology is very different from the anatomy and physiology of the fungus and/or algae and/or cyanobacteria that make up the lichen when growing apart from the lichen, either naturally, or in culture. The fungal partner is called the mycobiont . The photosynthetic partner, algae or cyanobacteria, is called the photobiont . The body of a lichens that does not contain reproductive parts of the fungus is called the thallus . The thallus is different from those of either the fungus or alga growing separately. The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations. In many species the fungus penetrates the algal cell wall, forming penetration pegs or haustoria similar to those produced by pathogenic fungi. [1] [2] Lichens are capable of surviving extremely low levels of water content (poikilohydric). [3] However, the re-configuration of membranes following a period of dehydration requires several minutes at least.

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The algal or cyanobacterial cells are photosynthetic, and as in plants they reduce atmospheric carbon dioxide into organic carbon sugars to feed both symbionts. Both partners gain water and mineral nutrients mainly from the atmosphere, through rain and dust. The fungal partner protects the alga by retaining water, serving as a larger capture area for mineral nutrients and, in some cases, provides minerals obtained from the substrate. If a cyanobacterium is present, as a primary partner or another symbiont in addition to green alga as in certain tripartite lichens, they can fix atmospheric nitrogen, complementing the activities of the green alga.

Although strains of cyanobacteria found in various cyanolichens are often closely related to one another, they differ from the most closely related free-living strains. [4] The lichen association is a close symbiosis. It extends the ecological range of both partners but is not always obligatory for their growth and reproduction in natural environments, since many of the algal symbionts can live independently. A prominent example is the alga Trentepohlia which forms orange-coloured populations on tree trunks and suitable rock faces. Lichen propagules (diaspores) typically contain cells from both partners, although the fungal components of so-called "fringe species" rely instead on algal cells dispersed by the “core species.” [5]

Lichen associations may be examples of mutualism, commensalism or even parasitism,[ citation needed ] depending on the species. Cyanobacteria in laboratory settings can grow faster when they are alone rather than when they are part of a lichen.

In tests, lichen survived and showed remarkable results on the adaptation capacity of photosynthetic activity within the simulation time of 34 days under Martian conditions in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center (DLR). [6] [7]

Symbionts

Schematic cross section of lichen, a symbiosis between green algae and a fungus. 1. Thick layers of hyphae, called the cortex 2. Green algae 3. Loosely packed hyphae 4. Anchoring hyphae called rhizines. Lichen Cross Section Diagram.svg
Schematic cross section of lichen, a symbiosis between green algae and a fungus. 1. Thick layers of hyphae, called the cortex 2. Green algae 3. Loosely packed hyphae 4. Anchoring hyphae called rhizines.
"Lichens are fungi that have discovered agriculture" - Trevor Goward [8]

Living as a symbiont in a lichen appears to be a successful way for a fungus to derive essential nutrients, as about 20% of all fungal species have acquired this mode of life. The fungal partner may be an Ascomycete or Basidiomycete. [9] Common algal partners are Trebouxia , Pseudotrebouxia , or Myrmecia . Common cyanobacterium partners include are Nostoc [1] or Scytonema . [9]

The largest number of lichenized fungi occur in the Ascomycota, with about 40% of species forming such an association. [10] Some of these lichenized fungi occur in orders with nonlichenized fungi that live as saprotrophs or plant parasites (for example, the Leotiales, Dothideales, and Pezizales). Other lichen fungi occur in only five orders in which all members are engaged in this habit (Orders Graphidales, Gyalectales, Peltigerales, Pertusariales, and Teloschistales). Lichenized and nonlichenized fungi can even be found in the same genus or species.[ citation needed ] 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. Comparatively few Basidiomycetes are lichenized, but these include agarics, such as species of Lichenomphalia , clavarioid fungi, such as species of Multiclavula , and corticioid fungi, such as species of Dictyonema .

The 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 organisms. Approximately 100 species of photosynthetic partners from 40 genera and five distinct classes (prokaryotic: Cyanophyceae; eukaryotic: Trebouxiophyceae, Phaeophyceae, Chlorophyceae) have been found to associate with the lichen-forming fungi. [11] The prokaryotes belong to the Cyanobacteria, whose representatives are often called bluegreen algae. The bluegreen algae occur as symbionts in about 8% of the known lichens. The most commonly occurring genus is Nostoc . [12] The majority of the lichens contain eukaryotic autotrophs belonging to the Chlorophyta (green algae) or to the Xanthophyta (yellow-green algae). About 90% of all known lichens have a green alga as a symbiont, and among these, Trebouxia is the most common genus, occurring in about 40% 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 are probably able to exist independently in nature as well as in the lichen. [12]

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 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. [12]

Both the lichen and the fungus partner bear the same scientific name, and the lichens are being integrated into the classification schemes for fungi. The alga bears its own scientific name, which bears no relationship to that of the lichen or fungi. [10]

Fungus component

Depending on context, the entire lichen, or just the fungus that is part of the lichen. Both the lichen and the fungus that is a part of the lichen are currently (2014) given the same species name, which creates an ambiguity. An example of when "lichenized fungus" refers to just the fungus is when the fungus is grown in culture without a phycobiont. An example where "lichenized fungus" refers to the entire lichen is in a list of classified lichens.

Some fungi can only be found living on lichens (obligate parasites), but are not considered part of the lichen. These are referred to as lichenolous fungi.

Photosynthetic component

The photosynthetic component of a lichen is called the photobiont or phycobiont . [13] Sometimes the photobiont is a green algae (chlorophyta), sometimes a blue-green aglae (cyanobacteria, not really an algae), and sometimes both. The layer of tissue containing the cells of the photobiont is called the "photobiontic layer". [13]

"Clorococcoid" means a green algae (Chlorophyta) that has single cells that are globose, which is common in lichens. [14] This was once classified in the order Chlorococcales, which you 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. Trebouxia, once included here, is now considered to be in a separate class, Trebouxiophyceae. "trebouxioid" refers to members of this class or algae resembling them.

" Trebouxioid " means a clorococcoid green algae photobiont is in the genus Trebouxia , or resembles a member of that genus, and is therefore presumably a member of the class Trebouxiophyceae. [13]

Cyanolichens

A cyanolichen is a lichen with a cyanobacteria as its main photosynthetic component (photobiont). [14] Many cyanolichens are small and black, and have limestone as the substrate. Another cyanolichen group, the jelly lichens ( e.g., from the genera Collema or Leptogium ) are large and foliose (e.g., species of Peltigera , Lobaria , and Degelia . These lichen species are grey-blue, especially when dampend or wet. Many of these characterize the Lobarion communities of higher rainfall areas in western Britain, e.g., in the Celtic Rainforest.

Related Research Articles

Algae Diverse group of photosynthetic eukaryotic organisms

Algae is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular microalgae, such as Chlorella,Prototheca and the diatoms, to multicellular forms, such as the giant kelp, a large brown alga which may grow up to 50 metres (160 ft) in length. Most are aquatic and autotrophic and lack many of the distinct cell and tissue types, such as stomata, xylem and phloem that are found in land plants. The largest and most complex marine algae are called seaweeds, while the most complex freshwater forms are the Charophyta, a division of green algae which includes, for example, Spirogyra and stoneworts.

Chlorophyta Phylum of green algae

Chlorophyta or Prasinophyta is a taxon of green algae informally called chlorophytes. The name is used in two very different senses, so care is needed to determine the use by a particular author. In older classification systems, it refers to a highly paraphyletic group of all the green algae within the green plants (Viridiplantae) and thus includes about 7,000 species of mostly aquatic photosynthetic eukaryotic organisms. In newer classifications, it refers to the sister clade of the streptophytes/charophytes. The clade Streptophyta consists of the Charophyta in which the Embryophyta emerged. In this latter sense the Chlorophyta includes only about 4,300 species. About 90% of all known species live in freshwater. Like the land plants, green algae contain chlorophyll a and chlorophyll b and store food as starch in their plastids.

Lichen Symbiosis of fungi with algae or cyanobacteria

A lichen is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship. Lichens have properties different from those of their component organisms. They come in many colors, sizes, and forms and are sometimes plant-like, but lichens are not plants. They may have tiny, leafless branches (fruticose); flat leaf-like structures (foliose); crust-like, adhering tightly to a surface (substrate) like a thick coat of paint (crustose); a powder-like appearance (leprose); or other growth forms.

Green algae Paraphyletic group of autotrophic eukaryotes in the clade Archaeplastida

The green algae are a large, informal grouping of algae consisting of the Chlorophyta and Charophyta/Streptophyta, which are now placed in separate divisions, together with the more basal Mesostigmatophyceae, Chlorokybophyceae and Spirotaenia.

Cladoniaceae Family of fungi

The Cladoniaceae are a family of lichenized fungi in the order Lecanorales. It is one of the largest families of lichen-forming fungi, with about 560 species distributed amongst 17 genera. The reindeer moss and cup lichens (Cladonia) belong to this family. The latter genus, which comprises about 500 species, forms a major part of the diet of large mammals in taiga and tundra ecosystems. Many Cladoniaceae lichens grow on soil, but other can use decaying wood, tree trunks, and, in a few instances, rocks as their substrate. They grow in places with high humidity, and cannot tolerate aridity.

Lithophiles are micro-organisms that can live within the pore interstices of sedimentary and even fractured igneous rocks to depths of several kilometers.

Cyanolichen

Cyanolichens are lichens that apart from the basic fungal component ("mycobiont"), contain cyanobacteria, otherwise known as blue-green algae, as the photosynthesizing component ("photobiont"). Overall, about a third of lichen photobionts are cyanobacteria and the other two thirds are green algae.

<i>Xanthoria parietina</i> Species of lichen in the family Teloschistaceae

Xanthoria parietina is a foliose, or leafy, lichen. It has wide distribution, and many common names such as common orange lichen, yellow scale, maritime sunburst lichen and shore lichen. It can be found near the shore on rocks or walls, and also on inland rocks, walls, or tree bark. It was chosen as a model organism for genomic sequencing by the US Department of Energy Joint Genome Institute (JGI).


Vernon Ahmadjian was a distinguished professor at Clark University in Worcester, Massachusetts. He specialized in the symbiosis of lichens, and wrote several books and numerous publications on the subject.

Dictyochloropsis is a genus of unicellular green alga of the phylum Chlorophyta. This genus consists of free-living algae which have a reticulate (net-like) chloroplast that varies slightly in morphology between species, and that when mature always lacks a pyrenoid. Dictyochloropsis is asexual and reproduces using autospores.

Trebouxia is a unicellular green alga. It is a photosynthetic organism that can exist in almost all habitats found in polar, tropical, and temperate regions. It can either exist in a symbiotic relationship with fungi in the form of lichen or it can survive independently as a free-living organism alone or in colonies. Trebouxia is the most common photobiont in extant lichens. It is a primary producer of marine, freshwater and terrestrial ecosystems. It uses carotenoids and chlorophyll a and b to harvest energy from the sun and provide nutrients to various animals and insects.

Coccomyxa is a genus of green algae, in the family Coccomyxaceae. This genus is defined by their small, elliptical to spherical shape, and the presence of a simple parietal chloroplast. These features, along with their occurrence in various lifestyles such as free-living, parasitic, or as photobionts, have been used to identify over 40 species. Using additional morphological features, such as brown akinetes formation, allows for the differentiation between Coccomyxa and the genus Pseudococcomyxa, as they tend to share some morphological characteristics like the general cell shape and one-sided mucilage cap. Recent molecular analysis, however, indicates that the genus Pseudococcomyxa is contained within different Coccomyxa clades, signaling the fact that the two genera are the same. Coccomyxa has often been used as a model organism, and its genome is being completely sequenced. The genus is also an attractive candidate for biofuels.

Cyanobionts are cyanobacteria that live in symbiosis with a wide range of organisms such as terrestrial or aquatic plants; as well as, algal and fungal species. They can reside within extracellular or intracellular structures of the host. In order for a cyanobacterium to successfully form a symbiotic relationship, it must be able to exchange signals with the host, overcome defense mounted by the host, be capable of hormogonia formation, chemotaxis, heterocyst formation, as well as possess adequate resilience to reside in host tissue which may present extreme conditions, such as low oxygen levels, and/or acidic mucilage. The most well-known plant-associated cyanobionts belong to the genus Nostoc. With the ability to differentiate into several cell types that have various functions, members of the genus Nostoc have the morphological plasticity, flexibility and adaptability to adjust to a wide range of environmental conditions, contributing to its high capacity to form symbiotic relationships with other organisms. Several cyanobionts involved with fungi and marine organisms also belong to the genera Richelia, Calothrix, Synechocystis, Aphanocapsa and Anabaena, as well as the species Oscillatoria spongeliae. Although there are many documented symbioses between cyanobacteria and marine organisms, little is known about the nature of many of these symbioses. The possibility of discovering more novel symbiotic relationships is apparent from preliminary microscopic observations.

Dictyochloropsis reticulata is a species of green algae in the Trebouxiales. It is a known as a photobiont with several lichen species, like Lobaria pulmonaria, but also as a free-living soil alga as well. Phylogenetic analysis of rRNA sequence data revealed that the species shares a sister group relationship with two other green algae that lack motile stages, Chlorella saccharophila and C. luteoviridis.

<i>Cryptothecia rubrocincta</i> Species of fungus

Cryptothecia rubrocincta is a species of lichen in the fungal family Arthoniaceae. The species is distributed in subtropical and tropical locations throughout the southeastern United States, as well as Central and South America, and has been collected infrequently in a few locales in Africa. The body of the lichen forms continuous, circular crust-like patches on dead wood, readily recognizable by the prominent red pigment. The older, central region is covered with red, spherical to cylindrical granules. Moving outwards from the center, zones of color may be distinguished, the first gray-green, the second white, and finally a bright red cottony rim. The red and green colors of this unmistakable woodland lichen give the appearance of a Christmas wreath, suggestive of its common North American name, the Christmas wreath lichen. The red pigment, called chiodectonic acid, is one of several chemicals the lichen produces to help tolerate inhospitable growing conditions.

<i>Pilophorus acicularis</i> Species of fungus

Pilophorus acicularis, commonly known as the nail lichen or the devil's matchstick lichen, is a species of matchstick lichen in the family Cladoniaceae.

Dendriscocaulon is a taxonomic name that has been used for a genus of fruticose lichen with a cyanobacteria as the photobiont partner of the fungus. Dendriscocaulon is considered a taxonomic synonym of the genus Sticta, a foliose lichen, which generally has a green alga as the photobiont partner. Lichens that have been called Dendriscocaulon or Sticta involve the same fungal species. They show dramatically different morphology, may grow side-by-side, and mixed forms exist where different algae are growing within different portions of the same fungal thallus. The biochemistry of the two forms is very different, but the DNA sequences from the fungus and the photobiont can be distinguished using different primers for DNA sequencing.

Lichen morphology

Lichen morphology includes lichen growth forms used to group lichens by "vegetative" thallus types, and forms of "non-vegetative" reproductive parts. Some lichen thalli have the aspect of leaves ; others cover the substrate like a crust, others such as the genus Ramalina adopt shrubby forms, and there are gelatinous lichens such as the genus Collema.

Symbiosis in lichens

Symbiosis in lichens is the mutually helpful symbiotic relationship of green algae and/or blue-green algae (cyanobacteria) living among filaments of a fungus, forming lichen.

Some types of lichen are able to fix nitrogen from the atmosphere. This process relies on the presence of cyanobacteria as a partner species within the lichen. The ability to fix nitrogen enables lichen to live in nutrient-poor environments. Lichen can also extract nitrogen from the rocks on which they grow.

References

  1. 1 2 F.S. Dobson (2000) Lichens, an illustrated guide to the British and Irish species. Richmond Publishing Co. Ltd., Slough, UK
  2. ↑ R. Honegger (1988) Mycobionts. Chapter 3 in T.H. Nash (ed.) (1996) Lichen Biology. Cambridge University Press. ISBN   0-521-45368-2
  3. ↑ Nash, Thomas H., ed. (2008). Lichen Biology (2nd ed.). Cambridge University Press. pp.  5–6. ISBN   978-0-521-69216-8.
  4. ↑ Sciencemag.org
  5. ↑ A.N. Rai; B. Bergman; Ulla Rasmussen (31 July 2002). Cyanobacteria in Symbiosis. Springer. p. 59. ISBN   978-1-4020-0777-4 . Retrieved 2 June 2013.
  6. ↑ Baldwin, Emily (26 April 2012). "Lichen survives harsh Mars environment". Skymania News. Archived from the original on 28 May 2012. Retrieved 27 April 2012.
  7. ↑ de Vera, J.-P.; Kohler, Ulrich (26 April 2012). "The adaptation potential of extremophiles to Martian surface conditions and its implication for the habitability of Mars" (PDF). European Geosciences Union. Archived from the original (PDF) on 8 June 2012. Retrieved 27 April 2012.
  8. ↑ LICHEN BIOLOGY AND THE ENVIRONMENT, LICHENS OF NORTH AMERICA, Sylvia and Stephen Sharnoff,
  9. 1 2 Lichens: Systematics, University of California Museum of Paleontology
  10. 1 2 Kirk et al., pp. 378–81.
  11. ↑ Friedl T, Büdel B. "Photobionts". In Nash III TH (ed.). Lichen Biology. Cambridge: Cambridge University Press.
  12. 1 2 3 Rikkinen J. (1995). "What's behind the pretty colors? A study on the photobiology of lichens". Bryobrothera. 4: 1–226.
  13. 1 2 3 Alan Silverside's Lichen Glossary (p-z), Alan Silverside
  14. 1 2 Alan Silverside's Lichen Glossary (a-f), Alan Silverside
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