Symbiosis in lichens

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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.

Symbiosis in lichens is the mutually beneficial symbiotic relationship of green algae and/or blue-green algae (cyanobacteria) living among filaments of a fungus, forming lichen. [1] [2] [3]

Contents

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 adopted this mode of life. [4] 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. [5] [4] [3] [2]

Overview of lichens

"Lichens are fungi that have discovered agriculture"— Trevor Goward [1]

A lichen is a combination of fungus and/or algae and/or cyanobacteria that has a very different form (morphology), physiology, and biochemistry than any of the constituent species growing separately. The algae or cyanobacteria benefit their fungal partner by producing organic carbon compounds through photosynthesis. 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. [5] [4] [3] [2]

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. Among these, Trebouxia is the most common genus, occurring in about 20% of all lichens. [6] 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. [7] Common algal partners are Trebouxia , Pseudotrebouxia , or Myrmecia . [5]

The prokaryotes belong to the Cyanobacteria, which are often called by their old name “bluegreen algae”. Cyanobacteria occur as symbionts only in about 8% of known lichens. The most commonly occurring genera of symbiotic cyanobacteria are Nostoc [7] and Scytonema . [4]

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 cyanobacterium bears its own scientific name, which has no relationship to either the name of the lichen or the fungus. [8]

Fungus component

About 20% of all fungal species are able to form lichens. The fungal partner may be an Ascomycete or Basidiomycete. [4] 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 largest number of lichenized fungi occur in the Ascomycota, with about 40% of species forming such an association. [8] 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 ]

Photosynthetic component

The photosynthetic component of a lichen is called the photobiont or phycobiont . [9] The layer of tissue containing the cells of the photobiont is called the “photobiontic layer”. [9]

Approximately 100 species of photosynthetic partners from 40 genera and 5 distinct classes (prokaryotic: Cyanophyceae; eukaryotic: Trebouxiophyceae, Phaeophyceae, Chlorophyceae) have been found to associate with the lichen-forming fungi. [10]

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. [7]

Green algae

About 90% of all known lichens have a green alga as a symbiont. [11]

Cyanolichens

Although the photobionts are almost always green algae (Chlorophyta), sometimes the lichen contains Cyanobacteria, taxonomically bacteria, 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). [12] 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 dampened or wet. Many of these characterize the Lobarion communities of higher rainfall areas in western Britain, e.g., in the Celtic Rainforest.

Lichenization

The process by which the fungus and the photobioant comes together is called "lichenization". There are five steps to this process: [14]

  1. The fungus and the alga start in the enviornment, apart from each other. Each part produces chemical signals to help the other part find it. At this point, the fungus displays increased hyphae branching.
  2. The fungus makes initial contact with the alga by growing appressoria and haustoria.
  3. The fungus grows hyphae to completely engulf the alga.
  4. The alga cell is now incorporated into the lichen. It starts increasing in cell size in response. The hyphae continue growing and swelling.
  5. The fungal structure continues differentiating into a full thallus.

Scientists have successfully replicated lichenization in the laboratory; it takes anywhere from a few months to a few years for the isolated fungus and photobiont of a lichen to grow back into a complete thallus. It is unclear how long the process takes in the wild. [14] Throughout lichenization (and after its completion), the fungus and the alga continue to exchange different chemical signals. [14]

Parasitic fungi

Some fungi can only be found living on lichens as obligate parasites; They are not considered part of the lichen. These are referred to as “lichenolous fungi”.

Some of these parasitic lichenolous fungi form their own thalli and become lichen themselves; they are called "lichenicolous lichens". They steal the symbioant of another lichen (kleptosymbiosis) into their own structure (trans-lochenization). [14]

Related Research Articles

<span class="mw-page-title-main">Lichen</span> 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 are important actors in nutrient cycling and act as producers which many higher trophic feeders feed on, such as reindeer, gastropods, nematodes, mites, and springtails. Lichens have properties different from those of their component organisms. They come in many colors, sizes, and forms and are sometimes plant-like, but are not plants. They may have tiny, leafless branches (fruticose); flat leaf-like structures (foliose); grow crust-like, adhering tightly to a surface (substrate) like a thick coat of paint (crustose); have a powder-like appearance (leprose); or other growth forms.

<span class="mw-page-title-main">Green algae</span> Paraphyletic group of autotrophic eukaryotes in the clade Archaeplastida

The green algae are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as a sister of the Zygnematophyceae. Since the realization that the Embryophytes emerged within the green algae, some authors are starting to include them. The completed clade that includes both green algae and embryophytes is monophyletic and is referred to as the clade Viridiplantae and as the kingdom Plantae. The green algae include unicellular and colonial flagellates, most with two flagella per cell, as well as various colonial, coccoid and filamentous forms, and macroscopic, multicellular seaweeds. There are about 22,000 species of green algae, many of which live most of their lives as single cells, while other species form coenobia (colonies), long filaments, or highly differentiated macroscopic seaweeds.

<i>Xanthoria parietina</i> Species of lichen

Xanthoria parietina is a foliose lichen in the family Teloschistaceae. 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.

<i>Trebouxia</i> Genus of algae

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.

<i>Lobaria pulmonaria</i> Species of lichen

Lobaria pulmonaria is a large epiphytic lichen consisting of an ascomycete fungus and a green algal partner living together in a symbiotic relationship with a cyanobacterium—a symbiosis involving members of three kingdoms of organisms. Commonly known by various names like tree lungwort, lung lichen, lung moss, lungwort lichen, oak lungs or oak lungwort, it is sensitive to air pollution and is also harmed by habitat loss and changes in forestry practices. Its population has declined across Europe and L. pulmonaria is considered endangered in many lowland areas. The species has a history of use in herbal medicines, and recent research has corroborated some medicinal properties of lichen extracts.

Dictyochloropsis reticulata is a species of green alga 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.

<span class="mw-page-title-main">Verrucariaceae</span> Family of mostly lichenised fungi

Verrucariaceae is a family of lichens and a few non-lichenised fungi in the order Verrucariales. The lichens have a wide variety of thallus forms, from crustose (crust-like) to foliose (bushy) and squamulose (scaly). Most of them grow on land, some in freshwater and a few in the sea. Many are free-living but there are some species that are parasites on other lichens, while one marine species always lives together with a leafy green alga.

<i>Buellia</i> Genus of lichens

Buellia is a genus of mostly lichen-forming fungi in the family Caliciaceae. The fungi are usually part of a crustose lichen. In this case, the lichen species is given the same name as the fungus. But members may also grow as parasites on lichens (lichenicolous). The algae in the lichen is always a member of the genus Trebouxia.

<span class="mw-page-title-main">Cephalodium</span> Morphological structure found in some lichens

A cephalodium (pl. cephalodia) is a small gall-like structure found in some lichens. They occur only lichens which contain both cyanobacterial and green algal partners. Cephalodia can occur within the tissues of the lichen, or on its upper or lower surface. Lichens with cephalodia can fix nitrogen, and may be an important contributor of nitrogen to the ecosystem.

<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.

<span class="mw-page-title-main">Lichen growth forms</span> Gross morphological classification

Lichens are symbiotic organisms made up of multiple species: a fungus, one or more photobionts and sometimes a yeast. They are regularly grouped by their external appearance – a characteristic known as their growth form. This form, which is based on the appearance of vegetative part of the lichen, varies depending on the species and the environmental conditions it faces. Those who study lichens (lichenologists) have described a dozen of these forms: areolate, byssoid, calicioid, cladoniform, crustose, filamentous, foliose, fruticose, gelatinous, leprose, placoidioid and squamulose. Traditionally, crustose (flat), foliose (leafy) and fruticose (shrubby) are considered to be the three main forms. In addition to these more formalised, traditional growth types, there are a handful of informal types named for their resemblance to the lichens of specific genera. These include alectorioid, catapyrenioid, cetrarioid, hypogymnioid, parmelioid and usneoid.

<span class="mw-page-title-main">Lichen morphology</span>

Lichen morphology describes the external appearance and structures of a lichen. These can vary considerably from species to species. Lichen growth forms are 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.

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. Lichens are capable of surviving extremely low levels of water content (poikilohydric). However, the re-configuration of membranes following a period of dehydration requires several minutes at least.

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.

Trebouxia decolorans is a widespread and common symbiotic species of green alga that is found in association with different species of lichen-forming fungi. Some lichens in which it is the photobiont partner are Xanthoria parietina and Anaptychia ciliaris.

Trebouxia arboricola is a symbiotic species of green alga in the family Trebouxiaceae. Described as new to science in 1924, it is usually found in association with different species of lichen-forming fungi and has a broad global distribution.

Trebouxia gelatinosa is a common symbiotic species of green alga in the family Trebouxiaceae. Formally described as new to science in 1975, it is usually found in association with different species of lichen-forming fungi.

Racoleus is a genus of two species of lichen-forming fungi of uncertain familial placement in the order Capnodiales. Both species of Racoleus are filamentous lichens with Trentepohlia as the photobiont partner. The lichens form thalli in the form of felt-like, blackish-brown cushions that grow under shaded conditions on siliceous rocks.

Asterochloris is a genus of green algae in the family Trebouxiophyceae. It is a common photobiont in lichen, occurring in the thalli of more than 20 lichen genera worldwide. Asterochloris is distinguishable from the morphologically similar genus Trebouxia, primarily due to its deeply lobed chloroplast, the placement of the chloroplast along the cell's periphery before the initiation of zoospore or aplanospore formation, and its tendency to primarily reproduce asexually through the production of aplanospores.

References

  1. 1 2 Sylvia Sharnoff; Stephen Sharnoff. "Lichen Biology and the Environment". Lichens of North America Information (formerly www.lichen.com). Archived from the original on 2023-11-09. Retrieved 2023-11-09.
  2. 1 2 3 "What is a lichen?". Australian National Botanical Garden.
  3. 1 2 3 "Introduction to Lichens – an Alliance between Kingdoms". University of California Museum of Paleontology.
  4. 1 2 3 4 5 "Lichens: Systematics". University of California Museum of Paleontology.
  5. 1 2 3 Dobson, F.S. (2000). Lichens, an illustrated guide to the British and Irish species. Slough, UK: Richmond Publishing Co. Ltd.
  6. ↑ Lichen Biology. Nash, Thomas H. Cambridge University Press. 2008. ISBN   978-1281751393. OCLC   819628036.{{cite book}}: CS1 maint: others (link)
  7. 1 2 3 Rikkinen J. (1995). "What's behind the pretty colors? A study on the photobiology of lichens". Bryobrothera. 4: 1–226.
  8. 1 2 Kirk et al., pp. 378–81.
  9. 1 2 3 Silverside, Alan. "Entries 'P'-'Z'". Alan Silverside's Lichen Glossary.
  10. ↑ Friedl, T.; Büdel B. "Photobionts". In Nash III, T.H. (ed.). Lichen Biology. Cambridge, UK: Cambridge University Press.
  11. ↑ Lichen Biology. Nash, Thomas H. Cambridge University Press. 2008. ISBN   978-1281751393. OCLC   819628036.{{cite book}}: CS1 maint: others (link)
  12. 1 2 Silverside, Alan. "Entries 'A'-'F'". Alan Silverside's Lichen Glossary.
  13. ↑ Krienitz, L. (2009). "Algae". Encyclopedia of Inland Waters. pp. 103–113. doi:10.1016/B978-012370626-3.00132-0. Most members of the order Trebouxiales are edaphic or live in symbiosis with fungi in lichen thalli.
  14. 1 2 3 4 Pichler, Gregor; Muggia, Lucia; Carniel, Fabio Candotto; Grube, Martin; Kranner, Ilse (May 2023). "How to build a lichen: from metabolite release to symbiotic interplay". New Phytologist. 238 (4): 1362–1378. doi: 10.1111/nph.18780 .
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