Lichen - Wikipedia
The lichen symbiosis is thought to be a mutualism, since both the fungi and the Up to half of the carbon fixed by algae is immediately converted to fungal. Yeast emerges as hidden third partner in lichen symbiosis. Date: July The fungus, in turn, forms the main structure of the lichen and offers its. to lichen-forming lineages, many of the other fungi that are found have close even before lichens were recognized as a fungal-algal symbiosis D.; Egidi, E. ; Zucconi, L.; Gueidan, C.; de Hoog, G.S.; Onofri, S. Mountain tips.
As with most organisms, lichen fungi are most diverse and least studied in the tropics.
For example, the genus Arthonia is comprised of a mix of lichenised and non-lichenised species and includes many which are specialist parasites, only found on one or a few closely-related host lichens.
In a single genus, then, we have a case of lichen parasites evolving from lichen fungi! Other non-lichen fungi arose from lichenised ancestors, such as Stictis and Ostropa. Fungi are classified in part by the type of spore-producing structures they produce, with the cup fungi ascomycetes named for the open, cup-shaped structures which often bear the sexual spores of the fungi. Not all ascomycetes have these cup-shaped structures, however, and, easily observed morphological characteristics like fruit type cup-like apothecia versus flask-shaped perithecia, for example cannot always be used to assess relationships.
Unfortunately, this means that not all fungi sharing a single characteristic are likely to be related. However, some order can be distilled. The bulk of lichen diversity belongs to the class including the well-known genera Lecanora, Cladonia, Parmelia and Peltigera Lecanoromycetes, or the Lecanora-groupwhere spores are borne mostly in open or cup-shaped fruits apothecia.
This group of fungi is very old, estimated to have evolved during the Carboniferous period. The very first lichens probably date back to before the origin of land plants, when most of the biodiversity of Earth was in the sea.
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Many Arthonia relatives also have open cup type fruits, but their development is quite different, giving a clue that they are not closely related to the Lecanora-group. Instead, they are more closely related to other ascomycetes that have flask-shaped spore-bearing structures perithecia. Similarly, for still other lichen groups, morphological similarities have been confirmed by molecular evidence to point to their widely disparate origins in the ascomycete tree of life.
For examples of these, students would be advised to visit the tropics, where the members of the Arthonia- Trypethelium- and Pyrenula- groups form conspicuous and sometimes colourful crusts.
In Britain, the smooth barked trees of the western districts are good places to see some of our Arthonia and Pyrenula species.
What is a Lichen? | The British Lichen Society
Students of lichenology will probably not be surprised to read that lichen fungi can be difficult to identify, partly due to the paucity of morphological characters to go on, but also due to the repeated and independent evolution of such characters. For example, the fruticose habit has evolved repeatedly within the Lecanora-group, but also within the distantly related Arthonia-group. Unrelated fungi repeatedly evolve similar morphologies to succeed under similar conditions, making morphological identification especially difficult in some groups.
Lichen Photobionts Fungi are heterotrophic, meaning that, like animals, they require a carbon source to survive. The lichen fungi share a common ecological strategy of hosting an internal population of photosynthetic cells, from which they obtain their carbon source in the form of simple sugars. These photosynthetic cells can either be green algae Chlorophyta or cyanobacteria or sometimes both, in which case the cyanobacteria are localised in distinct areas of the thallus. As the photosynthetic partners come from divergent parts of the tree of life green plants vs bacteriathe term photobiont is used as collective term for any of them.
The role of the photobiont in lichens is clear — to provide carbon in the form of simple sugars. These sugars are used by the fungi to maintain physiological functions, to grow, and reproduce. However, in the case of lichens with both green algae and cyanobacteria, the lichen gets an added nutrient input from the cyanobacteria in the form of fixed nitrogen.
Although lichens can probably access inorganic nitrogen from the atmosphere directly, it can be a limiting nutrient, so having an internal source can be an advantage especially in heavily leached environments.
Only about species of photobionts are commonly found across all known lichens, representing 4 main genera. The vast majority of photobionts are from the genus Trebouxia, followed by Trentopohlia both ChlorophytaNostoc and Scytonema both Cyanobacteria.
Most green-algal photobionts are unicellular green forms, but small colonial types and filamentous algae occur as well. Within the lichen thallus, most photobionts have a different morphology than they would when grown in isolation, so few photobionts can be reliably identified using traditional microscopic methods. Instead, it is best to rely on culturing studies, and more often, on molecular methods, as many different strains have very similar morphology.
The jelly-lichens are one exception, where the chain-of-pearls structure of Nostoc is very clear under the microscope. For example, the same fungal species will use different photobionts in different ecological settings, even within similar geographic areas.
Some individual lichens even contain more than a single photobiont strain, a situation that has been better explored in other symbiotic systems.Lichens and Mycorrhiza
In corals and other marine reef organisms, the animals can regulate the populations of their varying photobionts to maximize photosynthetic output according to environmental variation. These are referred to as lichenicolous fungiand are a different species from the fungus living inside the lichen; thus they are not considered to be part of the lichen.
When the cortex is more transparent, the algae show more clearly and the lichen looks greener. Metabolites, metabolite structures and bioactivity[ edit ] Lichens can show intense antioxidant activity. Life span[ edit ] Lichens may be long-livedwith some considered to be among the oldest living organisms. In an experiment led by Leopoldo Sancho from the Complutense University of Madrid, two species of lichen— Rhizocarpon geographicum and Xanthoria elegans —were sealed in a capsule and launched on a Russian Soyuz rocket 31 May Once in orbit, the capsules were opened and the lichens were directly exposed to the vacuum of space with its widely fluctuating temperatures and cosmic radiation.
After 15 days, the lichens were brought back to earth and were found to be unchanged in their ability to photosynthesize. Many lichens reproduce asexually, either by a piece breaking off and growing on its own vegetative reproduction or through the dispersal of diaspores containing a few algal cells surrounded by fungal cells. Fruticose lichens can easily[ citation needed ] fragment, and new lichens can grow from the fragment vegetative reproduction. Following dispersal, such fungal spores must meet with a compatible algal partner before a functional lichen can form.
Some lichen fungi belong to Basidiomycetes basidiolichens and produce mushroom -like reproductive structures resembling those of their nonlichenized relatives.
Most lichen fungi belong to Ascomycetes ascolichens. Among the ascolichens, spores are produced in spore-producing structures called ascomata. When apothecia are shaped like squiggly line segments instead of like discs, they are called lirellae. They usually bear the fungal pycnidia or apothecia or both.
Instead, the lichen-forming fungi of these species reproduce sexually by self-fertilization i. This breeding system may enable successful reproduction in harsh environments. Lichen species are given the same scientific name binomial name as the fungus species in the lichen. 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 fungus. This may cause confusion without context.
What is a Lichen?
A particular fungus species may form lichens with different algae species, giving rise to what appear to be different lichen species, but which are still classified as of as the same lichen species. Neither the ascolichens nor the basidiolichens form monophyletic lineages in their respective fungal phyla, but they do form several major solely or primarily lichen-forming groups within each phylum.
Geosiphon is not usually considered to be a lichen, and its peculiar symbiosis was not recognized for many years. The genus is more closely allied to endomycorrhizal genera. Fungi from Verrucariales also form marine lichens with the brown algae Petroderma maculiforme,  and have a symbiotic relationship with seaweed like rockweed and Blidingia minimawhere the algae are the dominant components. The fungi is thought to help the rockweeds to resist desiccation when exposed to air.
The mycobiont may be an Ascomycete or Basidiomycete. But the same mycobiont with different photobionts may also produce very different growth forms. Although each lichen thallus generally appears homogeneous, some evidence seems to suggest that the fungal component may consist of more than one genetic individual of that species.