What is a Lichen? | The British Lichen Society
The nature of the lichen symbiosis is not clear. It is generally thought to be mutualistic but this concept is not supported by experimental. Discusses parasitic and mutualistic relationships of fungi. Two common mutualistic relationships involving fungi are mycorrhiza and lichen. When both members of the association benefit, the symbiotic relationship is called Fungi form mutualistic associations with many types of organisms, including . This cross-section of a lichen thallus shows the (a) upper cortex of fungal.
Probably more than ten distinct major lineages of fungi within the ascomycetes are lichenised. Current estimates suggest that one fifth of all known fungi and half of all ascomycetes are lichenised, with about 28, species worldwide.
Lichen - New World Encyclopedia
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.
- MUTUALISMS BETWEEN FUNGI AND ALGAE
- What is a Lichen?
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.
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. 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.
Symbiosis in lichens
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.
Most lichens grow slowly, probably because they live in environments where water is available for only short periods. They tend to live for many years, and lichens hundred of years old can be used to date the rock surfaces on which they grow. Lichens spread mostly by small pieces of their body being blown around.
All the partners in the original lichen body are present in the fragment, so growth can begin immediately. Some lichens create soredia, balls of tissue made just for dispersal. Although the fungus is the major partner, dispersal by spores is rare. Uses for Lichens Lichens have many uses. They differ in their sensitivity to air pollution, and the presence or absence of different lichens in an area has been used to map concentrations of pollutants.
Foliose lichens are used to represent trees in model train layouts. Lichens also make about known "secondary products". It is thought that these chemicals are produced by lichens as defenses against disease and parasites, and, in some cases, to make the lichen taste unpleasant to animals. Some of these compounds are now used as anti-viral and anti-bacterial medications. Other secondary products are used to make everyday life more colorful and pleasant.
Some are used to scent Wool Dyed with Lichenssoaps and make perfumes. Others were used in the past to dye woolen cloth. Most colors were some shade of brown or yellow, but blue was produced from a few species. The discovery of synthetic dyes ended the demand for lichen dyes. The synthetic dyes provided many more colors, and did not fade.
Lichen dyes are still used by some craft weavers who like their soft, quiet colors. Today, the only commercially important lichen dye is used to make litmus paper, to test the acidity of liquids. The litmus dye turns blue in "basic" low-acid solutions like ammonia, and red in acid solutions like vinegar.
The Lapp people, who live above the Arctic Circle in Scandinavia and Russia, harvest lichens as winter food for their reindeer, just like farmers in temperate zones stockpile hay. Sheep in the deserts of Libya survive, in part, by eating crustose lichens growing on rocks.
Lichens are also important in making soil. Soil is made up of organic matter, such as decayed plants, and minerals.