Lichen - Wikipedia
Mutualism is a type of relationship where both organisms benefit from being Crustose lichen example makes a crust over the surface in which it grows. Lichens are an example of a symbiotic relationship between algae and certain fungi. They are capable of producing their own food. The alga that is associated. Lichens comprise a fungus living in a symbiotic relationship with an the West of Scotland, gives the example of the fungus Sticta canariensis.
However, while symbiotic, the relationship is probably not mutualisticsince the algae give up a disproportionate amount of their sugars see below. 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 a green alga as in certain tripartite lichens, they can fix atmospheric nitrogencomplementing the activities of the green alga. In three different lineages the fungal partner has independently lost the mitochondrial gene atp9, which has key functions in mitochondrial energy production. The loss makes the fungi completely dependent on their symbionts. Phycobionts algae produce sugar alcohols ribitolsorbitoland erythritolwhich are absorbed by the mycobiont fungus.
What is a Lichen? | The British Lichen Society
The absence of this third partner could explain the difficulties of growing lichen in the laboratory. The yeast cells are responsible for the formation of the characteristic cortex of the lichen thallus, and could also be important for its shape. The body thallus of most lichens is different from those of either the fungus or alga growing separately.
When grown in the laboratory in the absence of its photobiont, a lichen fungus develops as a structureless, undifferentiated mass of fungal filaments hyphae. If combined with its photobiont under appropriate conditions, its characteristic form associated with the photobiont emerges, in the process called morphogenesis.
Quite naturally, these alternative forms were at first considered to be different species, until they were found growing in a conjoined manner. Evidence that lichens are examples of successful symbiosis is the fact that lichens can be found in almost every habitat and geographic area on the planet.
There is evidence to suggest that the lichen symbiosis is parasitic or commensalisticrather than mutualistic. Photobiont cells are routinely destroyed in the course of nutrient exchange. The association is able to continue because reproduction of the photobiont cells matches the rate at which they are destroyed. In many species the fungus penetrates the algal cell wall,  forming penetration pegs haustoria similar to those produced by pathogenic fungi that feed on a host.
Miniature ecosystem and holobiont theory[ edit ] Symbiosis in lichens is so well-balanced that lichens have been considered to be relatively self-contained miniature ecosystems in and of themselves. Lichens have been used in making dyesperfumes and in traditional medicines. 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.
The most commonly occurring genus of symbiotic cyanobacteria is Nostoc. 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.
The fungal partner may be an Ascomycete or Basidiomycete. Next to the Ascomycota, the largest number of lichenized fungi occur in the unassigned fungi imperfecti.
Comparatively few Basidiomycetes are lichenized, but these include agaricssuch as species of Lichenomphaliaclavarioid fungisuch as species of Multiclavulaand corticioid fungisuch as species of Dictyonema. Other lichen fungi occur in only five orders in which all members are engaged in this habit Orders GraphidalesGyalectalesPeltigeralesPertusarialesand Teloschistales. 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.
What Are Lichens?
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.