Fungi Symbiosis ( Read ) | Biology | CK Foundation
Lichens are quite a large group of organisms. Fungi constitute one of the five kingdoms of living organisms and of all fungi about 20 per cent are lichens. The symbiotic relationship between algae and fungi is lichen. The fungal component is called mycobiont while the algae component is called phycobiont. The fungus, in turn, forms the main structure of the lichen and offers its in B. tortuosa enable the lichen to produce the acid that helps defend it.
Rarely, the reverse can occur, and two or more fungal species can interact to form the same lichen. Chlorococcales is now a relatively small order and may no longer include any lichen photobionts.
Algae that resemble members of the Trebouxia are presumed to be in the class Trebouxiophyceae and go by the same descriptive name Trebouxioid.
Cyanolichens[ edit ] Although the photobionts are almost always green algae chlorophytasometimes the lichen contains a blue-green alga instead cyanobacterianot really an algaand 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. Another cyanolichen group, the jelly lichens e. 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. Weird World of Lichen: On his website dedicated to lichen, Alan Silverside, now retired from the University of the West of Scotland, gives the example of the fungus Sticta canariensis.
This fungus is capable of forming two different lichen associations with an alga and cyanobacterium, yet both lichens are referred to as Sticta canariensis. This is how early lichens might have looked like million years ago. It is the thallus that gives lichens their characteristic outer appearance. Lichen thalli come in many different forms. Examples on Silverside's pages include foliose lichen, which look flat and leafy; fruticose lichen, which have a wiry, tufted appearance; squamulose lichen, which have flat, overlapping scales; and crustose lichen, which as the name suggests, form a tightly attached crust over the surface it inhabits.
In general, the inside of the lichen thallus appears stratified, with the mycobiont and photobiont cells arranged in layers. According to the U. Forest Servicethe outer layer or cortex is made up of thick, tightly packed fungal cells. This is followed by a segment with the photobiont either green algae or cyanobacteria.
If a lichen has both an algal and a cyanobacterial partner, the cyanobacteria can be seen within little compartments above the upper cortex. The final layer is the medulla, with loosely arranged fungal cells that look like filaments. Extensions below the medulla, which are called basal attachments, enable lichens to adhere to various surfaces. Typical basal attachments include rhizines, which are fungal filaments extending from the medulla, and a single, central structure called the holdfast, which latches onto rocks.When Algae Met Fungi: A lichen love story
The Forest Service gives the example of a foliose lichen called the umbilicate lichen, where the holdfast resembles an umbilical cord. As an exception to the general thallus structure, jelly lichens do not have a layered or stratified thallus. The mycobiont and photobiont components sit together in a single layer.
As a result, jelly lichens look like jelly; for example, Collema auriforme.
Symbiosis in lichens - Wikipedia
Appearance When dry, lichens simply take on the color of the mycobiont the fungus itself or can be drab and gray. In fact, a legacy of exclusion from accepted mycological research persisted until the s, despite their obvious affinities with non-lichen fungi. With the advent of molecular biology, the shared history of lichens and non-lichens has been elucidated and acceptedand we now know that the fungi that form lichens have evolved from many only distantly related lineages across the fungal tree of life, uniting them and their non-lichen relatives in the Kingdom Fungi.
Lichen fungi are a heterogeneous group; they are similar only ecologically, in that they share the nutritional strategy of gaining carbon from an internal symbiotic photosynthetic partner, the photobiont. In the study of lichens, the name and classification belongs to the fungal partner, which in most cases is the dominant member of the association, at least in terms of biomass. Lichen fungi have evolved independently several times within the mushroom-forming fungi and relatives the basidiomycetesbut much more commonly, from within the cup fungi the ascomycetes.
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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.
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. 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.