Fungi Symbiosis ( Read ) | Biology | CK Foundation
A lichen is not a single organism, but the result of a partnership (mutualistic symbiosis) between a fungus and an alga or cyanobacteria. Some lichens are. In nature, interactions between prokaryotic and eukaryotic organisms are common. The prokaryote may form a pathogenic or symbiotic relationship with the. Accommodation of the asymmetry of gains between the host (the fungus) and the fungus only forms an association with cyanobacteria without green algae (a.
Lichens producing apothecia are discussed further in the section dealing with lichen-forming discomycetes. In the case of L.
- Symbiosis in lichens
- MUTUALISMS BETWEEN FUNGI AND ALGAE
Although appearing whitish, the margin contains green algae, the photobiont, and will be seen to be green if cut lengthwise. Some lichens produce their asci in perithecia such as those in the photograph of Hydropunctaria maura at right.
Perithecia are flask-shaped structures that open to the outside by a pore called an ostiole. Lichens producing apothecia are discussed further in the section dealing with lichen-forming pyrenomycetes.
The ostioles in both perithecia at right can be seen as small circular areas at their tops. There are some other ways that asci can be produced by lichens, but perithecia and apothecia are by far the most common.
The photographs above illustrate two other types you may encounter. The first are structures called lyrellae. These are similar to apothecia but are greatly elongated. In the case of Graphis scripta, shown in the photo, the lyrellae are highly branched and may resemble some kind of mysterious writing. The second photograph, of Calicium trabinellum, illustrates a mazaedium, a kind of stalked apothecium in which the asci dissolve and leave the ascospores to pile up in a powdery mass.
A few basidiomycetes are also capable of forming lichens. These are not generally considered to be highly-developed relationships yet there is no doubt they function as lichens. The first of the two photos above shows Multiclavula mucida. In this species the basidia and basidiospores line the surface of the upright "fingers" and under cool moist conditions release the spores to drift in the wind.
The photobiont, a green alga, forms a thick crust of the the substrate, in this case rotten wood. The algae are enclosed by the hyphae of the mycobiont.
In the second picture the mycobiont is Lichenomphalia umbellifera, a mushroom. The photobiont and its relationship with the phytobiont are the same as in M.
Since these sexual structures reproduce only the fungus, the resulting spores must be fortunate enough to land on an appropriate alga, or perish.
However, there is another way. If the lichen can disperse propagules containing both myco- and photobionts then it will be able to develop in any suitable habitat. However, this type of reproduction is strictly clonal and does not allow for the kind of genetic recombination that occurs during sexual reproduction. Clonal reproduction of lichens can occur in several ways.
The simplest of these is simply to separate a piece of the thallus containing both alga and fungus and send it off by wind or water to develop in a new place. This kind of reproduction is common among lichens and generally effective. There are more highly developed forms of clonal reproduction, two of which are represented in the photographs above.
In the first the lichen has produced soredia. Soredia are small bundles of algae held together by fungal hyphae.
They are small enough to be carried by wind yet guarantee the presence of both partners. The illustration above left shows a young thallus of the foliose lichen Peltigera didactyla. In this species the upper surface becomes dotted with soralia, special structures for the production of soredia. In the photograph, the soralia have released granular masses of soredia. The other photograph above is a highly magnified view of isidia, small coral-like branches containing both mutualists that can break off and drift to a new habitat.
Lichens | Herbarium | USU
The lichen in the picture is Xanthoparmelia conspersa, a common lichen on exposed rock in New Brunswick. Lichen habitats One of the fascinating aspects of lichen biology is the ability of these organisms to occupy habitats that would be totally in inhospitable to other organisms.
Thus we can find them growing on the ground in deserts, on the sides of dry rock, hanging from the branches of trees and and even growing on the backs of turtles. They are nearly as easy to find and study in the middle of winter as during the warmer months. The first of the three photographs above was taken in Saskatchewan, out in an open prairie.
The rock in the forground is the highest point in the immediate area; animals sitting there get a panoramic view of the grassland and all that is taking place there. It is a favourite place for birds, especially birds of prey waiting for a mouse or vole that might be moving through the grass. The orange lichen is a species of Xanthoria that thrives on nitrogen-rich bird droppings left on the rock.
Similar species of Xanthoria, as well as members of the related genus Caloplaca, can be found on our seacoast on rocks frequented by gulls and cormorants. The second of the two pictures above is of White Horse Island, a small island in the Bay of Fundy supporting large colonies of nesting birds.
The white colour of the rock is due to a thick layer of bird droppings; the orange material is a species of Caloplaca. The gravestone at left marks the resting place of Roland ThaxterProfessor at Harvard University and brilliant mycologist, known in particular for his monumental studies on the Laboulbeniales.
Beside Roland's grave is that of his brother Karl. Both gravestones have become colonized by lichens and are now difficult to read. Click on the photograph to get an enlarged version of Roland's gravestone Another interesting thing about our coastal lichens is that some of them are highly tolerant of salt, a substance that is toxic to most fungi, including lichenized ones.
The picture at right depicts some coastal rocks on the Bay of Fundy near Saint John. At the bottom of the picture are bunches of brown algae, mostly Fucus vesiculosus and Ascophyllum nodosum, commonly called rockweed.
These rockweeds grow in areas along the shore where they will be immersed in seawater, at least at high tide. At the very top of the rock is a patch of orange, probably Xanthoria parietina. In between is a black zone consisting of the custose lichen Hydropunctaria maura. Hydropunctaria maura can grow where it is periodically immersed in seawater but is also able to grow in an area just above that where it receives only splash from waves. Nitrification cannot occur in the presence of oxygen, so nitrogen is fixed in specialized cells called heterocysts.
These cells have an especially thickened wall that contains an anaerobic environment. You can see these larger cells among the filaments of Nostoc, shown at right. Many plants, especially legumes, have formed symbiotic relations with nitrifying bacteria, providing specialized tissues in their roots or stems to house the bacteria, in return for organic nitrogen. This has been used to great advantage in the cultivation of rice, where the floating fern Azolla is actively distributed among the rice paddies.
The fern houses colonies of the cyanobacterium Anabaena in its leaves, where it fixes nitrogen. The ferns then provide an inexpensive natural fertilizer and nitrogen source for the rice plants when they die at the end of the season.
Cyanobacteria also form symbiotic relationships with many fungiforming complex symbiotic "organisms" known as lichens. The cyanobacterium Spirulina, shown at right, has long been valued as a food source; it is high in protein, and can be cultivated in ponds quite easily. In tropical countries, it may be a very important part of the diet, and was eaten regularly by the Aztecs; it is also served in several Oriental dishes.
What is a Lichen?
In the US, the popularity of Spirulina is primarily as a "health food", being sold in stores as a dried powder or in tablet form. Many other species of cyanobacteria produce populations that are toxic to humans and animals. Blue-green pond scums have been linked to the poisoning of cattle and dogs, and occasionally people. It is therefore not recommended that wild populations be gathered and eaten without some knowledge of the organisms involved.
Cyanobacteria may cause other problems as well; a species of Lyngbya is responsible for one of the skin irritations commonly known as "swimmer's itch.