Symbiosis: Mycorrhizae and Lichens
They are a combination of a fungus -- non-green, plant-like organisms about the exact benefits passed between fungus and algae in lichens. A lichen is not a single organism; it is a stable symbiotic association between a fungus and algae and/or cyanobacteria. . parts of the tree of life (green plants vs bacteria), the term photobiont is used as collective term for any of them. Symbiosis in lichens is the mutually helpful symbiotic relationship of green algae and/or blue-green algae (cyanobacteria) living among filaments of a fungus, forming lichen. Living as a symbiont in a lichen appears to be a successful way for a fungus nonlichenized fungi that live as saprotrophs or plant parasites (for example.
The plant-like appearance of lichens hides their true identity. 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 formed of three or more partners. The basis of the mutualistic symbiosis in lichens is similar to the mycorrhizal partnership between some species of fungi and the roots of most plants.
Symbiosis in lichens - Wikipedia
The lichen fungus provides its partner s a benefit protection and gains nutrients in return. The complexity of lichen partnerships has caused lichens to be described as "small ecosystems".
They are classified as members of the Fungus Kingdom by systematists because the fungus partner is always the major partner.
The algal and bacterial partner s each have their own scientific names, but the lichen symbiosis is known only by the name of its fungus. Cross Section of Lichen The great majority of the 13, species of lichenized fungi are Ascomycetes, the "cup fungi".
About 20 species in the tropical and temperate rain forests are Basidiomycetes, the "mushrooms". About 40 genera of algae and cyanobacteria are found in lichen partnerships. How do Lichens Grow?
They also provide vitamins to the fungus. Cyanobacteria can make amino acids directly from the nitrogen gas in the atmosphere, something neither fungi nor algae can do.
The fungus, in turn, protects its partners from drying out and shades them from strong sunlight by enclosing the photosynthesizing partners within the body of the lichen.
This life habit has allowed lichens to successfully colonize many different habitats. Lichen photobionts are the green algae or cyanobacteria that provide the simple sugars to their fungal partners.
About species of photobionts are known, and the commonest ones are from four main groups. Lichen fungi specialise on particular photobionts. Typically they only associate with a small group of related species, though they may associate flexibly with different photobionts according to their environmental situation.
The Lichen Symbiosis Lichens are made up of two or more closely interacting organisms, a fungus, and one or more partners, called photobionts. In contrast, fungi are 'heterotrophic' and require an external source of food. The fungi build the structure of the lichen thallus, within which they provide conditions for a long term, stable association with their photobionts, the basis of the lichen symbiosis.
There is some debate about the exact nature of the symbiotic association between the lichen-fungi and their photobionts. There is good evidence for the lichen symbiosis as a mutualism, in which both partners benefit from the relationship. It is clear that fungi obtain their carbon-source in the form of simple sugars, but the photobionts seem also to be provided with optimal living conditions, in which their populations are often much larger than outside lichens.
The photobiont probably also benefits from improved access to mineral nutrients which are provided because of fungal digestion outside their cells.
Last, but not least, the interior of lichens is often a place richly infused with complex secondary fungal chemicals found nowhere else in nature, and these compounds are likely to play a role in protection from UV radiation, desiccation, and grazing by herbivores as well.
However, there are also good arguments in favour of the controlled parasitism camp. Up to half of the carbon fixed by algae is immediately converted to fungal sugars which are inaccessible to the alga itself. In fact, it is thought that many early stages of developing lichen spores may survive using such a parasitic or saprophytic strategy. Lastly, there are many lineages of lichen fungi that are parasitic on other lichens — the so-called lichenicolous lichens!
There was a problem providing the content you requested
In some cases, non-lichen fungi have evolved from lichenised forms. These can be specialised opportunistic parasites or saprophytes or even symbionts, competing for nutrients with other fungi in the lichen thallus. The symbiosis may be more complex than this. Recent work by Spribille et al has found yeasts embedded in the cortex of ascomycete macrolichens, and their abundance correlates with previously unexplained variations in phenotype.
There is also convincing evidence for a consistent presence of non-photosynthetic bacteria within the thalli of all lichens, although the role of these bacteria is as yet unknown. Interestingly, a role for non-photosynthetic bacteria was suspected for many years, as the relichenization of separately cultured fungi and algae in the lab was facilitated by the presence of bacteria.
In fact, a legacy of exclusion from accepted mycological research persisted until the s, despite their obvious affinities with non-lichen fungi.
- MUTUALISMS BETWEEN FUNGI AND ALGAE
- Symbiosis in lichens
- What is a Lichen?
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.
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.
Fungi Symbiosis ( Read ) | Biology | CK Foundation
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.
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.