Fungi Symbiosis ( Read ) | Biology | CK Foundation
The lichen symbiosis is thought to be a mutualism, since both the fungi and the fungal tree of life, uniting them and their non-lichen relatives in the Kingdom Fungi. Fungi are heterotrophic, meaning that, like animals, they require a carbon. Mutualism is a positive reciprocal relationship between two species. The relationship is obligate, meaning at least one of the species must be involved in the An example of this is Spanish moss growing on the branches of an oak tree . There are many species of lichen within the Caledonian Forest. Among Mycorrhizas are symbiotic relationships between certain fungi and the roots of plants.
Three subcategories are recognized, arbutoid, ericoid and monotropoid. We will briefly cover the latter two groups. Ericoid Mycorrhizae This group is probably the most important, with respect, to its potential applications.
Ericoid mycorrhizae have evolved in association with plants that are continually stressed by factors within the soil. The soil is typically extremely acid, low in available minerals because mineralization is inhibited.
Plants with ericoid mycorrhizae seem to have a high tolerance to these stresses and there is good reason to believe that this is related to the presence of the mycorrhizal fungus and that the survival of the host is dependent upon the fungus. Monotropoid Mycorrhizae One of the characteristics that we always attribute to plants is that they have chlorophyll and can produce their own food through the process of photosynthesis.
However, this is not true of all plants. The Monotropaceae and Pyrolaceae are two families of plants which are achlorophyllous. Thus, plants in these families are more dependent upon their mycorrhizal partners than plants which can carry out photosynthesis. The means by which food is obtained by these plants is similar to that of the epiphytic orchids described above. However, morphologically, they are very different.
The achlorophyllous host has mycorrhizae roots that appear to be formed by an ectomycorrhizal fungus, but the epidermal and outer cortical cells are penetrated by the fungus.
The fungus also forms an ectomycorrhizal relationship with a tree which is capable of photosynthesis. So, as in the case of the epiphytic orchids, the photosynthetic tree indirectly provides carbohydrates to these achlorophyllous plants, as well as to the fungus. Both hosts probably obtain their mineral requirements through the fungus.
Lichens The most well known example of a symbiosis between fungi and plants is the lichen, if you will allow me to include algae as plants. The concept of what constitutes a lichen has broaden significantly in the last 25 years to include some species of mushrooms, slime molds, and some members of the Zygomycota.
However, we will discuss lichens in the traditional sense, as an association between a fungus and an alga that develops into a unique morphological form that is distinct from either partner. The fungus component of the lichen is referred to as the mycobiont and the alga is the phycobiont. Because the morphology of lichen species was so distinct, they were once thought to be genetically autonomous until the Swiss Botanist Simon Schwendener described their dual nature in Prior to that time, because of the morphology of many of the "leafy" species of lichens, they were considered to be related to bryophytes, i.
Although, lichens are now known to be composite organisms, they are still named for the fungus part of the association since that is the prominent part of the lichen thallus. A thallus is an old botanical term used to describe "plants" that do not have leaves, stems and roots, and its origin goes back to a time when only two kingdoms were recognized in classifying organisms, i.
Prior toorganisms such as algae, bacteria and fungi, were included in the plant kingdom. InWhitaker, proposed a five kingdom system which we still presently use. Although, this term is now dated, it is still used to describe the "bodies" algae, fungi and of course lichens. The only group of organisms that are still considered to be plants, in which we still use the term thallus, to refer to the plant body, are the bryophytes.
Although the lichen thallus is composed of an algal and fungal component, lichens are not studied in mycology or phycology that part of botany that studies algae.
Instead, they are studied in their own discipline, lichenology. There are relatively few lichen researchers. Of these most are taxonomist. As a result, there are still some basic questions concerning this symbiosis that are unanswered or at least up for debate.
One of the most basic questions, that has been asked since the discovery of the lichen symbiosis, concerns whether lichens represent a true mutualistic symbiosis or nothing more than a variation of a host-parasite relationship. There is evidence supporting both sides.
What is a Lichen?
That it represented a mutualistic symbiosis, in which the alga was believed to contribute the food supply through photosynthesis, and the fungus protected the alga from dessication, harmful solar radiation and provided the alga with water and inorganic nutrients, was postulated by Beatrix Potter, the writer and illustrator of Peter Rabbit, soon after Schwendener had determined the true nature of the lichen thallus.
In order to understand both sides of the issue, lets look at the morphology and anatomy of lichens. The Lichen Thallus In the traditional sense of lichens, their thallus can be artificially divided into four forms: Foliose Lichens Lichen thallus which is generally "leaf-like", in appearance and attached to the substrate at various points by root-like structures called rhizines.
Two examples of this thallus type is Pseudocyphellaria anthraspsis and Hypogymnia imshaugii.
Symbiosis in the Forest
Because of their loose attachment, they can easily be removed. These are the lichens which can generally be mistaken for bryophytes, specifically liverworts. It is possible, or even probable, that herbaria still contain lichens that have been mistakenly identified as liverworts.
If we look at these a foliose lichen in longitudinal section, from top to bottom, we would be able to distinguished the following layers: Often composed of tightly interwoven mycelium, which gives it a cellular appearance. This cellular appearance is referred to as pseudoparenchymatous.
Composed of interwoven hyphae with the host algal cells. This is the ideal location for the algal cells. Beneath the upper cortex so that it receives the optimal amount of solar radiation, for photosynthesis, but not direct solar radiation which would be harmful.
Composed of loosely interwoven mycelium. Layer is entirely fungal. Usually same composition as the upper cortex and attached to the substrate by root-like structures called rhizines.
The rhizines are entirely fungal, in origin, and serve to anchor it to the substrate. Thus, the foliose lichens also have what is referred to as a dorsiventral thallus, i. Crustose Lichens Lichen thallus which is very thin and flattened against the substrate.
The entire lower surface is attached to the substrate. These lichens are so thin that they often appear to be part of the substrate on which they are growing. The following link shows an image of several lichen thalli.
The most prominent is Buellia aeruginascenswhich has a light, olive-green thallus with small, dark, black fruiting bodies. Species that are brightly colored often give the substrate a "spray-painted" appearance. The thallus has the upper cortex, algal and medullary layers in common with the foliose lichens, but does not have a lower cortex.
- Symbiosis in lichens
The medullary layer attached directly to the substrate and the margins are attached by the upper cortex. Fruticose Lichens The thallus is often composed of pendulous "hair-like or less commonly upright branches finger-like. The thallus is attached at a single point by a holdfast. In cross section, the thallus can usually be seen to be radially symmetrical, i.
The following link shows an image of a typical fructicose lichen, Usnea arizonica. The layers that can be recognized are the cortex, algal layer, medullary layer, and in some species the center has a "cord" which is composed of tightly interwoven mycelium.
Other species have a hollow center that lack this central cord. Biology of Lichens In looking at the anatomy of the lichen, it is obvious that there is interaction between the phycobiont and mycobiont, but what kind of interaction is occurring.
One school of thought is that the alga produces the food material and that the fungus protects alga from desiccation, high light intensities, mechanical injuries and provides it with water and minerals. This is the reasoning that many introductory text books have adopted and they define a lichen as a mutualistic symbiosis. However, in studies that have been done that examines the alga-fungus interface, it can be clearly seen that haustoria, specialized feeding structures present in parasitic fungi, penetrate the alga cells.
Thus, many lichenologist have defined this relationship as a controlled form of parasitism. There is more evidence and I would like to go over some of these. If we think about fungi and algae in general, we know that they are normally going to be found in a moist to wet environment where they are not receiving direct solar radiation.
Conditions outside these parameters will usually be fatal for most species of fungi and algae. However, lichens occur all over the world. They even occur in arctic and hot, dry desert areas where few organisms can live or even survive. 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.
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.
Symbiosis in lichens - Wikipedia
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. For example, the fruticose habit has evolved repeatedly within the Lecanora-group, but also within the distantly related Arthonia-group.
The algae or cyanobacteria benefit their fungal partner by producing organic carbon compounds through photosynthesis. In return, the fungal partner benefits the algae or cyanobacteria by protecting them from the environment by its filaments, which also gather moisture and nutrients from the environment, and usually provide an anchor to it.
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.