Symbiosis in lichens - Wikipedia
The symbiotic relationship between algae and fungi is lichen. The fungal component is called mycobiont while the algae component is called phycobiont. The lichen symbiosis is thought to be a mutualism, since both the fungi and the . However, in the case of lichens with both green algae and cyanobacteria, the. About 90% of all known lichens have a green alga as a symbiont. has single cells that are globose, which is common in lichens.
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.
What Are Lichens?
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.
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. Appearance When dry, lichens simply take on the color of the mycobiont the fungus itself or can be drab and gray.
Fungi Symbiosis ( Read ) | Biology | CK Foundation
But when wet, they are completely transformed. This is because the fungal cells in the upper cortex become transparent and the colors of the algal or cyanobacterial layers can shine through. Green algae bestow lichens with a bright green color, while cyanobacteria give hues of dark green, brown, or black, according to the Forest Service.
Photosymbiodeme with green [algal] lobes growing from cyanobacterial ones. It actively seeks out the photobiont by chemical recognition. Acceptance occurs when the two lichen partners interact without negatively influencing one another. He notes that fitness and how the lichen partners work together are dependent on environmental conditions.
Usually, once a lichen association has been established the mycobiont does not switch partners. In this case the fungus associates with a cyanobacterium in shady, humid conditions to form small, shrub-like thalli.
However in drier or more exposed conditions, the fungus associates instead with green algae to form large, flat lobes. Mosses are also not lichensaccording to the Forest Service. Though at first glance some may superficially resemble a lichen, mosses are actually primitive versions of plants and are capable of independent photosynthesis. 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.
MUTUALISMS BETWEEN FUNGI AND ALGAE
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. 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. This "black zone" occupies an area that often goes for days or even weeks without immersion in seawater but will eventually get splashed.
This is a tough place to live: Just the place for a lichen! The picture at right depicts yet another species of Verrucaria mucosa, a close relative of H.
What Are Lichens?
In fact, it releases its ascospores when it is above the water and thus depends upon being exposed to air. However, it does not grow in the upper areas of the tide like H. In the picture V. On parts of the rock that have dried it is harder to see but you may notice that it is slightly green, revealing the presence of the photobiont. The red spots are the alga Hildenbrandia polytypa, similar is size and growth habit to V.
The last picture again shows Verrucaria mucosa, this time growing under water at high tide. Note that even this lichen has its limits; most of the rocks in the picture have no lichens at all.
This may be because the rocks are too small and may be moved by currents as the tide ebbs and flows or it may be that their surfaces are unsuitable for lichens. Another problem that lichens face is being eaten by animals. Many contain acids and other compounds that make them unpalatable to animals but V.
Notice the large rock above the one with lichens on it. On its surface is a small snail called a periwinkle.