Symbiosis in lichens - Wikipedia
Relationship between fungus and alga in the lichen Cladonia cristatella Tuck. Vernon Ahmadjian; & Jerome B. Jacobs. Nature volume Lichens are a mutualism formed between fungi and various groups of algae. The photobiont and its relationship with the phytobiont are the same as in M. A lichen is a composite organism consisting of a fungus and an alga functioning in a symbiotic relationship.
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.
Mutualisms between fungi and algae
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.
Comparatively few Basidiomycetes are lichenized, but these include agaricssuch as species of Lichenomphaliaclavarioid fungisuch as species of Multiclavulaand corticioid fungisuch as species of Dictyonema. Other lichen fungi occur in only five orders in which all members are engaged in this habit Orders GraphidalesGyalectalesPeltigeralesPertusarialesand Teloschistales.
Lichenized and nonlichenized fungi can even be found in the same genus or species. TrebouxiophyceaePhaeophyceaeChlorophyceae have been found to associate with the lichen-forming fungi.
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.
What is a Lichen?
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.
What is a Lichen? | The British Lichen Society
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.
- Symbiosis in lichens
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. 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. Some periwinkles, notably the rough periwinkle, eat V.
This has not happened here yet but there are in fact several periwinkles present, as well as the white barnacles and a mussel. How many periwinkles are here? Not many at first glance, but you might be surprised. Click on the picture to get an enlarged view and see how many periwinkles you can count. One of the more intriguing mutualisms found in our region is the one between the brown alga Ascophyllum nodosum and the fungus Mycophycias ascophylli.
MUTUALISMS BETWEEN FUNGI AND ALGAE
Ascophyllum nodosum, commonly called rockweed, occurs in the intertidal zone where it is left exposed to the air when the tide goes out. Mycophycias ascophylli, a member of the lichen-forming order of fungi Verrucarialesgrows within the body thallus of A. In return the fungus has access to carbohydrates and other nutrients within its protective environment.