Q.3. What are lichens? Detail the biotic relationship between the components of lichen thallus.(BSc 1st Year Botany Lichens)
Ans.3. A lichen is a composite organism, consisting of a specific fungus, usually an ascomycete, living in association with one-or sometimes two-species of algae or cyanobacteria. The fungus component is called the mycobiont (Gr. mykes = fungus + bios = life) and the algal component is called the phycobiont (Gr. phykos = sea weed i.e., Alga + bios = life). The two components are intertwined so as to form a single thallus. Hawksworth et al. (1983) have defined a lichen as “a stable self-supporting association of a mycobiont and a photobiont”.
The association between the two types of components results in a new distict morphological entity which is a self-reproducing functional unit behaving as a single organism having no obvious similarities with either of its components. About 13,500 species of lichens are known.
The term ‘lichen’ was first used by Theophrastus (371-284 B.C.) to denote a superficial growth on the bark of olive trees.
Components of Lichens
A lichen consists of two components, an alga (photobiont) and a fungus (mycobiont). The nycaliant is usually a member of Ascomycotina, less commonly of Basidiomycotina and only rerely of Deuteromycotina. The photobionat is usually a member of Myzophyce (blue-green algae), and less commonly of Chlorophyceae (green-algae). In most of the earlier literature, however, instead of ‘protobiont’ the word “Phycoliont’ (Gr. phykos, alga; bios, life) is used, which indicates its algal nature. But the recent trend is to treat blue-green algae as Cyanobacteria and not as algae. Based on this trend Hawksworth and Hill (1984) divided the photo bionts into ‘phycobionts (Green algae) and ‘Cyanobionts’ (Cyanobastes).
A majority of the ascomycetous lichens belong to either Discomycetes or Pyrenomycetes. Some also belong to Loculoascomycetes. But none belong to Mastigomycotina, Zygomgioting, Hemiascomycetes, Plectomycetes and Laboulbeniomycetes.
Nature of association between the mycobiont and the phycobiont
There has been a lot of difference of opinion regarding the nature of association between the mycobionts and the phycobionts in the lichen thalli.
1. Helotism
Some persons believe that the lichens represent a condition of slavery enforced on the phycobiont by the mycobiont. The fungus act as master and has an upper hand over the algae, which has to function as slave. This type of association has been termed helotism.
2. Parasitism
According to some botanists, the fungus is simply parasitic on the enclosed algal cells, which are penetrated by the haustoria of the fungi, and are ultimately injured or killed.
3. Consortium or mutualistic symbiosis
However, nowadays, most of the botanists believe that the lichen thallus represents a mutualistic symbiosis, from which both organisms benefit. The two components behave like two friends or husband and wife benefitting one another. This type of association is called consortium. Following lines of evidences support this opinion:
(i) When the two components of a lichen thallus are grown separately in pure cultures, and are then placed together again, the two components combine to form lichens only on that medium which will not support either component separately. On media, which favour the growth of either or both components separately, on lichenisation takes place.
(ii) Ultrastructural studies have revealed that there are three kinds of physical relationship between the mycobiont and the phycobiont. Usually, there is simple wall to wall contact with very close attachment between the hypha and the algal cell, and there may also be fusion of the two walls, those of both fungus and alga becoming thinner over the area of contact. In the second kind of interaction, the hypha grows into the algal cell wall, while it does not rupture, it becomes deeply invaginated. Finally, in the third type of interaction, hyphae do enter algal cells by penetrating the cell wall to cause in vagination of the algal plasmalemma.
However, the penetration of algal cells by the fungus does not seem to be necessary in lichens. Many lichens lack penetrant hyphae, and in those lichens which possess them, their frequency is very low.
The penetrant hypha, il present, does not penetrate algal cell wall and further, it penetrant hyphae may not be called haustorial matrix. So, these penetrant hyphae may not be called haustoria. Further, it is now known that these penetrant hypha are usually only mature and post-mature cells indicating that these are more necrotrophic than absorptive organs. Thus, parasitism of the algal cells is ruled out.
(iii) In the fruiticose lichen, Cornicularia normoerica, the algal cells possess channels which originate from the chloroplast and then extend through the cell wall, opening at its periphery. These channels contain small tubules. It is believed that these tubules facilitate exchange of metabolites between the algae and the fungi.(BSc Botany Lichens Sample Model Practice Question Answers)
(iv) In Lichinia pygmaea, the outer layers of the algal cell walls and the plasmalemma are crenulate. Vesicles bounded by a single membrane are present in the fine ground substance between the algal cell and the fungal hypha. The vesicles serve as a mechanism of metabolite exchange. Crenulations increase the surface area of the protoplasts to increase the efficiency of exchange.
(v) Studies using labelled carbon dioxide (C14O2) have revealed that the fungal partner directly interferes with the metabolism of photobiont the growth of which is severely restricted. Even then the rate of photosynthesis remains unaffected. As a result, I conditions for the production and release of surplus carbohydrates are created. The carbohydrates move to the fungus. However, the glucose is not released directly, but is used to form plasmalemma, and that this glucan is then hydrolysed by the fungal glucan outside the algal enzyme, glucanase. The free glucose, so formed, is then taken up by the fungus and utilised. Thus there is no doubt that the fungus derives benefit from the photobiont in the form of carbohydrates.
(vi) In the lichens having blue-green algae, nitrogen fixation also takes place. Studies using labelled nitrogen (15N2) have shown that if the thalli bearing cephalodia (containing blue-green algae) are exposed to 15N2, then the isotopic nitrogen content of the cephalodia rises first but afterwards, remains at constant level. The isotopic nitrogen content of thallus also rises, but continues to rise, at a steady rate, indicating that nitrogen released from cephalodia passes into the thallus. Thus, there is no doubt that the lichen thallus (the mycobiont and non nitrogen-fixing algae) are benefitted by N2 fixation by azotodesmic (N2-fixing) algae.(BSc Botany Lichens Sample Model Practice Question Answers)
(vii) The photobionts on the other hand, are physically supported and effectively displayed to the light by the fungi. Algae are very susceptible to desiccation. Mannitol, stored by fungi, lowers the osmotic potential so that water accumulation could occur in environments of low water availability. Because of the intimate association of the fungus and the autotroph, some of the water becomes available to the autotroph; allowing it to continue its activities. The fungal medulla provides water to the rest of the thallus, when the latter is drying out.
(viii) Presence of dark substances like anthraquinone, parien protect algae from strong sunlight by changing the quality of light reaching the
(ix) Many substances like lecanoric acid, physodic acid are formed by The mycobionts or the phycobionts cannot produce these individually. It is believed that these are symbiotically produced and protect both the fungi and algae from insects, sunlight and make them unpalatable to many herbivores. These features help lichens in extending their distribution to the areas with highly unfavourable environmental conditions, where none of the components could survive individually.
BSc 1st Year Botany Lichens Sample Model Practice Question Answer Papers
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