Introduction
A Mutualistic or symbiotic relationship whereby two organisms of related species co-exist and benefit from each other. In this case, the lichen is as a result of a symbiotic relationship between the algae and fungi. The symbiotic relationship resulting from fungi and algae is that algae prepare food for the fungi because they are green and they contain chlorophyll, and in return, the fungus provides shelter for the algae, and it also absorbs nutrients and water from the soil. (“Lichen Ireland: Where Are Lichens Found?”)
Symbiotic relationships are beneficial to both partners. Examples of symbiotic relationships in nature include Ants and aphids, bacteria in cow’s stomachs, mycorhiza fungi and tree roots. However, lichens exhibit uniqueness because each partner loses its identity and different or even new dual organisms are formed. (“Lichen Ireland: Where Are Lichens Found?”)
Lichens are slow growing, long-living organisms, that are symbiotic and comprise of the fungus and photosynthesizing partner. The fungus is referred to as the mycobiont, and the photosynthesizing partner is known as the photobiont. The photobiont is green, unlike the mycobiont. Other photosynthesizing partners could be cynobacterium. Lichens are considered of being fungi. (“Yeast Emerges As Hidden Third Partner In Lichen Symbiosis”)
Lichens growing on rocks help in new soil generation by producing chemicals which facilitate the degradation of the rock. Most lichens are either arctic or temperate however; there are many tropical and desert species. Lichens appear to survive better in drier environments where they are not left on stagnant water oftenley. (“Lichen Ireland: Where Are Lichens Found?”)
In areas of cool forests, large lichens resembling a beard of an old man can be seen hanging from branches of trees .There could be no water in these habitats but however, the air is saturated and breezes that are dry may facilitate the desiccation of arboreal organisms. Lichens can be mostly noticed on the tundra aeas.Tundra areas are those that the ground is covered by mosses, lichens, and also liverworts majorly. This type of ground cover, facilitates insulation on the ground, and could also provide good foraging feed for animals like for instance the Reindeer “moss”. (“Lichen Ireland: Where Are Lichens Found?”)
Lichens have got the ability to survive dry conditions, and also due to their complex structure, they are able to survive scorching deserts. During times when they lack moisture, Lichens may dry up completely but, complete loss of moisture makes them to become brittle. (“Lichen Ireland: Where Are Lichens Found?”)
During moist conditions, they can quickly absorb water and become fleshy and soft again. When they are dry, and brittle, lichen pieces break off and later they grow to form new lichens. (“Lichen Ireland: Where Are Lichens Found?”)
Lichens differ in their growth partners. Examples of how they grow include foliose, fructicose, and Crutose. Crutose lichens resemble a crust like structure and are usually attached tightly to their substrate. Crust-Like lichens have an upper surface but no lower surface. Foliose lichens are usually flat and resemble a leaf. Foliose lichens can either be loosely attached or tightly connected. They also have an upper and lower surface. Fructose lichens are usually shrubby and upright, and often sometimes, they hang downwards. As for fructose, they do not have an upper and lower surface; however, they have an outer surface. Foliose and fructicose are referred to as macronutrients as they can be seen with the naked eye. Crutose lichens are times known as micro lichens. (“The Lichens”)
Classification of Lichens
Lichens are classified based on habitat, group of fungal partner, thallus structure, distribution of algal component in the thallus.
Based on habitats
The following are the classifications based on where they are found;
Saxicolous – These are lichens that are found on rocks a good example is the Peltigera.
Corticolous – These are those that live on the back of trees an example is Parmelia.
Terricolous – These are those that live in soil, for instance, the Cladona.
Lignicious – Theses are lichens that live on wood an example is the Cyphelium. (“Classification of Lichen – Types of Lichen”)
Based on the group of fungal partners
The following are lichens grouped according to their fungal partners;
Ascolichen-The fungal partner is the ascomycete.
• Gymnocarpae –this is a fruiting body apothecium, and an example is the Usnea.
• Pyenocarpae –This is also a fruiting body perithecium, and an example is the verrucaria.
Basidiolichen-Fungal partner is a basidiomycete, and an example is the Dictyonema.
Based on the thallus structure
Leprose lichen –this is a case of the Lepraria.The. The Leprose lichen has tiny scales attached superficially to the substratum.
Crutose-These are crust like lichens thallus that is attached closely to the areas of the substratum, for example, graphis. (“Classification of Lichen – Types of Lichen”)
`Foliose-Theses are lichen thallus that is leaf-like and are lobbed to the substratum at either one-point o at many areas. (“Classification of Lichen – Types of Lichen”)
Fructose – They are bushy in appearance, pendulous and are also branched. They have a basal disc which attaches them to the substratum. The Fructose has a radical arrangement of its tissues, and also a central axial strand which is significantly used to distinguish it from others in its medulla. The central axis is usually dense and is comprised of paraplectechymatous tissue that gives the thallus the skeletal strength. However, the central axis does not feature in gelatinous like Cladona, Ramalina, and Alctoria.In these three, the axial strand is replaced by the spongy tissues or even hollow or gelatinous tissues. Thalli in fructose are arranged in a netlike or periline way hence, forming a palisade tissue, and the thalli are held upright by the tube-shaped cortex. (“Classification of Lichen – Types of Lichen”)
Based on distribution of algal component in the thallus
Homoisomerous thalli-Algal cells and fungal hyphae are usually distributed uniformly; an example is the Coliema. The Homoisomerous lichens are characterized by extremely short and hair like thalli which consist of filamentous alga that has got strands which are wrapped tightly together in the hyphae of the fungi. Species of collema consist mainly of the blue-green alga. Hyphae as the mycobiont usually grow inside the gelatinous matrix of the phycobiont. Only a few lichen genera have this type of thallus. (“Classification of Lichen – Types of Lichen”)
Heteromerous Thalli-They is thalli with two distinct layers formed by algal and the other layer by hyphae and an example is Parmelia. The lichens are however usually complex in their structure and the algae are restricted to a particular layer in the thallus. The two layers comprising this thallus are medulla and algal layers. The medulla layer does not contain algae in it. Other layers that could be developed in the process are the cortex. The Heteromerous has a stratified arrangement. (“Lichen Ireland: Where Are Lichens Found?”)
The algal layer usually has a mitotic division of cells and is caused by alplanospores. For example, in the Treboixia, the algal cell often divides to form many protoplasts which each of them secrete a cell wall. Rapture of the wall of the mother cell causes the alplanospores to be freed. It is possible to observe the stages of these processes in the thallus. However, there has not been a sexual reproduction by zoospores within the lichen thallus, though the motile stages produced in pure cultures of the phycobiont are generated by the algae. (“Lichen Ireland: Where Are Lichens Found?”)
The thickness of the algal layer could vary in different species of lichens, but the algal zone in the thallus does not vary. In the medulla, there consist of hyphae that are loosely interwoven periclinally. These hyphae have a cottony or fibrous appearance and are weakly gelatinized. The water holding capacity of the medulla layer is high compared to other tissues, and also the medulla is a food storage area. The loose interweaving of the hyphae, within the thallus, facilitates the exchange of gasses and also because they are not quickly moistened. Most of the substances of the lichen are deposited in the medulla. Despite the deposits of the Lichen, the thallus may also be occupied by large calcium oxalate crystal clusters, and example of where this occurs is the Cladona, porina, and Usnea. (“Classification of Lichen – Types of Lichen”)
Contact between the mycobiont and phycobiont in the algal layer is established. The contact could be direct or indirect; however, the relations between algae and hyphae would vary considerably. (Glimmer)
Aspersoria-In this kind of lichen, the mycobiont cells are pressed against the algal walls. In other genera, unlike the Aspersoria, the haustoria penetrating the algal cell membrane could sometimes kill the algae. They however in some lichens, they divide themselves simultaneously. After division, the two daughter cells are formed, and two haustoria branches hold them. (“Classification of Lichen – Types of Lichen”)
Lichens are usually classified with the fungi and can sometimes be referred to as lichenized fungi. Fungi found widely in lichens are ascomycete; however, few basidiomycete are involved. Many lichens are easy to identify out in the field, but he identifications of many lichens require microscopic observations as heir structures are small for example their reproductive structures, cellular features, and spores. Also, the chemical test can at times be used for identification. Chemical tests are carried out by applying reagents to the lichen tissues and presence, or absence of color change is seen. (“The Lichens”)
Lichens majorly grow in left over spots of natural world. Lichens are mostly found on bare rocks, Laval flows, cleared soils, dead wood and newly emerged volcanic islands in the sea. Lichens have the ability to shut down metabolically, under favorable conditions; hence this enables them to survive extreme conditions of heat, drought and even cold conditions. Lichens, having 15000 species, have had successful relationship with fungus and even algae at times or even both. The fungal partner usually constitutes to 90 to 95% of the lichens biomass and it encloses the cells of photosynthetic symbiont within a network of filaments. “Lichen Facts, Information, Pictures | Encyclopedia.Com Articles About Lichen”)
The non-photosynthetic fungus in return provides a sturdy structure while the algae and cynobacterium contribute to the product needed for photosynthesis, cynobacterium also, usually fix atmospheric nitrogen. Species of lichens that contain both algae and cynobacterial symbiont, are over 500.Lichens mostly produce vegetatively .Also they reproduce using spores produced in fruiting bodies that are made by the fungus which when they geminate (spores) , they have to capture new algae or cynobacterial cells so that they can form new lichens.
Evolution of lichens
The earliest evidence of lichen was dated 400-million years old and was discovered in Scotland. Researchers found that, plants living on land had evolved on earth by about 700 million years ago, and fungi living on land about 1300 million years ago based on the fossil found belonging to these organisms. Before the research, it is said that the earth landscape then was covered by barren rocks, on which nothing much but bacteria and some fungi inhabited. (Dimijian)
The discovery however raised new questions about the lichen evolution because they were only able to find few fossil records. The earliest Lichen fossils described were discovered by Professor Taylor and they originated from non-marine deposits, during the times when plants were majorly the colonizers of the land. (“Evidence of 600-Million-Year Old Fungi-Algae Symbiosis Discovered In Marine Fossils”)
Lichens are commonly formed through a symbiotic relationship with an association between the algae and fungi, whereby, the survival of the fungi depends on its association with the green algae (cynobacterial which is the (photobiont) and feeding on the sugars and nutrients the algae produces by photosynthesis. (Science and Earth)
Lichens have been seen to evolve many times and they arise from parasitic, Mutualistic and free-living fungi. The Mutualistic relationship of some lichens is encouraged by the fact that neither of the partners enjoys the same type of survival success in extreme conditions. However, a free-living fungus may overtake a colony of the cynobacterium.Nostoc also incorporate themselves into the lichens and in this case; the fungal partner appears to be parasite to the bacterium. (Mowat)
Through this some lichens appear to be a case of “controlled parasitism” whereby their hostages provide a resistance measure. Hence, lichens are considered to be the end points o the symbiotic relations Plants that develop on land are likely to have evolved from marine life particularly the green algae. Therefore, plants and green algae remain very closely related at a genetic level. With lichens containing both the algae and fungi, most scientists believe that lichens played a key role in the transfer of genetic information of the marine organisms to the inheritors of plant life on land. Researchers have however been able to find evidence of lichen-like symbiosis in 600-million year fossils from south China. Fossil records show that lichens had been having this type of relationship between algae and fungi for more than 400-million years, when it is thought that plants evolved from green –algae. However, scientist do not yet have complete certainty through what organisms predicated and evolved during the “bio-invasion” of life on land; though the assumption is that bacteria, algae and fungi were previously present and thus lead to the development of lichens, ferns, mosses. Therefore while the fossil record shows that lichens are old, it is not easy to come up with a conclusion that they were present during the era when algae’s genetic code was passed on to the life of plants on land. This is because, the fossil record from this time is limited and this could have been caused by the adverse environmental conditions or rather also because , any existing organisms that were primitive then, would have found it hard to leave the rock impressions.(“Evidence of 600-Million-Year Old Fungi-Algae Symbiosis Discovered In Marine Fossils”)
Those that were able to survive those adverse environmental conditions were characterized by versatility and hence, they would be able to cope with the fluctuating saline conditions, heat or desiccation. Lichens have been found to be able to exhibit the versatility trait in areas characterized by high temperatures. Radiations, repetitive tidal changes and also wave action areas. Some lichens have been discovered to be adapted at a switch between symbiont over time, and this attributes, would majorly help them in the survival of a changing environment. As for the Algae, some are capable of being transported by air onto the land. Research has shown that once algae have been dispersed, they are able to colonize new areas in the crust of the soil. Early lichens were not only responsible or the colonization of new areas on land but also they were capable of contributing highly to the formation of soil by rock erosion. . (“Evidence of 600-Million-Year Old Fungi-Algae Symbiosis Discovered In Marine Fossils”)
Besides the fact that scientists have known that lichens are made up of two species that are special for some time now, and that these two species are important for the biological transfer of life to land from the sea, research also shows that lichens also posses the qualities that are necessary to enable them to be able to survive through adverse environmental conditions which were thought to have been present during those times. Today examples of such relationships exist a lot in the sea hence giving an example from ancient ocean life. At a place where a lot of algae are known to have lived in an area that is shallow and that is also characterized by sub-tidal environment, about 600 million years ago, two scientists, were able to find 3specimens that today, have an evidence of two partners in the same type of relationship. (“Evidence of 600-Million-Year Old Fungi-Algae Symbiosis Discovered In Marine Fossils”)
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