Why is lichen symbiotic

This cross-section of a lichen thallus shows its various components. The upper cortex of fungal hyphae provides protection. Photosynthesis occurs in the algal zone. The medulla consists of fungal hyphae. The lower cortex also provides protection. The rhizines anchor the thallus to the substrate. In some ways, the symbiotic relationship between lichens and algae seems like a mutualism a relationship in which both organisms benefit.

The fungus can obtain photosynthates from the algae or cyanobacterium and the algae or cyanobacterium can grow in a drier environment than it could otherwise tolerate. However, most scientists consider this symbiotic relationship to be a controlled parasitism a relationship in which one organism benefits and the other is harmed because the photosynthetic organism grows less well than it would without the fungus.

It is important to note that such symbiotic interactions fall along a continuum between conflict and cooperation. Lichens are slow growing and can live for centuries.

They have been used in foods and to extract chemicals as dyes or antimicrobial substances. Some are very sensitive to pollution and have been used as environmental indicators. Lichens have a body called a thallus, an outer, tightly packed fungal layer called a cortex , and an inner, loosely packed fungal layer called a medulla Figure 1.

Lichens use hyphal bundles called rhizines to attach to the substrate. Lichens are classified as fungi and the fungal partners belong to the Ascomycota and Basidiomycota. Lichens can also be grouped into types based on their morphology. There are three major types of lichens, although other types exist as well. Lichens that are tightly attached to the substrate, giving them a crusty appearance, are called crustose lichens. The bulk of lichen diversity belongs to the class including the well-known genera Lecanora , Cladonia , Parmelia and Peltigera Lecanoromycetes, or the Lecanora -group , where 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. Unrelated fungi repeatedly evolve similar morphologies to succeed under similar conditions, making morphological identification especially difficult in some groups.

Fungi are heterotrophic, meaning that, like animals, they require a carbon source to survive. The lichen fungi share a common ecological strategy of hosting an internal population of photosynthetic cells, from which they obtain their carbon source in the form of simple sugars. These photosynthetic cells can either be green algae Chlorophyta or cyanobacteria or sometimes both, in which case the cyanobacteria are localised in distinct areas of the thallus.

As the photosynthetic partners come from divergent parts of the tree of life green plants vs bacteria , the term photobiont is used as collective term for any of them. The role of the photobiont in lichens is clear — to provide carbon in the form of simple sugars. These sugars are used by the fungi to maintain physiological functions, to grow, and reproduce. However, in the case of lichens with both green algae and cyanobacteria, the lichen gets an added nutrient input from the cyanobacteria in the form of fixed nitrogen.

Although lichens can probably access inorganic nitrogen from the atmosphere directly, it can be a limiting nutrient, so having an internal source can be an advantage especially in heavily leached environments. Only about species of photobionts are commonly found across all known lichens, representing 4 main genera.

The vast majority of photobionts are from the genus Trebouxia , followed by Trentopohlia both Chlorophyta , Nostoc and Scytonema both Cyanobacteria. Most green-algal photobionts are unicellular green forms, but small colonial types and filamentous algae occur as well.

Within the lichen thallus, most photobionts have a different morphology than they would when grown in isolation, so few photobionts can be reliably identified using traditional microscopic methods. Instead, it is best to rely on culturing studies, and more often, on molecular methods, as many different strains have very similar morphology.

Many organic acids are obtained from lichens. Harmful activities of Lichens Some harmful activities of lichen are as given below: Many lichens produce skin diseases in human beings.

Lichens destroy the glass of window, wood and stones of the walls. After drying lichens burn and produce fire for the forests, i. So, dear friends, these are some important detail of Lichens. You can find this lichen at natural places and lichen is a good species to preserve in herbarium. Being the most popular educational website in India, we believe in providing quality content to our readers.

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