Q.1 Give an account of the structure and economic importance of lichens. (2005, 12)
Related Question -
Q. Write short note on general structure of Lichens. (2013, 15)
Ans. Structure: -
The body (thallus) of most lichens is quite different from those of either the fungus or alga growing separately, and may strikingly resemble simple plants in form and growth. The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations. In many species the fungus penetrates the algal cell wall, forming penetration pegs or haustoria similar to those produced by pathogenic fungi. Lichens are poikilohydric, capable of surviving extremely low levels of water content. However, the re-configuration of membranes following a period of dehydration requires several minutes at least. During this period a “soup” of metabolites from both the mycobiont and phycobiont leaks into the extracellar spaces. This is readily available to both bionts to take up essential metabolic products ensuring a perfect level of mutualism. Other epiphytic organisms may also benefit from this nutrient rich leachate. This phenomenon also points to a possible explanation of lichen evolution from its original phycobiont and mycobiont components with its subsequent migration from an aquatic environment to dry land.During repeated periods of dehydration in an alga and the resultant leakage of beneficial metabolites to the adjacent aquatic fungus, the mutalistic “marriage” slowly becomes constant.
The algal or cyanobacterial cells are photosynthetic, and as in higher plants they reduce atmospheric carbon dioxide into organic carbon sugars to feed both symbionts. Both partners gain water and mineral nutrients mainly from the atmosphere, through rain and dust. The fungal partner protects the alga by retaining water, serving as a larger capture area for mineral nutrients and, in some cases, provides minerals obtained from the substrate. If a cyanobacterium is present, as a primary partner or another symbiont in addition to green alga as in certain tripartite lichens, they can fix atmospheric nitrogen, complementing the activities of the green alga.
Algal and fungal components of some lichens have been cultured separately under laboratory conditions,but in the natural environment of a lichen, neither can grow and reproduce without a symbiotic partner. Indeed, although strains of cyanobacteria found in various cyanolichens are often closely related to one another, they differ from the most closely related free-living strains. The lichen association is a close symbiosis: It extends the ecological range of both partners and is obligatory for their growth and reproduction in natural environments. Propagules (diaspores) typically contain cells from both partners, although the fungal components of so-called “fringe species” rely instead on algal cells dispersed by the “core species.”
Lichen associations may be considered as examples of mutualism, commensalism or even parasitism, depending on the species. Cyanobacteria in laboratory settings can grow faster when they are alone rather than when they are part of a lichen. The same, however, might be said of isolated skin cells growing in laboratory culture, which grow more quickly than similar cells that are integrated into a functional tissue. However, from the work of Coxson (see above) mutualism would appear to best summarise our current knowledge.
Morphology and Structure: -
Some lichens have the aspect of leaves (foliose lichens); others cover the substrate like a crust (crustose lichens), others such as the genus Ramalina adopt shrubby forms (fruticose lichens), and there are gelatinous lichens such as the genus Collema. Although the form of a lichen is determined by the genetic material of the fungal partner, association with a photobiont is required for the development of that form. When grown in the laboratory in the absence of its photobiont, a lichen fungus develops as an undifferentiated mass of hyphae. If combined with its photobiont under appropriate conditions, its characteristic form emerges, in the process called morphogenesis. In a few remarkable cases, a single lichen fungus can develop into two very different lichen forms when associating with either a green algal or a cyanobacterial symbiont. Quite naturally, these alternative forms were at first considered to be different species, until they were first found growing in a conjoined manner.
There is evidence to suggest that the lichen symbiosis is parasitic or commensalistic, rather than mutualistic. Under magnification, a section through a typical foliose lichen thallus reveals four layers of interlaced fungal filaments. The uppermost layer is formed by densely agglutinated fungal hyphae building a protective outer layer called the cortex, which can reach several hundred ìm in thickness. This cortex may be further topped by an epicortex 0.6-1ìm thick in some Parmeliaceae, which may be with or without pores, and is secreted by cells - it is not itself cellular. In lichens that include both green algal and cyanobacterial symbionts, the cyanobacteria may be held on the upper or lower surface in small pustules called cephalodia. Beneath the upper cortex is an algal layer composed of algal cells embedded in rather densely interwoven fungal hyphae. Each cell or group of cells of the photobiont is usually individually wrapped by hyphae, and in some cases penetrated by an haustorium. Beneath this algal layer is a third layer of loosely interwoven fungal hyphae without algal cells. This layer is called the medulla. Beneath the medulla, the bottom surface resembles the upper surface and is called the lower cortex, again consisting of densely packed fungal hyphae. The lower cortex often bears rootlike fungal structures known as rhizines, which serve to attach the thallus to the substrate on which it grows. Lichens also sometimes contain structures made from fungal metabolites, for example crustose lichens sometimes have a polysaccharide layer in the cortex. Although each lichen thallus generally appears homogeneous, some evidence seems to suggest that the fungal component may consist of more than one genetic individual of that species. This seems to also be true of the photobiont species involved.
Growth Form: -
Lichens are informally classified by growth form into:
- crustose (paint-like, flat), e.g., Caloplaca flavescens
- filamentous (hair-like), e.g., Ephebe lanata
- foliose (leafy), e.g., Hypogymnia physodes
- fruticose (branched), e.g., Cladonia evansii, C. subtenuis, and Usnea australis
- leprose (powdery), e.g., Lepraria incana
- squamulose (consisting of small scale-like structures, lacking a lower cortex), e.g., Normandina pulchella
- gelatinous lichens, in which the cyanobacteria produce a polysaccharide that absorbs and retains water.
Economic Uses: -
Food: -
Umbilicaria esculenta) are collected from cliffs for use in a variety of traditional Korean and Japanese foods. Lichen flour is used by the Tarahumara as an ingredient of Tesguino.
Other Uses: -
Many lichens produce secondary compounds, including pigments that reduce harmful amounts of sunlight and powerful toxins that reduce herbivory or kill bacteria. These compounds are very useful for lichen identification, and have had economic importance as dyes such as cudbear or primitive antibiotics.
There are reports dating almost 2000 years of lichens being used to extract purple and red colors. Of great historical and commercial significance are lichens belonging to the family Roccellaceae, commonly called orchella weed or orchil. Orcein and other lichen dyes have largely been replaced by synthetic versions. The pH indicator litmus is a dye extracted from the lichen genus Rocella tinctoria by boiling.
Q.2 Give an account of reproduction in lichens. (2006, 07)
Or Write a short note on reproduction in lichens. (2005, 13)
Ans. Lichen Reproductive Structures: - (2011)
Many lichens reproduce asexually, either by vegetative reproduction or through the dispersal of diaspores containing algal and fungal cells. Soredia (singular soredium) are small groups of algal cells surrounded by fungal filaments that form in structures called soralia, from which the soredia can be dispersed by wind. Another form of diaspore are isidia, elongated outgrowths from the thallus that break off for mechanical dispersal. Fruticose lichens in particular can easily fragment. Due to the relative lack of differentiation in the thallus, the line between diaspore formation and vegetative reproduction is often blurred. Many lichens break up into fragments when they dry, dispersing themselves by wind action, to resume growth when moisture returns.
Many lichen fungi appear to reproduce sexually in a manner typical of fungi, producing spores that are presumably the result of sexual fusion and meiosis. Following dispersal, such fungal spores must meet with a compatible algal partner before a functional lichen can form. This may be a common form of reproduction in basidiolichens, which form fruitbodies resembling their nonlichenized relatives. Among the ascolichens, spores are produced in spore-producing bodies, the three most common spore body types are the apothecia, perithecia and the pycnidia.
For reproduction, lichen possess is idia, soredia, and undergo simple fragmentation. These structures are also composed of a fungal hyphae wrapped around cyanobacteria. While the reproductive structures are all composed of the same components(Mycobiont and Photobiont) they are each unique in other ways. Isidia are small outgrowths on the exterior of the lichen. Soredia are powdery propagules that are released from the top of the thallus. In order to establish the lichen, the soredia propagules must contain both the photobiont and the mycobiont.
Lichens have two fundamentally different sorts of reproductive bodies. These are 1) spore forming bodies and 2) vegetative reproductive bodies.
Spore formation is a function of the fungal partner only and the form that the spores occur in reflects the type of fungus involved. As we already know that two main types of fungus are involved it will come as no surprise that each can be identified by its manner of spore production. The most common fungal partners in Lichens are the Ascomycetes. Ascomycetes are so called because they produce their spores in a bag or ascus. There are 8 ascospores per ascus and any of the fruiting bodies mentioned below will contain numerous asci (which is the plural of ascus). Just to confuse you some (about 8,000) Ascomycetes also produce spores not in an ascus, but on the sides of, or at the tips of, special hyphal filaments. These structures are called conidiomata and the spores they produce are called conidiospores. Conidiomata come in different forms but by far the commonest is the Pycnidium, see below.
The other main group of fungal partners are Basidiomycetes. Basidiomycetes are relatively rare as lichen partners, and one of the ways they differ
from Ascomycetes is that they produce their spores on a basidium, a special structure which normally holds four spores at its top.
Spores come in a great variety of sizes, shapes and forms. They are for instance, much larger in the genus Pertusaria than in the genus Acarospora. Many are simply a single whole spore while in others the spore may be divided up into 2 or more subsections. These spores are all microscopic in size and float easily in the air, this enables them to disperse widely when they are released. Quite how these spores meet up with the correct algal partners to form new Lichens is something nobody really understands, though as most of the algae occur in the wild on their own it may be that it is simply a matter of chance that some will come to rest on the right algae.
Spore producing bodies come in several forms, 3 of which are relatively common: Apothecia, Perithecia and Pycnidia.
Apothecia normally look like exposed disks which may be raised above
the surface of the thallus, level with it, or sunk below the surface. In some genera/species the apothecia look more like slits, i.e. Graphus sp. The Asci are held within a mass of special sterile hairs called paraphyses. These paraphyses are composed of fungal hyphae and often have coloured tips. These then give the Apothecia their coloured centres. Differences in the colours of Apothecia can be important in identification.
Perithecia here the spore bearing body is not open as apothecia but is in a chamber with only a small opening to the outside world, this opening is called an ‘ostiole’. Perithecia are often buried almost completely in the substrate on which the lichen is growing. Both these structures release ascospores - spores produced within an ascus (a sack).
Pycnidia are reproductive bodies which release conidiospores - spores produced from the end or side of special hyphal filaments called conidia. In form, pycnidia look a lot like perithecia.
Vegetative Reproductive Bodies-Vegetative reproduction is important to many lichens and has the advantage of dispersing both partners at the same time. Three main types of vegetative reproduction are Isidia, Soredia and Lobules.
Lobules are living lobes which grow on the edges of the thallus of foliose lichens. They are flat and break off from the thallus for wind or water distribution.
Isidia are extensions of the surface of the thallus and may be cylindrical, globular, brachiate (branched) or lobula (lobe-like). 20-30% of foliose and fruticose lichens have isidia. Isidia are really very small bits of the top of the thallus.
Soredia are small bundles of algal cells in a fungal hyphae mesh. Unlike Isidia, they do not contain any of the cortex. Instead they are more similar to the medulla portion of the thallus with some algal cells included.