Lichens - A general Account

Lichens - A general Account

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  LICHENS BY Dr P BMALLIKHARJUNA GFGC YB 25-4-2020
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  LICHENS  Lichens arethe most important symbiotic living organisms / plants on the earth.  Theophrastus (371-284 BC), the Father of Botany, to denote the superficial growth on the tree bark, coined the term ‘LICHEN’.  Lichenology is a branch of Botany that deals with the study of Lichens.  Acharius (1798), a Swedish lichenologist, is the Father of Lichenology.  Lichens are one of the largest group of plants consisting about 500 genera and 18000 species included in 9 orders.  A lichen can be defined as “the dual organism with symbiotic (mutualistic) association of a fungus with an alga”.
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   Mycobiont: isthe fungal partner. It is the exhabitant, dominant, colourless, protective and reproductive partner.  Phycobiont:is the algal partner. It is the inhabitant, recessive, coloured autotrophic partner. Although, lichens are the symbiotic associations, the relationship between a fungus and an alga is like a Master and Slave. It is known as “HELOTISM” (Crombie,1885). Fungus is the Master and alga is the Slave
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  Occurrence: Lichens areone of the most widely distributed plants ranging from the polar to the deserts, and aquatic bodies.  Lichens are the dominant vegetation both in the Arctic and Antarctic regions.  They will grow as lithophytes, xerophytes, epiphytes, mesophytes, and hydrophytes.  Some of these are Clandonia rangiferina (Reindeer moss), Peltigera (Aquatic lichen), Gyrophora (White lichen), Graphis, Parmelia, Haematomma, Cora, Dictyonema, etc.  They show great variation in their size, shape, structure, morphology, growth rate and ecology.  In a typical lichen, the fungal partner (the Mycobiont) is the most abundant component, which accounts for 90-95% of the total biomass, whereas the algal partner( Phycobiont) accounts for 5-10%.  The fungal hyphae form a close network resembling a tissue like mass with the algal cells often embedded in it.
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   Lichens basedon their habitat (distribution), external morphology, internal anatomy, and the partners involved (fungal and algal) are classified into several types. 1) Based on their Habitat: There are three types of lichens,viz., a) Saxicoles (which grow on the rocks), b) Corticoles (which grow on the Bark), and c) Terricoles (which grow on the soil).
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  1) Based onthe Habit / Morphology: There are three types of lichens viz., a) Crustose lichens: These are the most common type with thin mat like thalloid plants, firmly attached to the substratum. Ex: Graphis, Haematomma (More common on the barren rocks). The pioneers of the xeorsere type of plant sucession b) Foliose lichens: These are thalloid; flatten leaf like plants attached to the substratum through rhizines. Ex: Parmelia (More common in the tropical forests as corticoles) c) Fruiticose lichens: These are usually a small shrub like vertically or pendulously growing plants. They are common in the temperate and polar regions. Ex: Cladonia
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  1) Based onthe Internal Structure: Lichens may be of a) Homoimerous and b) Heteroimerous In Homoimerous lichens, the fungal hyphae are loosely arranged. Several crustose lichens belongs to this category. In Heteroimerous lichens, the fungal hyphae are thick and compact and thereby surrounding the alga.
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  1) Based onthe Fungal Partner: Lichens can be classified into three types viz., a) Ascolichens, b) Basidiolichens and c) Deuterolichens a) Ascolichens: The fungal partner belongs to the class Ascomycetes. These are the abundant type. Over 98% lichens are Ascolichens. Ex: Parmelia, Graphis. Apothecia, perithecia and pseudothecia are the common fruiting bodies. They may be (Apothecia) or (Perithecia). b) Basidiolichens: The fungal partner belongs to the class Basidiomycetes. They are very less in number (<1%). Some of these are Cora, Corella and DIctyoma. They produce pycnidial cups.
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  1) Based onthe algal partner: Generally, either green alga or the blue-green alga will be the algal partner of lichens. Hence, lichens are a) Chlorophycophyllous lichens and b) Cyanophycophyllous lichens. a) In Chlorophycophyllous lichens, the algal partner is green alga. 80% of lichens contain green alga as a phycobiont. Trebouxia (unicellular) is the most abundant partner followed by Chlorella. b) In Cyanophycophyllous lichens, the algal partner belongs to BGA or cayanobacteria which accounts for ~8%. Some of these are Nostoc, Stigonema, Rivularia, and Gleocapsa. However, Nostoc is the most common partner.
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  HOMOIMEROUS THALLUS OFLICHEN
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  HETEROMEROUS THALLUS OFLICHEN
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  REPRODUCTION IN LICHENS: Both vegetative/asexual reproduction and sexual reproduction are present in Lichens.  The Vegetative Reproduction: takes place very frequently either by the fragmentation process or by the formation of different types of diaspores (having both alga and fungus) such as Soredia and Isidia.  In the vegetative propagation, both the partners are involved and develop into a new daughter Lichen.  Fragmentation is the common mode of vegetation where the fragments are arise either due to the progressive death and decay of older portion or due to the involvement physical factor.
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   Soredia: Theseare the asexual/vegetative propagules of Lichens. o These are small, rounded granules or bud-like outgrowth, which develop, in the form of a greyish white or greyish green powder in extensive patches usually over the upper surface or edges of the thalli of many species of Lichens. o It consists of a small group of algal cells surrounded by a few fungal hyphae. o Several soredia are present into a cluster often referred as Sorelia.
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   Isidia: Theseare the asexual/vegetative propagules of Lichens. o These are small, conical warts developed on the upper thallus of many Lichens. o An isidium is usually surrounded by cortex and diasporic in composition o Their shape is highly variable among lichens ranging from rod shape to coral-like structure. Ex: Parmelia, Usnea.
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  Sexual Reproduction: Sexualreproduction occurs in many Lichens and in this, only the fungal partner (the mycobiont ) is involved. The role of algal partner is passive or very absent.  The mode of sexual reproduction in lichens maybe spermatogamy.  Ascolichens produce the sex organs namely Spermogonium (male) and Carpogonium (female). After fertilization, they produce the special fruiting bodies such as Apothecium, Perithecium and some times Puesdothecia. Ex: Parmelia  While, the Basidiolichens, produce basispores followed by the formation of the Pycnidia, as the sexual apparatus. After spermatogamy, the sexual fruiting body develops as Basidiocarp like in Agaricus. Ex: Corella.
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  Sexual reproduction inAscolichens:  Since majority of the lichens are with Ascomycetous fungal partner, it will reproduce sexually by the formation of two different types of sex organs.  The male reproductive structure is Spermogonium and the female reproductive sex organ is Carpogonium.  The spergonium is a flask shaped structure that usually present present upper surface of the thallus. It is with a pore called ostiole, and several muticelluar spmertiophores which produce the uni-nucleate spheroidal sermatia.  The carpogonium is a special cellular filament. It consists of two portions, the lower coiled portion usually called Ascogonium and it is multi-cellular and monokaryotic. It lies deep in the medullary region of the thallus.
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   The straightupper portion of the carpogonium is called trichogyne and it is multicellular. The component cells are elongated one with the porous cross septa.  The trichogyne receives the spermatia (the male gametes) which in turn penetrates and finally reaches the corpogonium and subject to fertilization resulting a diploid cell called ascus mother cell. This cell proliferates into several asci and whereby the diploid nucleus undergo meiosis and produce haploid ascospores  Likewise, so many filaments arise and protrude above the surface of the thallus and produces the characteristic fruiting bodies viz., 1) Apothecium 2) Perithecium and 3) Pseudothecia with or without stalk.
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  Lichen Apothecia
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  1) Apothecium: isa major type of sexual fruiting body of Ascolichens.  It is generally round and cup-shaped structure.  Their colour is highly variable ranging from reddish to black colour.  In some lichens, the apothecia contain only the mycobiont but the phycobiont is absent. Ex: Cladonia.  However, in majority of the lichens, the apothecia possess both the partners. Ex: Parmelia. Apothecium structure: Apothecium is lined with palisade-like layer cells called Hymenium.  The Hymenium consists of a series of elongated cells – the asci, intermixed with sterile hyphae – the paraphyses.  Each ascus usually contains eight ascospores (bi-celled). The liberated ascospores liberated, if come in contact with suitable alga, produce the new daughter thallus. Otherwise they perish.
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  1) Perithecium: Thisis another sexual fruiting body of the Ascolichens.  It is much smaller than the apothecium and flask shaped structure and appears as a small dot on the surface of the thallus.  The dark coloured wall bound it.  The cavity is lined with asci.  The paraphyses are few in number or totally absent.  It is embedded in the elevations of the thallus on the upper side.  It opens through a small pore called an Ostiole.
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  ECOLOGICAL ROLE OFLICHENS 1) Lichens are serving as the indicators of Air pollution. Especially they are very sensitive to the NO2 and SO2 gases. 2) They involve in the soil formation on the rocks called biological weathering. 3) They are the first/pioneer colonizers on a barren rock in the establishment of ecological plant succession. 4) Some lichens may be used as tools in order to determine the age of the rocks and the past events occurred called Lichenometry. ECONOMIC IMPORTANCE OF LICHENS  Since lichens are rich with secondary metabolites like alkaloids, oils and other substances, lichens are extensively used in medicine, besides serving as food and fodder to man and animals. Further, several acids, dyes (orecin, litmus) are extracted. Also using in perfume and dhoop industries