Lichens: General Characteristics (A Symbiotic Association Between Algae and Fungi)

Flavoparmelia caperata - lichen - Caperatflechte

Flavoparmelia: A Foliose Lichen (source wikipedia)

General Characteristics of Lichen

What are lichens?

Lichens are structurally organized permanent symbiotic association between fungi and algae. The fungal component of lichen is called mycobiont and the algal component of lichen is called phycobiont. Theophrastus (371 – 284 BC), who is known as the ‘Father of Botany’, for the first time used the term lichen to denote the superficial growth on tree barks. The branch of biology which deals with the study of lichen is called Lichenology and the one who studies is known as lichenologist. Acharius is known as the ‘Father of lichenology’.

Lichens are symbionts:

Lichens are symbiotic association between algae and fungi. The term Symbiosis (introduced by De-Bary, a mycologist) is used to specify the association or interaction of organisms where both partners are mutually benefitted. The fungus absorbs water and protects algae from unfavorable conditions such as drought and extreme temperature. The algal components, since they are autotrophs, in turn supplies organic food to fungus. This type of symbiosis is known as heliotism, a ‘master’ and ‘slave’ relationship.

Lichens do not show the morphology of fungi and algae:

Even though lichens are symbiotic association between algae and fungi, the vegetative morphology (plant body) of lichen neither resembles algal or fungal morphology. Fungal component is prominent vegetative part in lichen than algal component. Almost 90% of the plant body of lichen composed of fungal component. Due to the ill differentiated plant body, lichens are included in Thallophyta of Cryptogams along with algae and fungi.

Usnea australis.jpg

Usnea: Thallus with Appothecium (source wikipedia)

Lichens are indicators of air pollution and pioneers of ecological succession:

Lichens are very slow growing plants and they can survive on extreme environmental conditions such as high temperature and can bury in snow for long years. Lichen produce a special acid known as lichen acid, which helps to weather rocks and assist in soil formation. Because of these two reasons, lichens form the pioneer community (first community) in xerarch plant succession on rock surface. Lichens do not grow near air polluted industrial areas and thus they are considered as indicators of air pollution. Example, absence of Lobaria (an asco-lichen) in the area indicates the high degree of industrial air pollution in the area.

Lichens are of word wide distribution

Even though the group lichens contains limited number species, they are distributed all over the world including tropical, temperate and Polar Regions. About 500 genera and approximately 13,500 species of lichens are recorded till date.

Lichens have a variety of habitats:

Mostly lichens are found on tree barks, decaying wood and rock surfaces. Based on the substratum on which the lichens are growing, lichens are of different types:

Ø  Xaxicolous lichens: Lichens growing on the surface of rocks. Example: Caloplecta, Aspicilia

Ø  Corticolous lichens: lichens growing on the surface of barks of trees. Example: Evernia, Parmelia and Usnea

Ø  Follicolous lichens: lichens growing on the surface of leaves. Example: Calicium, Cyphelium and Strigula

Ø  Terricolous lichens: lichens growing on the surface of soil.

Ø  Muscicolous lichens: lichens growing along with mosses. Example: Cladonia

Very few species of lichens are aquatic.  Peltigera is one of the marine aquatic lichen. Cladonia rangiferina (reindeer moss) grows luxuriously in Tundra. In India, lichens are luxuriously found in Himalayas and higher hills of South India.

Lichens are highly pigmented plants:

Lichen has various colors such as green, bluish, yellow, orange, reddish etc. and they are highly pigmented organisms. Some lichens are white in colour. The colouration is due to the pigmentation of algal component in the lichens. In some lichens, a special pigment called usnic acid is present which give lichens a variety of colors, including red, orange, yellow, and brown, especially when exposed to dry habitats. When high moisture is there in the surrounding, lichens appears as greener. This is because the water absorbed fungal mater become more transparent and as a result of this the green colour algal pigments get exposed.


Cladonia (source wikipedia)

Composition of lichen plant body (mycobiont and phycobiont)

The fungal component or mycobiont of all lichens belong to either Ascomycotina or Basidiomycotina or Deuteromycotina. Lichen with fungal component belonging to Ascomycotina is called Ascolichens. Similarly, those with Basidiomycotina and Deuteromycotina are called as Basidiolichen and Deuterolichens respectively.  Deuterolichens are also known as lichen imperfectii. Majority of the lichens are Ascolichens. Only four genera of Basidiolichens are so far reported. Deuterolichens are also very rare in nature.

The algal component or phycobiont may belong to Cyanophyceae (blue green algae), Chlorophyceae (green algae), Xanthophyceae or Phaeophyceae. Algal component from Cyanophyceae and Chlorophyceae are most common phycobionts in lichens. Among Cyanophyceae, 8 genera including Gloeocapsa, Nostoc and Rivularia are reported. Similarly among Chlorophyceae, 18 genera including Trebouxia, Trentipohlia and Cladophora are reported. Single genera from both Xanthophyceae and Phaeophyceae are also reported to from lichens with some fungi.

Classification and Naming of Lichens is Based on its Fungal Component:

According to International Code of Botanical Nomenclature (ICBN) classification and naming of lichen should be on the basis of the fungal component. Based on the nature of fungal components, lichens are divided lichens to 3 classes

(1). Asco-lichen

(2). Basidio-lichen

(3). Lichen-inperfectii (deuterolichens)

(1). Ascolichen: The mycobiont belongs to Ascomycotina division of fungi. Sexual reproduction of Ascolichens is similar to those of Ascomycotina. They produce Ascus with Ascospores after sexual reproduction. Majority of lichens are Ascolichens.

(2). Basidiolichens: The mycobionts belong to the Basidiomycotina division of fungi. Sexual reproduction is similar to those of Basidiomycotina. They produce Basidia and Basidiospores during sexual reproduction. Only very few lichen (4 genera reported so far) belongs to Basidiolichen.

(3). Lichen Imperfecti: They are also called as Deuterolichens. The fungal partners belong to Deuteromycotina division of fungi. These lichens lack sexual reproduction, since Deuteromycotina members do not show sexual reproduction.

Thallus organization in lichens:

Thallus organization shows the diversity of morphology of lichens. They are also the different growth forms of lichens. Based on thallus morphology, lichens are divided into three major groups.

(1). Crustose (Crustaceous) lichen

(2). Foliose lichen

(3). Fruticose lichen

N2 Lichen

A crustose lichen (source wikipedia)

(1). Crustose (Crustaceous) lichen:

Crustose lichens do not have well organized thallus. They have flattened thallus, closely attached to substratum such as rocks, soil or bark of other trees as crusts. Thallus may be partially or completely embedded in the substratum. The attachment of thallus to the substratum is very firm so that it is very difficult to separate crustose lichen from the substratum without disturbing the thallus. Crustose lichens are considered as the most primitive type of thallus organization in lichens.

Examples of crustose lichens: Graphis, Lecanora, Lecidia

(2). Foliose Lichen:

Foliose lichens have flat dorsi-ventral, leaf like lobed thallus and they look like the thallus of liverworts (bryophytes). They are attached to substratum with the help of rhizoid like structures called Rhizines. Foliose lichens are more advanced than crustose lichens.

Examples of foliose lichens: Parmelia, Peltigera, Collema, Heterodermia, Physcia, Xanthoria, Cetraria

(3). Fruticose lichen:

Fructicose lichens have well developed shrub-like, cylindrical and branched thallus. They grow or hang from the substratum such as tree trunk or rock surface. Plants attached to the substratum with the help of a mucilaginous disc. Fructose lichens have well organized internal structures and they are considered as the most advanced thallus organization in lichens.

Example of fruticose lichen: Cladonia, Usnea

Reproduction of lichens:

Lichens reproduce by asexual (vegetative) and sexual methods. Asexual reproduction takes place by specialized types of diaspores such as soredia and isidia. Diaspores are vegetative or asexual reproductive lichenized structures in which algal and fungal components together act as separable autonomous subunits of the thallus. Only the fungal component of lichens reproduce sexually. Female sex organ is called carpogonium. Carpogonium is differentiated into a basal ascogonium and an elongated trichogyne. Male reproductive organ is called spermagonia which produce spermatia.

Appothecia and Perithecia are the Fruiting bodies in Lichens:

Fruiting bodies (fructifications) of lichen are disc shaped appothecium (appothecia) and flask shaped perithecium (perithecia).

Economic importance of lichen:

Ø  Lichens are the pioneers of rock vegetation

Ø  Lichens initiates xerarch type of  plant succession on rock

Ø  Lichen acid cause weathering of rock into soil particles

Ø  Some lichens are ecological indicators, they acts as indicators of pollution

Ø  Cladonia rangiferina, which is luxuriously grows in polar region act as the food source for some animals such as reindeers

Ø  Lecanora is consumed as food by human

Ø  Peltigera canina and Lobaria pulmonaria are medicinal

Ø  Prmelia is used as a spice or condiment in some parts of India

Ø  Rocella and Lecanora yield dye called orchil or orecin or Cudbear which are used for colouring woolen and silk fabrics

Ø  Orecin is an excessively used chromosomal stain, it is used for the ‘O’ banding of chromosomes

Ø  Lobaria pulmonaria is used in tanning, perfumery industry. It is also used as ‘hope’ in brewing industry.

Key questions:

  1.  What are lichens?
  2.  What is symbiosis?
  3.  What is the morphology of lichens?
  4.  What is the ecological significance of lichens?
  5.  What are Corticolous, Follicolous and Xaxicolous lichens?
  6.  What are the pigments in lichens?
  7.  What are the components of Lichens?
  8.  How lichens are classified?
  9.  What are Ascolichens, Basidiolichens and Deuterolichens?
  10.  How the thallus is organized in lichens?
  11.  How reproduction occurs in lichens?
  12. What are the economic importance of lichens?

Lichens Short Notes

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