In this article we will discuss about:- 1. Introduction to Lichens 2. Symbiotic Association of Lichens 3. Reproduction.
Introduction to Lichens:
Lichens are composite organisms consisting invariably of a fungus and an alga or a cyanobacterium. The two components are permanently associated with each other to form a lichen- body. The relationship is symbiotic. The algal or cyanobacterial component provides photosynthetic as well as products of atmospheric nitrogen fixation, if the partner is a cyanobacterium.
The fungal partner absorbs water and minerals from the substratum. The fungus also protects the photosynthetic component of lichen within a moist mycelial covering. The fungus sends haustoria into algal cells to draw nourishment. Thus, lichens represent an ideal example of symbiosis, in which two taxonomically separate organisms are permanently associated to form a single composite organism.
Only in a few lichens, it has been possible to separate the two partners and grow them independently. It has also been possible to synthesise a lichen by re-associating the original partners. When the two components are grown separately, the alga or cyanobacterium generally grows well in culture. But it is difficult to grow the fungal component. In nature, lichens are among the slowest growing organisms. There are about 25,000 species of lichens.
Lichens grow in greatly variable environmental conditions. Some can grow on solid rocks, while others in the permanently frosted arctic regions. However, majority of them prefer a moist humid shady environment such as one prevailing in the forests. In forests, lichens grow on the branches, barks and even on leaves of trees, as also on forest floors.
Lichens are often the first form of life to colonize a newly exposed rock or land. When they grow on solid rocks, they secrete organic acids which attack the rocky minerals causing weathering of rocks. This helps in genesis of soil, where subsequently plants can grow.
Lichens, though apparently insignificant in the overall biosphere, may be a considerable constituent in specialized geographical areas. For example, in the Tundra zones of Arctic regions, they form a major part of the vegetation. Cladonia rangiferina growing in such areas provides an important source of food for the reindeer and caribou. It is known as “reindeer moss”.
Certain lichens, like Cetraria islandica and Endocarpon miniatum, are used for human consumption. Leconera esculenta is also edible. Lichens have also other important uses. Rocella tinctoria is the source of erythrolitmin, an acid-base indicator dye which is the active ingredient of litmus. Lobaria pulmonaria and some species of Evemia are used as source of perfumes. Orcein, a dye used for staining chromosomes, is extracted from some species of Rocella. Usnic acid is obtained from Usnea species. It has antimicrobial activity. Letharia vulpine is a poisonous lichen.
Lichens are also of considerable importance as indicator organisms for detection of air pollution. Lichens can incorporate various cations into their thalli. The concentration of different cations in lichen thalli gives a measure of these cations in the atmosphere.
Moreover, certain lichens are specially sensitive to specific pollutants, like sulfur dioxide. By determining the species diversity in lichen population in an area, as well as by determining the disappearance of certain species at time intervals, it is possible to monitor the air quality.
Radioactive elements, like Cesium 137, escaping from nuclear plants, are absorbed by lichens. In the Chernobyl disaster occurring in the then U.S.S.R. in 1986, the reindeer population was found to have a much higher radioactivity, because they feed on radioactive lichens.
Symbiotic Association of Lichens:
In lichens, a photosynthetic component in the form of a green alga or cyanobacterium is permanently associated with a fungus, generally an ascomycetes and rarely a basidiomycete. Depending on the nature of the fungal partner, a lichen is designated as either an ascolichen or a basidiolichen. Some lichen-like forms do not have an algal component and they are called pseudo-lichens. Pseudo-lichens are mostly saprophytic or sometimes parasitic.
The photosynthetic component of a lichen is called a phyco-symbiont and the fungal component as myco-symbiont. The association always involves a single specific fungus and a single phyco- symbiont in any given species of lichen. The partners are not interchangeable which indicates that the symbiotic association is specific.
The phyco-symbionts may belong to about 30 different genera of algae and cyanobacteria. Among the algae, only green algae are components of lichen thallus. No other groups of algae are known to be present in lichens. The cyanobacteria may be unicellular or filamentous. The common unicellular cyanobacteria present in lichens are Chroococcus and Gloeocapsa.
Filamentous types include Nostoc, Scytonema, Rivularia, Stegonema etc. The green algae also include unicellular genera, like Chlorella, Cystococcus, Protococcus etc. or filamentous genera belonging to Chaetophorales, like Trentepohlia and Pleurococcus. Among the phyco-symbiont genera most commonly occurring in lichens are Trebouxia, Trentepohlia and Nostoc. These three genera are found in more than 90% of lichens.
The myco-symbionts of lichens are in the majority of cases members of ascomycetes belonging to the apothecia-forming group, called Discomycetes. Perithecia-forming ascomycetes (Pyrenomycetes) are rarely present in lichens (e.g. Dermatocarpori). Symbiotic association with basidiomycetes is also rare. The basidiolichens generally have some member of Thelephoraceae as myco-symbiont.
Some of the common ascolichens are Parmelia acetabula, Cladonia, rangiferina, Cetraria islandica, Usnea barbata, Lobaria pulmonaria, Rhizocarpon geographicum etc. Among basidiolichens, Cora pavonica is a well-known genus with Clavulinopsis is another genus with Clavaria as myco- symbionts. Some lichens found in India belong to the genera, Graphis, Lecanora, Parmelia, Physcia, Usnea, Cladonia etc.
An interesting aspect of lichen symbiosis is that certain chemical compounds are synthesized only when the phyco- and myco-symbionts are associated in a lichen. When the symbionts are separately grown, neither of the components is able to synthesise these compounds. Many lichens produce unusual phenolic compounds and lipids in substantial amounts in their thalli. Lichens are as yet not fully explored for obtaining new useful compounds.
Reproduction in Lichens:
Lichens may multiply vegetatively by fragmentation of the thallus. They also multiply by formation of several types of asexual structures. The most common of such structures are soredia. They are §mall globose deciduous bodies containing both the symbiotic partners. In these bodies, an algal or cyanobacterial cell is enveloped by fungal hyphae.
The soredia are formed on the surface of the thallus in very large numbers, giving a powdery appearance to the thallus surface. Soredia are easily detached by wind or splashing of rain drops. When they settle on a suitable substratum, they germinate to produce new thalli. Since both the symbionts are carried together in a soredium, the development of a new thallus is ensured (Fig. 5.30). The algal or cyanobacterial partners of lichen thalli generally multiply by cell division. Sometimes algae also multiply by formation of spores.
Sexual reproduction of lichens is restricted to the myco-symbiont only. The ascolichens produce asci and ascospores which are generally produced in well-differentiated ascocarps. As most of the ascolichens have Discomycetes, the ascocarps are commonly apothecia. Pyrenomycetes are much less common and so are perithecia.
Sometimes, defined fruit-bodies are-lacking and the asci are produced intermingled with loose sterile hyphae. In any case, the ascospores after they are dispersed, germinate under favourable conditions to produce a germ tube which develop into a hypha. When the hypha happens to encounter the appropriate algal partner, it establishes a symbiotic association and initiates a new lichen thallus formation. In case, the hypha fails to find its cognate partner, it degenerates.