In this article we will discuss about:- 1. Meaning of Lichens 2. Habitat and the Thallus of Lichens 3. Reproduction 4. Economic Importance 5. Resynthesis.
- Meaning of Lichens
- Habitat and the Thallus of Lichens
- Reproduction of Lichens
- Economic Importance of Lichens
- Resynthesis of Lichens in Laboratory
1. Meaning of Lichens:
Lichen is an association between an alga and a fungus, in which the two organisms jointly form a thallus that is distinct from either partner. The fungal partner is known as mycobiont and the algal partner as the phycobiont. Both the partners are benefitted by the association. Such an association is known as symbiosis or mutualism.
The fungus derives nutrition from the alga, which, in turn, is protected by the fungus. The benefit derived by the fungus is more than what it gives to the alga, but it happens in all symbiotic associations. The mutual interdependence is absolute; the components are incapable of free existence; except in laboratory cultures. It is not a casual association!
The mycobionts are usually Ascomycota and occasionally Basidiomycota or Anamorphic fungi. At least 26 genera of algae (8 blue-green, 1 yellow-green and 17 green algae) are found as phycobionts. Nostoc (blue-green) and Trebouxia (green alga) are the most commonly encountered phycobionts. The fungal and algal partners lose their identity in the lichen thallus. The lichens are designated by new generic and species names and the names of fungal and algal partners are not included in their nomenclature.
It is being increasingly felt that the fungi, forming lichen associations, should be classified like other fungi at their appropriate place in the fungal kingdom. Similarly phycobionts too should be included in the general algae. Cifferi and Tomaselli in 1953, named the lichenized fungi by adding myces to the name of each lichen genus. Thus the mycobiont of Collema was named as Collemomyces.
2. Habitat and the Thallus of Lichens:
Lichens can withstand extremes of climate and, thus, are found everywhere ranging from hot deserts to chilly mountains. They can colonize rocks, but are also found growing on fertile soils. The tree trunks on hills are the most common sites of lichen growth. In some places, the growth is so luxuriant that lichens are used as fodder for cattle. But, lichens are most sensitive to air pollution and probably that is the reason they are not found near cities.
The numerous types of thalli that the lichens exhibit, can be grouped under three morphological categories. These are Foliose (leaf-like), Crustose (crust-like) or Fruticose (branched-upright or pendulous). Anatomically, the three forms show much variation.
These leaf-like lichens, in a transverse section, show much resemblance to an angiosperm leaf. Starting from above, it has the upper cortex, an algal layer, medullary layer and lower cortex. The upper and lower cortex is made of ‘pseudoparenchymatous tissue’, formed by fungal hyphae. The medullary layer consists of loosely interwoven hyphae. Pores or hair like rhizoids are found on the lower surface, e.g., Physcia, Sticta, Parmelia, etc.
The crustose thalli do not show much differentiation. There are no cortical layers. The algal cells are surrounded by the loosely arranged hyphae, e.g., Graphis, Verrucaria.
There is a central medullary layer surrounded by the more compact mass of hyphae. The algal cells lie in a uniform layer or as patches in the hyphae, e.g., Usnea, Cladonia, Evernia.
Growth of Thallus:
Lichens grow at an extremely slow rate, usually less than 1 mm per year. The maximum growth that has been recorded is 4 mm in one year. The thalli mature and reproduce after 4-8 years of growth. Equally surprising is their age. It has been calculated to be 1,000 to 4,500 years. Lichens are in no hurry!
The slow rate of growth is due to the fastidious requirements of lichens. A low light intensity, cool temperature and moisture (in the form of fog) are the requirements for growth. These conditions occur only for a few hours in the morning on hills. The water requirement is the critical factor. The lichen thalli behave like agar gel and absorb moisture from the humid environment up to 100-300 times of their dry weight. They lose the water also with the same rapid rate.
3. Reproduction of Lichens:
Asexual reproductive methods are the main source of multiplication. The sexual reproduction is insignificant in the life of lichens.
Death and decay of old parts of the thallus produce smaller pieces which give rise to new thalli.
Soredia (Sing. Soredium):
These are minute outgrowths, formed on the upper surface of the thalli in distinct pustule-like sori. The large numbers of soredia form a powdery coating on the surface. Each soredium consists of a few algal cells surrounded by a mass of hyphae. These are dry and, therefore, easily blown by wind. Each soredium gives rise to a lichen thallus.
Isidia (Sing. Isidium):
These are stalked outgrowths of the thallus containing masses of algae and fungal hyphae, surrounded by a portion of the thallus cortex. These are not detachable. Accidental breakage results in their dissociation from the thallus. Conidia are formed in pycnidia but these stand poor chance (one in a million) of meeting the right type of algal partner for reconstituting the thallus.
Only the fungal partner shows sexual reproduction. Ascospores are formed in ascomata. Which are mostly apothecia (e.g., Physcia, Parmelia, Usena, Cladonia, etc.). The non-occurrence of sexual reproduction in the phycobiont is a weak point in the perpetuation of the lichens.
The ascospore has, in the words of the popular song, “one in a million” chance to come in contact with the right algal partner to initiate a new lichen association. Ahmadjian (1960) asked a very pertinent question. What happens to the spores (conidia and ascospores) of the mycobiont in nature? It has not yet been answered!
a. Use as Fodder:
The ‘reindeer moss’ in Tundra and the ‘Iceland moss’ in Iceland and some species of Lecanora, occurring on barren plains and mountains of Western Asia, are used as feed for cattle.
The “dog lichen” (Peltigera canina) is useful in the cure of hydrophobia. The lung wort-Lobaria pulmunaria is used in the treatment of lung infections. But an assessment of their efficacy is not done.
Orchill, obtained from species of Roccella and Lecanora, is used in dyeing of woollen and silk fabrics. The stain orcein, used for microscopic preparations, is a purified form of the orchill. The chemical indicator, litmus, is also derived from the above lichens.
d. First Colonizers of Rocks:
Lichens are the first to start colonization of bare rocks. Crustose lichens produce acids that dissolve stone particles in crevices of rocks and establish their thalli. Foliose lichens come next in their capacity to initiate rock colonization.
During hot seasons, the dry lichens on tree trunks catch fire quickly, which is soon transmitted to tree tops.
The algal and fungal components of several lichen genera have been isolated and grown in culture. It has been found that they grow extremely slowly-not more than 1-2 mm in a year. The mycobionts are dependent for growth on external supply of vitamins, e.g., thiamine and biotin. However, when both the components were grown together, the algae died. Sometimes, the dead algal cells were found to be filled with fungal hyphae.
Death of algal cells was noted when the mycobiont of the lichen Collema tenax was grown with its algal component, Nostoc or with any other alga isolated from other lichen genera. It was explained that the first reaction between the two partners is that of parasitism of the alga by the fungus. The algae which survive this parasitism enter into lichen association.
Ahmadjian, however, in 1962, succeeded in synthesizing the lichen Acarospora fuscata by growing its fungal and algal components together on a medium on which each alone failed to grow. This gave a valuable indication that the condition, which was unfavourable for each of them independently, was favourable for a joint growth and formation of lichen.
Thus, a very delicate balance exists between their associations and if there is any disturbance, dissociation results. Their association is a “marriage of convenience”. Under the luxuriant conditions in laboratory culture, the lichens dissociate and this creates a big hurdle in the study of their biology.