Lichen | Definition, Symbiotic Relationship, Mutualism

Lichens are dual (composite) organisms or entities which contain a permanent association of a fungal partner or mycobiont and an algal partner or phycobiont. Mycobiont is dominant partner and mostly belongs to ascomycetes (Ascolichens — e.g., Graphis, Cladonia, Parmelia, Usnea, etc.) or sometimes basidiomycetes (Basidiolichens —, eg., Corella, Cora, etc.). Phycobiont is mostly a member of Chlorophyceae (e.g., Chlorella, Trebouxia, Protococcus, Palmella, etc.) or can be a BGA (e.g., Nostoc, Chlorococcus, Scytonema, etc.). The term lichen was coined by Theophrastus (370 – 285 B.C.). also called Father of Botany. Lichens often grow in most inhospitable and uninhabited places like barren rocks (saxicolous), soil (terricolous), icy tundra or alpines, Sand dunes, roofs, walls, wood (lignicolous), tree bark (corticolous), leaves, etc. They commonly live under humid and exposed conditions but can tolerate extreme desiccation. However, lichens, cannot tolerate air pollution, especially due to sulphur dioxide (so are considered indicators of SO2 pollution).

Lichens are perennial. Their growth is slow. Lichens have greyish, yellowish, greenish, orange, dark brown or blackish colouration.

Structure

Based upon external morphology, the lichens are of three types

1. Crustose: Crust like, losely appressed to the substratum and attached to it at several places, e.g., Graphis, Lecanora, Rhizocarpon.
2. Foliose : The body of the lichen is flat, broad, lobed and leaf-like, which is attached to the substratum at one or a few places with the help of rhizoid like structures called rhizines, e.g., Parmelia, Peltigera.
3. Fruticose : The lichen is branched like a bush and attached to the substratum by means of a disc, e.g., Cladonia, Usnea, vernia, Bryonia.

The bulk of lichen body is formed by fungal partner called mycobiont. It includes the surface, medulla (or interior) and rhizines (attaching devices). The algal partner or phycobiont constitutes hardly 5% of the lichens body. It is generally restricted to a narrow zone (algal zone) below the surface

Relationship

The fungus performs following functions

1. Body structure and covering.
2. Anchoring
3. Absorption of water and minerals. It can absorb water from wet air (atmosphere), dew and rain. Minerals are picked up both from substratum and atmosphere. Special chemicals are excreted by the fungal partrner of the lichen to dissolve minerals from the substratum.
4. Sex organs and fruitifications are of fungal origin.

The major function of alga is photosynthesis.

The cyanobacterial alga additionally takes part in nitrogen fixation. The alga picks up water and mineral salts from the fungus while the fungus obtains part of the food manufactured by the alga. Therefore, in a lichen the association between alga and fungus is that of mutual benefit (mutualism) popularly called symbiosis.

Reproduction

Lichens multiply by four methods

1. Progressive death and decay resulting in the separation of a lichen thallus into two or more parts
2. Fragmentation caused by mechanical injury, due to wind or animal bites.
3. Isidia are superficial outgrowths of the lichens which are primarily meant for increasing surace area and photosynthetic activity. At time, they are broken off. Each isidium is capable of forming a new lichen because it has a core of a few algal cells surrounded by a sheath of fungal nypnae.
4. Soredia. These are most efficient means of asexual reproduction. They are microscopic lichen propagules which are produced in large numbers inside sori called pustules. Soredia are dispersed by air currents. After falling on a suitable substratum each soredium gives rise to a lichen, because it has a few algal cells surrounded incompletely by a weft of fungus.