2. Lichens are among the most successful symbiotic (biotrophic) associations in

Question

2. Lichens are among the most successful symbiotic (biotrophic) associations in nature.

a. First explain how a mature lichen thallus is formed in nature beginning with a germinating spore and photobiont (i.e., the process of lichenization)?

b. Describe the internal structure of a stratified lichen thallus (Trebouxia) and compare it with that of a non-stratified (Nostoc) lichen thallus.

c. Next explain how the hyphae of the mycobiont and cells of the photobionts are physically related to one another? Include examples of green algae and blue-green bacteria in your answer.

d. Finally, how do the mycobiont and photobiont in lichen thalli function in relation to one another, i.e., how do the fungi and algae obtain water, carbohydrates, mineral and other elements of nutrition?

Explanation / Answer

Lichens represent the symbiotic relationship between algae or cyanoba and fungi and possess the properties of both the organisms (bionts). Thus lichens are also referred to as holobionts. The lichens are found in a variety of habitats and can vary in shape and size. Primarily the thallus or the vegetative parts of the lichens are classified according to the shape of the fungi filaments. The fungi elements are known as mycobionts and the photosynthesizing elements, algae or cyanobacteria are known as photobionts.

(a) The process of lichen formation or lichenization can occur by vegetative as well as sexual means in nature.

A mature thallus may undergo fragmentation and give rise to a new thallus. These vegetative structures contain the photobiont cells in association with the fungal filaments. The means of vegetative propagation are by powdery granules called soredia or thallus outgrowths called isidia.

In the sexual form of reproduction, only the fungal spores undergo sexual reproduction and these spores are dispersed by agents of dispersal. The fungal spores are commonly produced in a disc-like structure called apothecia. These spores are contained in ascii and are formed by meiosis.

Once dispersed, the fungal spores must meet with a suitable algal partner to form a functional lichen. First, the fungal spore germinates in the suitable substratum and produces numerous germ tubes. When compatible algal cells are found in the vicinity, they are seized by the germ tubes. The branching initiates at this point. Some branches entrap the algal cells and form close associations and other branches penetrate the substratum to derive nutrients. The algal cells carry out photosynthesis and provide food for the entire holobiont.

This close association stimulates the development of the vegetative structure and framework of the lichen, that is the thallus. The thallus in its immature form provides fresh ramifications for the algal cells to divide and multiply. These algal partners derive water and nutrients from the fungal hyphae (branches) and provide with carbohydrates. This is the process which leads to the formation of the mature thallus, which can grow in size according to the availability of nutrients and suitable environment.

(b) Trebouxia, is a typical foliose structure lichen. The thallus is very well defined and hence known as stratified lichen thallus. It consists of a lower cortex which is closest to the substratum. Above it are the medulla or interwoven fungal hyphae, a layer of algal cells and topmost os the upper cortex.

Nostoc is a foliose gelatinous lichen which is non-stratified. The upper and lower surface are not very clearly defined. There is a presence of gelatinous matrix and the algal cells are spread throughout the network of fungal hyphae.

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