INTRODUCTION TO LICHENS

What are Lichens? Lichens are 'dual organisms.' Every lichen is a partnership between members of two different

kingdoms which live together in a special, mutually beneficial relationship (a symbiosis). Each lichen is made up of a fungus (usually an ascomycete) and an alga (green or blue-green).

• There are almost 20,000 lichens, each involving a different fungus, but the same algal partner can be found in many different lichens, so many fewer algae are involved.

• The body of the lichen is built up by tough fungal hyphae, and the algae live inside that framework.• The fungus protects the alga from the harsh world outside, and provides it with water and mineral

nutrients. The alga makes its own food by photosynthesis, and leaks some of this food, which is then absorbed by the fungus (which cannot make its own food).

• This partnership is so tough and self-reliant that lichens can grow in places like bare rock in deserts, where nothing else can survive. When it is too dry, too hot, or too cold, lichens go into a state of suspended animation until conditions improve. Since the algae make up only about 5% of each lichen, and are out of action for much of the time, you can imagine that lichens grow very slowly -- only a few ] per year. They make up for this by living for centuries, or in a few cases, millennia.

• Lichens have only one serious weakness -- they must absorb their mineral nutrients from the rain. So if the air is polluted with sulphur dioxide, this dissolves in the rain and is absorbed by the lichens which often die as a result.

• The lichen is a symbiosis between two different species: (1) a host fungus (i.e. mycobiont) and (2) endosymbiotic algae or cyanobacteria (photobiont). The host fungus provides the algae with protection from dessication, while the algae/cyanobacteria provides the host with carbohydrates produced from photosynthesis. The relationship is mutually beneficial (i.e. symbiotic), as both parties stand to gain from the association.

crustose

foliose

fruticose

Growth And Development In Lichens

• Like all living things lichens need nutrients and energy to grow.

• Lichens will and do grow on just about everything, natural or manmade. Different species of lichens prefer, or only grow on different substrates. Thus some species will be found on smooth barked trees, some on rough barked and some on only one species of tree. Also some lichens grow on basic rocks while others only grow on acidic rocks and some have particular mineral requirements, thus Acarospora sinopica only grows on rocks with a high iron content. When young and very small they grow slowly, then once they are reasonably well established they grow much more quickly, obviously when they are dying, for what ever reason they grow more slowly again, or not at all.

all lichens contain algae

or cyanobacteria

upper cortex

algal layer

medulla

lower cortex

Reproduction In Lichen• Lichens have two fundamentally different sorts of

reproductive bodies. These are 1) spore forming bodies and 2) vegetative reproductive bodies.

• The other main group of fungal partners are Basidiomycetes. Basidiomycetes are relatively rare as lichen partners, and one of the ways they differ from Ascomycetes is that they produce their spores on a basidium, a special structure which normally holds four spores at its top.

• Spores come in a great variety of sizes, shapes and forms. They are for instance, much larger in the genus Pertusaria than in the genus Acarospora. Many are simply a single whole spore while in others the spore may be divided up into 2 or more subsections. These spores are all microscopic in size and float easily in the air, this enables them to disperse widely when they are released. Quite how these spores meet up with the correct algal partners to form new Lichens is something nobody really understands, though as most of the algae occur in the wild on their own it may be that it is simply a matter of chance that some will come to rest on the right algae.

• Apothecia normally look like exposed disks which may be raised above the surface of the thallus, level with it, or sunk below the surface. In some genera/species the apothecia look more like slits, i.e. Graphus sp.. The Asci are held within a mass of special sterile hairs called paraphyses. These paraphyses are composed of fungal hyphae and often have coloured tips. These then give the Apothecia their coloured centres. Differences in the colours of Apothecia can be important in identification.

• Perithecia here the spore bearing body is not open as apothecia but is in a chamber with only a small opening to the outside world, this opening is called an 'ostiole'. Perithecia are often buried almost completely in the substrate on which the lichen is growing. Both these structures release ascospores - spores produced within an ascus (a sack).

• Pycnidia are reproductive bodies which release conidiospores - spores produced from the end or side of special hyphal filaments called conidia.

• Vegetative Reproductive Bodies - Vegetative reproduction is important to many lichens and has the advantage of dispersing both partners at the same time.

Anatomy Of Lichens• Most commonly, photobionts are located in a

layer within the fungal tissue. The layer is generally oriented in a manner that maximises photosynthesis, and is protected from rapid changes in water availability. Each cell or group of cells of the photobiont is usually wrapped by hyphae, and in some cases penetrated by a haustorium. Moribund cells may be digested by the fungus, but for the most part, the photobiont remains healthy during the functional period of the symbiosis. The increased size of cells of the photobiont indicates that reproduction is regulated by the symbiosis.

• The primary photobiont is commonly a green alga. This symbiont is found within a layer below the surface of the lichen. Cyanobacteria may also be held in small eruptions of or under the surface called cephalopodan. Cyan bacteria can fix atmospheric nitrogen, and thus complement the primary activities of the photobiotic, energy fixation.

• In fruiticoselichens, the central core of stems may be hollow. The core may have hyphae oriented in a woven pattern, and the hyphae may be thick-walled and multi-layered.

• The thallus is commonly layered. The thallus may be covered by or enmeshed in extracellular matrix expressed by the fungus. For instance, some crustose lichens have a polysaccharide layer on the surface. The photobiont is located at the base of the polysaccharide layer. Polysaccharide layers may also be found within the cortex of the thallus where their function may be different. The thallus is commonly interleaved by hyphal layers. Some thalli have hydrophobic layers on the surface or within the thallus. The hydrophobicity appears to be related to the presence of hydrophobins expressed by the fungus.Indeed, different hydrophobins act in different parts of the thallus. Finally, the lower layer of crustose lichens lack hydrophobic materials, indicating a role in the uptake of water and solutes to the tissue.

• The matted anatomy of most lichens is particularly important for uptake and storage of water. Though water can be taken up rapidly, even from condensation at night, water is also lost. Thus the anatomy is closely linked to the functioning of the thallus. Water is necessary for metabolic processes, and in the absence of water, the lichen slows or stops its metabolic processes.

Uses Of Lichens• Because they are capable of colonizing bare

rocks and other mineral substrates, lichens are important in soil formation during some ecological successions. For example, lichens are among the first organisms to colonize sites as they are released from glacial ice. In such situations lichens can be important in the initial stages of nitrogen accumulation and soil development during post-glacial primary succession.

• Lichens are an important forage for some species of animals. The best known example of this relationship involves the northern species of deer known as caribou or reindeer (Rangifer tarandus) and the so-called reindeer lichens (Cladina spp.) that are one of their most important foods, especially during winter.

• Some species of lichens are very sensitive to air pollutants. Consequently, urban environments are often highly impoverished in lichen species. Some ecologists have developed schemes by which the intensity of air pollution can be reliably assayed or monitored using the biological responses of lichens in their communities. Monitoring of air quality using lichens can be based on the health and productivity of these organisms in places variously stressed by toxic pollution. Alternatively, the chemical composition of lichens may be assayed, because their tissues can effectively take up and retain sulfur and metals from the atmosphere.

• Some lichens are useful as a source of natural dyes. Pigments of some of the more colorful lichens, especially the orange, red, and brown ones, can be extracted by boiling and used to dye wool and other fibers. Other chemicals extracted from lichens include litmus, which was a commonly used acid-base indicator prior to the invention of thepH meter.

• In addition, lichens add significantly to the aesthetics of the ecosystems in which they occur. The lovely orange and yellow colors of Caloplaca and Xanthoria lichens add much to the ambience of rocky seashores and tundras. And the intricate webs of filamentous Usnea lichens hanging in profusion from tree branches give a mysterious aspect to humid forests. These and other, less charismatic lichens are integral components of their natural ecosystems. These lichens are intrinsically important for this reason, as well as for the relatively minor benefits that they provide to humans.

Ecology• Lichens play an important role colonising

new surfaces. Among the metabolites excreted by some lichens are acids. Acids have the capacity to degrade the surfaces on which they are located, thus releasing minerals for uptake by the thallus. Acidic digestion has the effect of causing the slow disintegration of the surface, especially of limestone and other calcareous materials.

• Lichens grow extremely slowly. Any one thallus may be many decades old. The outer edge is probably the only active component of the thallus, unless the lichen has started to overgrow itself. The inner part is commonly inactive.

• Lichens have the potential to withstand a wide range of environments. Thus they adapt rapidly to local and seasonal changes in temperature and water availability: they are found in bleak artic and desert environments.

• The thallus has the capacity to cope with the frequent aridity of the environment. Foliose thalli will curl as the thallus dries, and then flatten as it rehydrates. Photosynthesis follows the pattern of wetting and drying. While changes in form enable a return from dehydration, the presence of trehalose, and possibly a range of polyols, is also important. These compatible metabolites enable the cytoplasm to desiccate, while protecting the functionality of the enzymes.

• The slow rate of growth and the reliance on minerals in rain or high humidity has consequences for survival of lichens in polluted environments. Lichens absorb all minerals in rain, and the presence of pollutants, including sulphur, will result in the decline of the thallus. Because of their sensitivity to pollutants, most lichens are uncommon in areas affected by acid rain and aerial pollutants. However, some lichens grow on surfaces containing high concentrations of metals, and must be adapted to those metals: single pollutants will select lichens that can tolerate the pollutant. Changing pollution will remove most. In cities, the pollution profile is variable and changing over time. Thus lichens are disappearing from cities.

Conclusion• Lichens are slow growing

associations between fungi and photosynthetic symbionts. They are widespread, and commonly found as primary colonisers on soil-less surfaces. They utilise rainfall for the water and dissolved minerals in the air. The absence of lichens indicates a polluted environment.

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