Lichens are stable, self-supporting associations between fungi and photobionts. The fungi are most commonly Ascomycota. A few Holobasidiomycetes are known to form the association. The photobionts are either algae or cyanobacteria. The algae may be single celled or, rarely, filamentous. In most cases, neither partner is found living alone in the wild LINK. The fungus most commonly forms the majority of the biomass of the lichen.
Lichens are common primary colonisers of stressful habitats. They are major structural determinants of extremely cold environments, and extremely dry climates. Thus, we commonly see lichens in the Arctic and Antarctic, and as crusts on desert soils. They are also seen as the surface layer of trees, rocks, roofing tiles and exposed surfaces in many different environments. In moist environments, the foliar forms are more obvious, being attached to limbs and either growing up or falling down from the plant. LINK Lichens are not found in heavily shaded sites because they require light to support their photosynthesis.
In general terms, the photobiont supplies the association organic carbon from photosynthesis, and the mycobiont ensures protection, and regulates the supply of minerals and water. The nutritional exchange between partners is probably much more complex than exchange of water and minerals for organic carbon. For example, cyanobacteria commonly fix dinitrogen. Thus the lichens containing cyanobacteria obtain their organic nitrogen directly. The source of nitrogen in the absence of fixation is presumed to be the air especially following the reduction of dinitrogen by lightening. In filamentous and gelatinous lichens, the photobiont may form a larger proportion of the lichen. In gelatinous lichens, the cyanobacterium exudes a polysaccharide that functions to absorb and retain water, a function normally attributed to the fungus.
While each lichen appears to be a homogenous structure, a single thallus may contain several different species of photobiont and fungus. Even in a single species of fungus, several different genotypes may coexist in the one thallus. Thus, description of the form and function of lichens are generalisations that need not reflect the complexity within a single unit.
Lichens may be flat structures that are appressed to a surface, leafy forms that have multiple attachments to their surface, or foliar forms that have a single attachment to a surface with the larger part either erect or hanging from the attachment. The fungus primarily determines the form, but the photobiont may also influence it. Thus, complex forms are possible within one lichen.
CRUSTOSE: The simplest form of lichen is a crust on the surface. Crustose lichens are highly variable in anatomy. However, they all tend to be adnate or attached directly to their surface. Their growth tends to be radiate, in that the mitotic regions are at the margins, and the centre is more likely to be dying.
FOLIOSE: Foliose lichens have a sheet-like structure, and are attached to their surface by root-like rhizines. The thallus is highly differentiated, with the lower surface being an absorptive tissue and the photobionts being held in a manner that maximises photosynthesis. Commonly, the upper surface is fungal tissue, with the mid-layer containing the photobiont. Growth takes place at the margins, and these tend to be lobed.
FRUTICOSE: Fruticose or umbilicate lichens are attached to their surface by a holdfast. The main body of the lichen is either erect or pendulous, and commonly highly branched. Growth takes place at the ends of the “stems” and may be quite complex.
EVOLUTION: Colonies of rock-surface fungi may be found living in close association with free-living algae. In co-culture, the organisms have been observed to initiate a close association. Hyphae branch increasingly as they near the photobiont, presumably because the fungus detects an increase in organic energy leaking from the photobiont. The fungus releases mucilage around the contact zone, and close specific contact may result. Interestingly, no evidence of an antibiotic effect has been reported. These observations led to speculation that lichens are the result of the interactions between rock-surface fungi and algae. However, many fungi form lichens and few have relatives among the rock-surface fungi.
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 cephalopodia. Cyanobacteria can fix atmospheric nitrogen, and thus complement the primary activities of the photobiont, energy fixation.
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. LINK 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.
In fruticose lichens, 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 core may serve a number of functions, including strength and stability.
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.
Lichens may reproduce vegetatively or sexually. The fungi form typical reproductive units. LINK Sexual reproduction tends to result in fungal and algal propagules that following germination must meet with a compatible partner before a functional lichen can form. Sexual reproduction is not considered to be a common means of reproduction to form new units. However, propagules may become entangled with an existing lichen. By initiating growth within an existing matrix, genetic complexity may be added to the unit. Reproduction of basidiomycetous lichen fungi appears to be largely sexual, as these lichens appear to lack specific asexual units.
Asexual reproduction can take place by release of diaspores or vegetative fragmentation. In diaspores, called soredia, small groups of photobionts become surrounded by hyphae in a crack or zone of the surface called a soralia. The soredia are dispersed when the soralia are opened, either by drying of the lichen surface, or by death of the surrounding tissue. Wind lifts the soredia away from the upper layer, and the propagules are deposited on a new surface.
Isidia are another type of diaspore. These form as elongated outgrowths of the thallus. The isidia break off and can be dispersed as for soredia. Intermediate forms are also found, and isidia may also function as photosynthetic units while still attached to the lichen. The fragmentation of a dried thallus can lead to the dispersal of that organism. This is particularly so with fruticose lichens. Their large thalli are readily shattered and fragments dispersed by wind. Some lichens appear to fragment during drought, resulting in a considerable subsequent increase in lichen units. The fragments have an innate capacity to survive dry conditions. On absorbing moisture, they can regenerate and re-establish a living thallus.
Overall, the patterns of asexual reproduction tend to blend one into the other. Numerous terms have been used to describe these forms leading to some confusion for the beginner. Due to the flexibility of the thallus, the importance of the differences among these forms is limited.
Some 95% of lichens are formed by fungi in the Ascomycota. The fungi are paraphyletic, and have been placed in at least 12 orders. The remainder are formed by Holobasidiomycetes. Sexual structures formed by the fungi are similar to closely related non-lichenised fungi, indicating that lichenisation has evolved independently many times.
Lichens are renowned for the extraordinary diversity of their secondary metabolites. All are formed by the fungal partner. Further, production of secondary metabolites by a basidiomycete partner is unknown, and thus these comments relate to the Ascomycota. Most secondary metabolites are formed as part of the Acetate-polymalonate biosynthetic pathway. Compounds formed from the Mevalonic acid pathway and the Shikimic acid pathway are also common. The compounds are detected in the wall or as a layer outside the wall in most cases.
The function of many of these compounds is unknown. However, some have been used in industry, such as for the production of litmus. LINK. The distribution of compounds may change within the thallus. Compounds commonly extracted from the surface are not necessarily found in the middle layers or at the base of any one thallus. Chemotaxonomy has been used for a long time in lichens. However, the taxonomic significance of the data is still unclear, especially given that a single thallus may be formed from many species of either or both fungus and alga.
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.
"Rusting" of surfaces is probably unimportant in terms of the total uptake of minerals by lichens. Most minerals are extracted from solutes in rain or surface water flow.
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. LINK Thus, primary production of lichens is highly dependent on the moisture levels of the environment, and thalli can survive desiccation.
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.
Remnants of lichen communities within cities are associated with protected habitats. Church yards for instance may house a wide diversity of lichens. Lichens are not welcome inhabitants of city surfaces, however. The capacity of lichens to "rust" the surface leads to loss of the structural integrity of stone and concrete. Attempts to remove lichens, and prevent the re-colonisation of grave stones and other surfaces, are rarely successsful.
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.
Baron G (1999) Understanding Lichens. Richmond Publishing, Slough, England. Chapters 10, 11 and 12 of The Mycota, Vol IX, Fungal Associations, Springer, Berlin.
Lumbsch HT (1998) Taxonomic use of metabolic data in lichen forming fungi. In Chemical Fungal Taxonomy, eds Frisvad JC, Bridge PD and Arora DK. Marcel Dekker, New York.
Nash TN (1996) Lichen Biology. CUP, Cambridge, UK.