LICHENS: CHARACTERISTICS, TYPES AND SPECIES - BIOLOGY - 2025

The lichens are symbiotic associations between fungi (mycobiont) and a green alga or a cyanobacterium (photobiont). Lichen-forming fungi cannot survive alone in nature, nor can they generate the great diversity of lichen growth forms or secondary substances without their photobiont.

Most mycobionts belong to a group of Ascomycota called Lecanoromycetes. Most of the photobionts belong to the genera Trebouxia and Trentepohlia (green algae) and Calothrix, Gloecapsa and Nostoc (cyanobacteria).

Lichen. Source: pixabay.com

At first glance, lichens look like plants, but through the microscope we can see the association of millions of photobiont cells intertwined within a matrix formed by the filaments of the fungus. The fungus forms a thallus, which houses the photobiont.

About 8% of terrestrial ecosystems are dominated by lichens. In these ecosystems, vascular plants are at their physiological limit. Lichens have an advantage in their ability to survive extreme cold, heat and water stress, which is why they can remain in a state of torpor.

Lichens are characterized by their distribution, propagation and reproduction, morphology, metabolism, symbiotic interactions, and ecology.

characteristics

Distribution

Lichens are found almost everywhere in the world, mainly in extreme environments such as the desert and high mountains. There is a close relationship between the shape of the thallus (also called the body of the lichen) and its distribution. The thallus has three different growth forms: crustose, foliose, and fructose.

The crustose thallus resembles a bark closely attached to the surface. They cannot be removed without causing destruction of the lichen. Lichens with this shape resist drought and are well adapted to dry climates, such as the desert. An example is Arthopyrenia halodytes that lives in the Mediterranean Sea on calcareous substrates.

The leafy (or leafy) thallus resembles a small shrub. Lichens with this shape grow best in areas of frequent rain. An example is the genus Physma, which lives in the tropical rain forest of Australia, on the bark of trees.

The fruticus (or fruticulous) thallus is filamentous, leaf-shaped. Lichens with this shape use atmospheric water vapor. They live primarily in humid environments, such as cloudy areas on the ocean coast and mountainous regions in the tropics. An example is Ramalina pollinaria that lives on a fir tree (Abies alba) in Switzerland.

Propagation and reproduction

The most common reproduction of lichens is the sexual one of the mycobiont. In this type of reproduction, the mycobiont releases numerous spores that after germination must find a compatible photobiont.

Because spores are genetically diverse, the union of a fungus and a green algae to form a lichen generates great genetic variability in lichens. It should be noted that the photobiont reproduces only clonally, with the exception of photobionts belonging to Trentepohliales.

If the mycobiont reproduces asexually, the photobiont is passed on to the next generation with its mycobiont through specialized vegetative propagules, such as soredia and isidia. These are outward growths through cracks and pores in the surface of the thallus cortex.

Soredia are small clumps of algae cells and fungal mycelia. This mode of propagation is typical of folious and fruity lichens. For example, the Lepraria thallus consists entirely of soredia.

Isidia are small extensions of the thallus that also serve for asexual propagation if they are cut from the thallus. For example, the thallus of Parmotrema crinitum is covered with isidia.

Morphology

The morphology and anatomy of lichens respond to the restrictions imposed by the environment on symbiosis. The mycobiont is external and the photobiont internal. The appearance of the thallus is determined by the mycobiont.

All lichens have a similar internal morphology. The body of the lichen is made up of mycobiont filaments.

The density of these filaments defines the layers of the lichen. On the surface, which is in contact with the environment, the filaments are very compacted forming the crust, which reduces the intensity of light, preventing damage to the photobiont.

Under the bark is a layer formed by algae. There, the density of the filaments is low. Beneath the algae layer is the pith, which is a loose layer made up of filaments. In crustose lichens, the pith makes contact with the substrate.

In foliose lichens, under the medulla, there is a second cortex, called the inner cortex, which is attached to the substrate by hyphae of the fungus that resemble roots, which is why they are called rhizines.

In fruit lichens, the bark surrounds a layer of algae. This in turn surrounds the medulla.

Metabolism

About 10% of the total lichen biomass is made up of the photobiont, which synthesizes carbohydrates through photosynthesis. Between 40% and 50% of the dry mass of lichens is carbon fixed by photosynthesis.

The carbohydrates synthesized in the photobiont are transported to the mycobiont, where they are used for the biosynthesis of secondary metabolites. If the photobiont is a cyanobacterium, the synthesized carbohydrate is glucose. If it is a green algae, the carbohydrates are ribitol, erythrole or sorbitol.

The main classes of secondary metabolites come via the:

- Acetyl-polymalonyl

- Mevalonic acid

- Shikimic acid.

The first pathway products are aliphatic acids, esters, and related derivatives, as well as aromatic compounds derived from polyketides. The products of the second pathway are triterpenes and steroids. The products of the third way are terphenylquinones and derivatives of pulvinic acid.

The photobiont also provides the mycobiont with vitamins. For its part, the mycobiont provides it with water obtained from the air and exposes the photobiont to light so that it can carry out photosynthesis. The pigments or crystals present in the crust act as filters, absorbing certain wavelengths necessary for photosynthesis.

Symbiotic interactions

The terms selectivity and specificity can be used for symbiotic associations. Selectivity is when one organism preferentially interacts with another. Specificity refers to the cell-cell interaction in which there is absolute exclusivity.

It has been proposed that lichens could be considered as a highly selective symbiosis. Some observations that support this idea are:

- Of thousands of genera of algae, very few are photobionts.

- Certain free algae that colonize the same habitats, lichens are not incorporated into them despite being in direct contact.

It has been proposed that in some lichens, such as those of the genus Cladonia, there is strong selectivity and specificity of the mycobiont towards the symbiont alga. Other lichens, such as those of the genera Lepraria and Stereocaulon, exhibit only specificity (in both cases towards the alga Asterochloris).

In general, the specificity is low at the species or population level. Additionally, it must be taken into account that specificity is not the only determinant of composition: the association between individuals is influenced by local environmental conditions.

Ecology

Compared to vascular plants, lichens are poor competitors because of their small size and extremely slow growth. Despite this, the composition of lichen species can influence soil texture and chemistry, increasing coverage and biodiversity.

The presence and abundance of lichens is determined by factors such as the chemistry and stability of the substrate, the availability of light and the humidity of the environment. Thus, lichen communities can change as a result of temperature or water availability.

For this reason, lichens serve as bioindicators of climate change, which can be periodically monitored by analyzing the coverage and species richness of the lichens present in the study area.

Using lichens as bioindicators of climate change has the following advantages:

- Daily measurements are not required.

- Lichens have a long life and are widely distributed.

- Lichen monitoring can be done at stations located in regions with extreme environmental conditions.

The photobionts of some lichens also serve as bioindicators of environmental contamination. For example, the Coccomyxa photobiont is very sensitive to heavy metals.

Types

Lichens exhibit a marked resilience, being capable of establishing themselves in inhospitable environments for other living beings. However, they can also be highly susceptible to human-caused disturbances to the environment.

Lichens can be classified according to the environment in which they grow, their pH requirements, or the type of nutrients they take from the substrate. For example, based on the environment, lichens are divided into saxicoles, corticosteroids, marine, freshwater, and follicles.

Saxicultural lichens grow on rocks. Example: Peltula tortuosa, Amandinea coniops, Verrucaria elaeina.

Corticultural lichens grow on the bark of trees. Examples: Alectoria spp., Cryptothecia rubrocincta, Evernia spp., Lobaria pulmonaria, Usnea spp.

Marine lichens grow on rocks where the waves beat. Examples: Arthopyrenia halodytes, Lichina spp., Verrucaria maura.

Freshwater lichens grow on rocks on which there is moving water. Examples: Peltigera hydrothyria, Leptosira obovata.

Follicular lichens grow on rainforest leaves. Species of this type serve as microclimatic bioindicators.

Taxonomy

Because they are polyspecific organisms and are considered as the sum of the mycobiont and mycobiont, lichens lack formal status in the taxonomy of living organisms. The ancient taxonomic classifications of lichens as single entities developed before their symbiotic nature was recognized.

The current taxonomy of lichens is based exclusively on the characters and phylogenetic relationships of the mycobiont. For this reason, all lichens are classified as fungi.

Currently, the orders, families and genera of lichen-forming fungi are delimited by the characters of the fruiting bodies. Lichens with thalluses, although they are morphologically different, remain united within the same family or genus. Other structures, such as isidiums and soredians, are also considered.

98% of species of fungi that form lichens belong to the Phylum Ascomycota. Most of the remaining species belong to the Phylum Basidiomycota. Regarding the photobionts, 87% of the species are green algae, 10% are cyanobacteria and 3% are a combination of green algae and cyanobacteria.

Molecular studies have made it possible to modify the concept of species based on morphology. Likewise, secondary metabolite studies have allowed the separation of morphologically similar species.

Representative species

Trophic chains

Because lichens are primary producers, they serve as food for herbivorous animals. In North America and Eurasia, large herbivorous mammals, such as reindeer and caribou, feed on the lichen Cladonia rangiferina. In winter, these herbivores can eat between 3 and 5 kg per day of this lichen.

C. rangiferina, known as reindeer lichen, belongs to the class Lecanoromycetes and to the family Cladoniaceae. C. rangifera can reach a size similar to that of typical vascular plants. It is gray with a fruit-like thallus.

Species belonging to the genus Cladonia are tolerant to high concentrations of metals, which is why they can store high concentrations of radioactive derivatives of strontium and cesium. The consumption of this lichen by animals represents a problem, because it can reach harmful levels in men who eat these animals.

Perfume industry

Evernia prunastri, known as oak moss, and Pseudevernia furfuracea, known as tree moss, are species of lichens important in the perfume industry. They belong to the Lecanoromycetes class and to the Parmeliaceae family.

Both species are collected in the south of France, Morocco and the former Yugoslavia, processing about 9000 tons per year. In addition to being useful for the perfume industry, P. furfuracea is sensitive to contamination, which is why it is used to monitor industrial contamination.

Applications

Lichens are rich in pigments that serve to block ultraviolet B (UVB) light. The cyanobacteria of the lichen Collema is rich in this type of pigments, which have been purified and patented as a product that gives 80% protection against UVB.

The cyanoliquen Collema cristatum, for example, has a pigment called collemin A (ʎ _max = 311 nm), a mycosporin that is the one that provides UVB protection (280-315 nm).

Roccellla montagnei is a fruity lique that grows on rocks, from which a red or purple dye is obtained in the Mediterranean region. Other lichens such as Heteroderma obscurata and Nephroma laevigatum contain anthraquinones used as dyes.

Lichens have substances that could be used by the pharmaceutical industry. Many species of lichens have active compounds that kill bacteria such as Staphylococcus aureus, Pseudomonas aeruginosa, Bacillus subtilis, and Escherichia coli. Additionally, lichens have high potential as a source of anticancer drugs.

References

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