Lichens are organisms that arise from the symbiosis between a fungus called mycobiont and an algae or cyanobacterium called a phycobiont. According to the nature of this association, numerous structural types of lichens can be distinguished: from the simplest, where fungus and algae meet casually, to the most complex, where the mycobiont and phycobiont give rise to a thallus morphologically very different from that which they form separately, and where the algae is forming a layer under the protection of the fungus.
Lichens are multicellular organisms, exceptionally resistant to adverse environmental conditions and, therefore, capable of colonizing very diverse ecosystems. The protection against desiccation and solar radiation provided by the fungus and the ability of photosynthesis of the alga give the symbiote unique characteristics within living beings. The synthesis of compounds only present in these organisms, the so-called lichenic substances allow a better use of water, light and the elimination of harmful substances.
[ hide ]
- 1 Taxonomy
- 2 Evolutionary History
- 3 The symbiosis
- 4 Type of bionts
- 1 Photosynthesis
- 2 Mycobionts
- 5 Organization of the lichenic thallus
- 6 Thallus morphology in the symbiote
- 1 Crustacean thallus
- 2 Foliose talus
- 3 Fruitful thallus
- 4 Heteromorphic talos
- 7 Reproduction
- 8 Ecology
- 9 Reference
The group of organisms that we call lichens is a polyphyletic group, that is, coming from a multitude of different ancestors that has evolved towards the same pattern based on different relationships; even so there is no classification for this group fully accepted by all experts. The classification of Ozenda and Clauzade (published in 1970) attends first of all to the type of fungus that forms the symbiosis, thus three classes are distinguished: Ascolichenes, Basidiolichenes and Hypholichenes; depending on whether the fungus is an ascomycete, a basidiomycete or a deuteromycete, respectively. Within the first group, two subclasses Pyrenolichenes and Discolichenes are differentiated according to whether they have perithecia or apothecia. Ascolichenes make up 96% of lichens with very few basidiolichenes.
The taxonomic position of lichens is currently under investigation and the tendency to group them within the ascomycotic fungi in the class Lecanoromycetes is widespread, which is what concentrates most of the known mycosymbionts; this classification, however, leaves out of the definition of lichen those formed by basidiomicotes and oomicotes. Another trend includes all known lichens within the Mycophycota Division of the Fungi kingdom, but considering that the only characteristic that allows this division to remain is the formation of symbiosis, not its cytological, genetic or phylogenic characteristics. The most recent classifications based on genetic studies offer more in-depth links between the different families of fungi, although there are still several of them whose phylogenetic position is not entirely clear, and amendments and rectifications are continually appearing for these models. It is possible that the genetic studies currently being carried out end this dilemma by confirming the group’s polyphilesis or by identifying the common origin of these organisms.
Fossil remains of lichen are extremely rare. It is known in the world of paleobotany that the incomplete fossil record cannot show at all the reality of the flora of the time to which it belongs; that is why it is necessary to deduce from the few conserved traces and from the phylogeny at which point many of the plant groups appear and in our case the symbiosis between an alga and a fungus.
The oldest specimen identified with a lichen Thuchomyces lichenoides dates from the Precambrian. It would be a marine species according to the sediments in which its fossils were found and although the mycobiont has been identified, the evidence for the existence of a photobiont associated with it is inconclusive. Even with this, the site of Rhynie Chert has given an example of a fossil lichen called Winfrenatia reticulata of extraordinary scientific value since it places this group in the Devonian era and can therefore consider this fossil as the oldest known to the group. Another representative of the group that appeared in the Middle Devonian is Spongiophyton although its affiliation is doubtful. Fossil remains of the Carboniferous, Permian and Triassic are missing and even the Paleogene are very few and inconclusive. From the Eocene the epigid species Strigula is known, from the Oligocene the species Anzia sp, Calicidum sp and Chaenotheca | and from the Miocene Chaenothecopsis bitterfeldensis. Co-evolutionary considerations and phylogenetic studies linking current distributions and continental movements have suggested that the lichen lifestyle is very old. It seems likely that many of the current families, genera, and in some cases species evolved in Permian / Triassic times, about 190-280 million years ago, from a few previous species. There are theories that maintain that lichens could be the first species to colonize the terrestrial environment, a theory that is too controversial and with tests that are still very inconsistent. Calicidum sp and Chaenotheca | and from the Miocene Chaenothecopsis bitterfeldensis. Co-evolutionary considerations and phylogenetic studies linking current distributions and continental movements have suggested that the lichen lifestyle is very old. It seems likely that many of the current families, genera, and in some cases species evolved in Permian / Triassic times, about 190-280 million years ago, from a few previous species. There are theories that maintain that lichens could be the first species to colonize the terrestrial environment, a theory that is too controversial and with tests that are still very inconsistent. Calicidum sp and Chaenotheca | and from the Miocene Chaenothecopsis bitterfeldensis. Co-evolutionary considerations and phylogenetic studies linking current distributions and continental movements have suggested that the lichen lifestyle is very old. It seems likely that many of the current families, genera, and in some cases species evolved in Permian / Triassic times, about 190-280 million years ago, from a few previous species. There are theories that maintain that lichens could be the first species to colonize the terrestrial environment, a theory that is too controversial and with tests that are still very inconsistent. Co-evolutionary considerations and phylogenetic studies linking current distributions and continental movements have suggested that the lichen lifestyle is very old. It seems likely that many of the current families, genera, and in some cases species evolved in Permian / Triassic times, about 190-280 million years ago, from a few previous species. There are theories that maintain that lichens could be the first species to colonize the terrestrial environment, a theory that is too controversial and with tests that are still very inconsistent. Co-evolutionary considerations and phylogenetic studies linking current distributions and continental movements have suggested that the lichen lifestyle is very old. It seems likely that many of the current families, genera, and in some cases species evolved in Permian / Triassic times, about 190-280 million years ago, from a few previous species. There are theories that maintain that lichens could be the first species to colonize the terrestrial environment, a theory that is too controversial and with tests that are still very inconsistent.
In an attempt to present together the orders of the Ascomycotina subdivision in relation to their biology, it was found that the Peltigerales order occupies a more or less central position. It was also observed that this order has a particular meaning in the evolution of ascomycetes, since it includes some genera that are essentially earthlings, although capable of spreading on rough bark and trees; that is, they generally occur in primary habitats, which have existed prior to the growth of phanerogams. Cyanobacteria happen to be one of the first photosynthetic organisms; present in the earth since the Precambrian, that is why this type of photobiont could have been present for fungi capable of associating with it from the evolutionary beginning of the group
The base of the symbiosis is the taking of nutrients by the fungus from the algae; For this reason, in almost all the lichens studied, some form of penetration of the fungus into the algal cells has been found, which is achieved by means of haustoria. Two types of haustoria or fungus penetration organs are distinguished: intracellular and intramembrane. In crustacean or crustose (crusting) lichens and in some more highly structured forms the penetrations are generally intracellular where the haustoriae penetrate the protoplast of the gonidial layer (where the algae are found); the wall of these haustoria is thinner than that of the rest of the hyphae, making it easier for them to penetrate the plant cell. In the most morphologically evolved lichens the haustoria are intramembrane; in these cases they penetrate the wall of the gonidial layer but not the cytoplasm, leaving an invagination in the wall of the alga. Lichen gets its food from the substances synthesized by the algae through photosynthesis; In this process, the carbohydrate called ribitol is synthesized, which is transferred to the fungus by diffusion. Inside the hypha, this ribitol is modified to mannitol, which is not transferable from the fungus to the algae. In this way the mycobiont ensures its food and the algae continues to synthesize. When the photobiont is a cyanobacterium, the synthesized carbohydrate is the glucose that the fungus also modifies to mannitol. The algae, for its part, obtains the necessary protection from the fungus against desiccation, an increase in its water absorption capacity thanks to the characteristics of the fungus hyphae. In short, the symbiosis allows the algae or cyanobacteria to colonize ecosystems where due to extreme weather it could not develop on its own.
The metabolites of lichens are usually a mixture of those produced by the algae for its own function and those produced by the fungus. There are few cases in which own substances are produced for lichen that none of the biontes could produce by itself; Among these substances are the lichene substances, a very heterogeneous set of specific products of the lichens produced by the fungus, many of them called lichene acids. About 200 types of lichen substances are known, although research constantly adds new types, so many are thought to be unique to a single species, meaning that it is the symbiosis itself that produces them. It is worth clarifying that these substances do not appear in the lower cortex or in the gonidial layer and in almost all cases they appear as tiny crystals and granules arranged on the surface of the hyphae. The function of the lichenic substances is not entirely clear; they are thought to act as a deterrent to herbivores or to protect against bacteria and other pathogens. It is also possible that they play an important role in the absorption of water or increasing the permeability of the algae membrane, thereby allowing metabolites to enter the cell interior. They are known to act as protection against various pollutants, UV radiation and even radioactivity. Some of them are simply wastes of the daily cellular activity of the symbiote organism.
It can be said that the formation of symbiosis, especially mutualistic by fungi with all kinds of photobionts is a characteristic that has given an enormous evolutionary advantage to the species that form them, at least this can be deduced from the data that on species that they form some kind of symbiosis we have, so of the total of 64,200 species of fungi, around thirty percent (19,000 species) have opted for this type of association, more than eight percent form symbiosis as mycorrhizae and twenty-one percent form the lichenic associations.
Among the largest groups of fungi, the number of species involved in lichene associations is enormous, for example 98% of these fungi belong to the Ascomycota subdivision, but of the forty-six known fungal orders sixteen form lichens and only six of them they only form lichens, that is, they are not known as free-living fungi. There are relatively few lichenized hymenomycetes, Dictyonema, Multiclavula, and Omphalina, a lichen mastigomycete (Geosiphon pyriformes), and about fifty genera of conidial fungi.
A priori it can be affirmed that from the evolutionary point of view the formation of lichene symbiosis is a very favorable evolutionary strategy and that it is constantly evolving in some fungi, for example in Hymenomycetes or Helotiales, although it seems that it is also being lost in others, such as in Callices and Lecanorales.
When the mycobiont is an ascomycete, it is called the symbiosis with the algae ascolichens, which presents the typical fructifications of ascomycetes, in the case of perithecia, they are called pyreneolichens (pyremyomycetes) and if they are apothecia, discolichens (discomycetes) ); in the event that the mycobiont is a basidiomycete, the symbiote basidioliquen is called.
The phycobiont is usually a chlorophyte, more rarely a cyanophyte; sometimes triple associations occur, as in the genus Lobaria, an ascolichen, with a chlorophyte as a phycobiont, which, however, has special structures, called cephalodes, in which an association with a cyanophyceae (Nostoc) occurs. These cephalodes develop in those lichene symbiotes in which an extra symbiosis with a cyanophyceae is necessary to be the main symbiote algae incapable of fixing the atmospheric nitrogen necessary for the fungus. Due to their dual nature, lichens have characteristics both of the fungus like algae, despite presenting its own and particular characteristics.
Approximately forty genera of algae and cyanobacteria are currently known to act as photobionts in lichene symbiosis. Of these three genera are the most frequent, Trebouxia, Trentepohlia and Nostoc, the first two green algae and the third cyanobacteria.
Eukaryotic photobionts are known as phycobionts while cyanobacterial photobionts are known as cyanobionts; the vast majority of phycobionts are green algae (phyllum Chlorophyta) that have chlorophyll a and b, and only two genera (phyllum Heterokontophyta) have chlorophyll a and c. The metabolic transfer between photobionts and mycobionts depends to a great extent on the type of photobiont present in this way. When it is a green alga, the shared carbohydrate has alcohol groups (ribitol), whereas in lichens with cyanobacteria it is glucose.
Identification of cyanobacterial photobionts in lichenic thallus is sometimes impossible because the thallus morphology of the cyanobacterium changes due to the presence of the mycobiont, therefore the filamentous forms as occurs with the genus Dichothrix are usually tremendously deformed; only branched filaments of genera such as Stigonema can be identified in the lichen thallus.
Cyanobacteria also do not show all the phases of their life cycle when they are part of a lichen, this makes their identification even more difficult, as happens in genera such as Chloroccidiopsis and Myxosarcina; The observation of all the vital forms of this type of organism is usually essential for its correct taxonomization at the species level, it is necessary in the work with phycobionts the isolation of the algae and its subsequent cultivation in free life. The type of vegetative reproduction is also a tremendously important character in the taxonomy, thus species with heterocysts (such as Nostoc) increase the frequency of their production when they are in symbiosis compared to when they live freely;
In green algae the organization of the thallus is always very simple when they act as photobionts, only structural forms are known in coconut, sarcines and filaments where the filamentous structures are usually very small in size. Identifying the green algae that forms lichen is often easier than when working with cyanobacteria, at least on many occasions it is not necessary to isolate the algae and cultivate it to be able to identify it at least up to the genus level, to fine-tune to the species itself. Cultures are essential because the morphology of chloroplasts and various stages of the algal life cycle are unchanged or absent in symbiosis with the fungus.
Lichen-forming fungi are largely obligate symbiote cases and are unable to live in isolation in the environment; they only prosper when they find a suitable photobiont, in isolated culture they show their imperfect form, being able to produce asexual spores but practically never produce organized reproductive structures such as non-lichen fungi or a thallus structure that resembles that of lichen.
Currently, there is no evidence to confirm that the fungi that are part of the lichen symbiosis are morphologically different from those of the free-living species. It is true that spherical organelles of unknown protein function and nature have been found in some groups of Ascomycotas. They are found in free fungi and that some authors have tried to grant functions related to symbiosis, but the latest research indicates that it may be more like organelles of the lichen-forming Ascomycota species in which it was found, and not due to symbiosis with the alga.
These concentric bodies have subsequently been located in the cyclasm of mycelia, ascogenic cells, paraphysis and all kinds of cells in pathogenic fungi of plants and saprobes, especially in dry environments, so it is probable that they have a function of storing substances or resistance. to desiccation, which is necessary, yes, in lichenized fungi.
The main difference between lichen and non-lichen fungi is of course the type of nutrition they perform. Many fungi appear in nature only as part of lichens, although in cultivation they have been isolated from the algae and have been able to survive.
Cultures of these lichen-forming fungi in isolation have almost always given a phenotype very different from that expressed when they form symbiosis, a large proportion of these fungi have presented a characteristic thallus formed by agglutinated cell masses with filamentous growth only in the periphery; However, some of the most interesting groups, such as the Perigerales, are only able to grow when they are part of a lichen, in this way spore cultures of various species in this group have been cultivated isolated from the algae and although they have germinated their mycelium has been unable to develop, only after the addition of a compatible photobiont the now lichen has grown and with it the fungus
Organization of the lichenic thallus
Homomeric thallus is the one in which the photobiont and mycobiont are uniformly distributed. It is the gelatinous lichens that mainly have a homomeric structure of their thallus. These lichens are capable of absorbing more water than non-gelatinous lichens in relation to their dry weight. This makes the gas exchange very limited in these organisms since it is possible that the thallus is saturated with water and therefore the diffusion of carbon dioxide (CO2) is greatly hindered. For this reason, the presence of CO2 in these lichens is a limiting factor for photosynthesis. Recently, various CO2 concentration methods have been described that allow it to accumulate in the thallus for subsequent use by the photobiont.
Heteromeric thalli are those in which the photobiont and mycobiont occupy different layers within the lichen. The thallus is divided into several layers, on the one hand a superficial cortex appears, with very tight fungal hyphae where traces of the algae are usually not found. Then the so-called gonidial layer appears, with lax hyphae mixed with algal cells, it is the region where photosynthesis occurs by the algae and its interaction with the fungus is most evident by the presence of haustoria. Finally the marrow with loosely packed hyphae of the fungus. The gas exchange in this type of lichens is carried out by means of structures called pores, cifelas and pseudocifelas, more or less depressed areas of the cortex with lax hyphae of the fungus that leave a narrow pore.
Thallus morphology in the symbiote
There are few examples of lichens whose morphology is determined by the photobiont (it occurs for example in the genera Coenogonium, Ephebe, Cystocoleus or Racodium), in most cases it is the mycobiont that sets the growth patterns for the symbiote. The development of a certain type of thallus is important to know the relationships that will be established in the symbiote, for this reason the group has traditionally been divided into the morphology of its thallus in crustacean, foliaceous and fruit-bearing lichens, despite this classification there are other biotypes possible in lichens, like the so-called gelatinous ones that some authors, however, include in the previous types.
The so-called crustacean thallus are those that grow strongly attached to the substrate, to the point that it is impossible to separate them from it without destroying it. The thallus characteristics of this type of lichens allow them to survive in very extreme environments and on exposed rock surfaces. They have both homomeric and heteromeric organization, especially on the margins of those that have large areolas or in intermediate species with foliose lichens. They do not have an inferior cortex; Its growth is marginal, many times several individuals can overlap and form characteristic patchy structures; the simplest form of organization is present in genera such as Lepraria in which the mycobiont hyphae wrap small groups of algae without the possibility of differentiating them, the thallus has a powdery appearance as affected by leprosy, reason for which the genre takes the name; In the genus of epiphytic lichens Vezdaea the photobiont is organized in small granules or soreds of just a millimeter in diameter. The structure of endolytic lichens (which live inside rock microcracks) or endophylodic (under the cuticle of plant leaves) is much more complex; in many cases there is a differentiation between the upper cortex and the rest of the thallus; various genera such as Buellia and Lecidea even develop thallus stratification in the absence of the substrate; The latter genus places the photobiont in the medulla and can extend up to two millimeters inside the sandy grains of the rocks in which they live, and many more enter, as happens in the Lecidea sarcogynoides species in South Africa where various individuals have penetrated up to 9.6 millimeters deep every hundred years. Crustacean lichens, which live strongly attached to the surface of rocks, can have very diverse morphology. In this way we find species with unlimited margins, outlined that hardly differ from the substrate. Lichens with well-defined edges, lighter or darker in color than the rest of the individual and well differentiated from the medium. Talos figurados, lobed radially and with edges loosely attached to the substrate and can even be separated from it.
The epilitic lichens are the most abundant within crustacean lichens, their thallus is usually perfectly limited in its margin or with imprecise limits. A areolado talo is one in which the symbiote is distributed in multiple portions (areolas) of polygonal shapes, in the dry season each of the areolas of this organism are perfectly distinguishable while in the wet season they are not; the areolas develop from a primary thallus by extension towards its entire circumference. The most complex thallus within crustaceans is the so-called squamous where the areolas grow until they partially separate from the substrate, forming the characteristic scales that give the phenotype its name.
Foliose lichens are those in which the thallus is partially detached from the substrate and not as intimately related to it as in the previous ones. The talos can be homomers or heteromers. The most usual is that they have a dorsiventral organization, distinguishing between ventral and dorsal areas. Within this type of lichen there is an enormous diversity in terms of shapes, organization and sizes.
The lacynose lichens are those that have the typical structure of foliose lichens; they adhere to the substratum in almost all its extension and they possess to a large extent of the lobe species whose distribution in the thallus is one of the most varied, radial, alternating, etc. In some species the lobes may be inflated by increased growth of the medulla. They are the lichens that reach larger sizes within the group and present a wide range of colors, consistency and shapes, the genera Xanthoparmelia, Physcia and Solorina being an example of them. An umbilized lichen is called when it has a circular stem with a single anchorage to the substrate in the center, as occurs in Umbilicaria.
A very particular type of foliose lichens grow in deserts and have an interesting hygroscopic movement, in times of drought they are able to roll up on themselves to show the least amount of surface possible and thus avoid desiccation by exposing their lower surface formed by hyphae of the fungus, also in a coiled state they are capable of being transported by the wind, usually to shady places such as stone bases or thickets waiting for the arrival of moisture, this occurs in species such as Xanthomaculina convoluta or Chondropsis semivirdis.
The thallus of the fruit lichens is elongated, cylindrical or very narrow in all cases resembling a head of hair, they generally have a single point of attachment to the substrate, leaving the rest of the organism far from it; they can branch, sometimes very profusely, have apical or intercalated growth and can be solid or hollow in the case of homomers and flattened heteromers. However, there are exceptions to this general morphology, so Sphaerophorus melanocarpus has dorsiventral symmetry although the width of the lobes is very small. The size of these lichens is highly variable according to the species, for example the genus Usnea grows several meters while other species barely grow a few millimeters. The union to the substrate is carried out by means of special fixing structures that in some species degenerate at maturity leaving the individual free of the substrate. Examples of this type of lichens are those belonging to the genera Stereocaulon and Roccella.
Some genera develop two types of morphology in their thallus, for example, in the genus Cladonia, the thallus on which apothecia is developed, called podethium, is fruitful while the union to the substrate is scaly or crustacean.32 They are called dimorphic thallus; in them, a difference is made between a horizontal thallus or primary thallus attached to the substrate and another vertical or secondary thallus bearing the fruiting bodies. It is possible that in the adult state the primary thallus is lost as occurs in Cladina.33 In some species the secondary thallus is formed by carpogenic tissue, this means that it is part of the fruiting body while in other species the development of the fruiting body is carried out. out at the end of the secondary thallus itself not being part of them.
Lichens can reproduce asexually from thallus portions with representatives of the two bionts in the so-called talin fragmentation or from specialized structures called sorediums (which in turn can be grouped into so-called soraliums) and isidia. A soredium is a grouping of hyphae of the fungus surrounding a few elements of algarises that detach from the surface of the lichenic thallus to be disseminated either by the wind or by splashes of rain, this structure completely lacks internal organization. On the contrary, the Isidia are structured in the same way as the lichenic thallus; They are thallium portions that develop on the surface, preserving the structure in layers and cortexes and that can be easily detached. These types of asexual reproduction are the only processes by which the entire structure of lichen is spread, and not just one of its components. It should be noted that these structures are created exclusively by the fungus since this is the only component that needs symbiosis for its survival, or at least the one that has the most advantage in the union. The evolutionary advantage that the formation of these structures implies shows the goodness of the symbiosis for the mycobiont and its need to ensure a photosynthetic organism for its survival. or at least the one with the most advantage in the union. The evolutionary advantage that the formation of these structures implies shows the goodness of the symbiosis for the mycobiont and its need to ensure a photosynthetic organism for its survival. or at least the one with the most advantage in the union. The evolutionary advantage that the formation of these structures implies shows the goodness of the symbiosis for the mycobiont and its need to ensure a photosynthetic organism for its survival.
The rest of the reproductive structures formed in the symbiote do not involve the two bionts. Probably due to limitations imposed by the mycobiont that the photobiotic element is not yet known, it is unable to reproduce itself while it is part of the symbiosis. For its part, the fungus is capable of reproducing asexually and sexually according to the characteristics of the group to which it belongs, ascomycete, basidiomycete or other. The fungal reproductive components are able to spread in search of an alga to associate with, which will be the same species with which it previously formed the symbiosis or a different one depending on the specialization of the fungus. Sexual spores formed in ascomycotes are characteristically found in perithecia and apothecia;
The fungus can also develop asexual reproductive structures on its own; a single species of lichen (Micarea adnata) presents the structure called sporodochium formed by a series of branched or simple conidiophores similar to acérvules and synemas. The most common asexual reproductive structure is pycnidium, an open or closed cup-shaped or sphere receptacle that contains a large number of conidia producing conidiophores inside. This particular type of spore that is produced in large numbers is able to remain in the medium for a long time waiting to find the right algae or chiantophyte to associate with, as is the case with sex spores.
These organisms are primary colonizers in almost all known ecosystems, their ability to adapt to low-nutrient media makes them capable of early development and soil formation for the subsequent arrival of other plant organisms. Lichens are very specific organisms regarding the substrate and the conditions of the environment in which they grow. It is possible to find lichene symbiotes in extremely hostile environments for life such as polar or desert areas where the characteristics that symbiosis gives them allow their development. According to an experiment carried out by the European Space Agency during 2005, two species of Antarctic lichens were able to survive in space without any protection.
This great survival capacity has allowed the diverse species of lichene symbiotes to colonize and prosper in practically all terrestrial ecosystems. There are several species of marine lichens corresponding to several rows of Ascomycotic fungi and that in a great part of the cases they possess the alb Trebouxia as photobiont. These lichens have as their main habitat the intertidal zone where the beating of the waves prevents the growth of many algae, in this way the symbiosis would benefit the algae not so much in terms of water storage but regarding mechanical protection. It is possible to recognize in these communities a vertical zoning of the environment where organisms develop,
Lichens are used as a biological indicator of air quality mainly due to their susceptibility to the presence of sulfur dioxide in the medium. Young thallus are much more sensitive to this environmental contamination than those that are fully developed. The production of herds is notably reduced in contaminated media, so the spread of these organisms is significantly reduced. Sulfur dioxide is the main responsible for the acidification of rainwater (acid rain) necessary for the growth of these organisms, the thallus response to this type of contamination is the creation of hydrophobic lichenic substances and the reduction of the surface exposed to rain so that photosynthesis is reduced and with it growth apart from the amount of usable water. The recovery, however, of these organisms has turned out to be spectacular once the environmental conditions return to normal. Thus, according to studies carried out after the elimination of a large part of the environmental pollutants of the city of London at the end of the 20th century, lichens have progressively returned to colonize those habitats that were appropriate for them