The Ancient World of the Colourful and Enduring Lichens
Lichens are composite organisms consisting of a mutualistic symbiotic association between a fungus and one or more photosynthetic partners. Unicellular green algae are by far the most common, but cyanobacteria (also called blue-green algae or blue-green bacteria) are also found in lichens as well both group of organisms can be found in the same lichen. The fungus is called the mycobiont while the photosynthetic partner is called the photobiont or phycobiont, when referring to algae only. Lichens are perhaps the most common aspect of the life of fungi visible to us. They grow on stones and on walls, painting them in vivid colours. They also hang in festoons from trees and grow in water and on rocks wet with salty spray from the oceans. They are found in all continents with the most diverse colours, forms and shapes. They cover hundreds of square miles of Arctic tundra, the floor of cool forests and stabilize the soil of deserts. Also, they literally live on the South Pole, under the most extreme and harsh conditions that no plant or animal can bear. Some lichens have been used for traditional medicine, for extracting natural dyes or making perfumes, and occasionally have been used as source of food. Most of these lichens were only eaten in times of famine, while others are a staple food or even a delicacy. However, for the most part lichens remain ignored and unknown to most people as one could conclude from the fact that very few lichens have common names.
A Successful Association
Lichens are in fact examples of the closest relationship that can exist between plants and fungi. Lichens have in fact evolved greatly before any true plant appeared on land. For many millions of years many species of unicellular algae were floating in the sea. About 400 million years ago, some of those algae were able to spread to some moist environments close to fresh water, lakes or seashore and possibly survived in the form of a thin green coat covering the soil under very humid conditions. That is when fungi similar to fibres were also beginning to colonize the land. Where plants live plants die, and fungi existed to consume what is left of plants. For this reason fungi are undoubtedly and mostly found in the soil. Because fungi lack chlorophyll, they cannot photosynthesize and produce complex organic substances that they might eventually need. However, as today, fungi were able to get some nutrients by secreting acids that dissolve minerals from rocks and soils. On the other hand, these minerals were not accessible to algae that had begun colonizing the land. Therefore, fungi and algae had what was also missing to one another. The first symbiotic associations were thus experimented in which fungi could absorb sugars and other organic molecules from algae and these could obtain minerals took by fungi. Fungi and algae have then established an intimate partnership where fungi surrounded and included unicellular algae in their tissues. The success was such that it flourished into about 22,000 different species of fungi (about 20% of the total species of fungi) associated with about 100 species of about 40 genera of photosynthetic algae and cyanobacteria living today in all continents and climates. It is so intimate that each pair looks like a single entity, and scientists act as such by giving it a name. They are what we know today as lichens.
Although being known to science for quite sometime, it was only in 1867 that the symbiotic structure of lichens was proposed by the Swiss botanist Simon Schwendener. However, Schwendener’s hypothesis of the dual nature of lichens lacked convincing experimental evidence, although his hypothesis arose from his extensive work on the anatomy and development of lichens, algae, and fungi using light microscopy. The true revelation of the dual and symbiotic nature of lichens was finally revealed by the Swiss botanist Eugen Thomas who published his results in 1939 on the first successful experiment on the re-synthesis of lichens.
The nature of this ancient and efficient symbiosis is quite simple in principle, but its structure and physiology are in fact quite complex. The photosynthetic members of lichens provide energy-rich carbon compounds for both partners, and they are protected from extreme environmental conditions by the fungus, which passes on to them mineral nutrients that reach the fungus from the environment. One fungus can associate with more than one photosynthetic partner, being in 90% or the cases green algae and the remaining cianobactéria, with a very small minority of lichenized fungi being able of associating with green algae and cyanobacteria simultaneously. Also, the same species of fungus can associate differently to photosynthetic partners in different environments and the same happens to the photosynthetic partners that can associate differently with several fungal species. Lichens are thus named with the same name as the fungus while the photosynthetic partners (photobionts) bear their own scientific name, with no relationship to that of the lichen or fungus.
Due to this versatility of lichenized fungi, there is an enormous diversity in colour, size, and shape of lichens. Some lichens grow in the form of vesicles; others grow in height, forming miniature shrubs and bushes. There are also many lichens that form a thin paint-like coating on rocks, trees, or walls, often with bright colours of red, blue, yellow, green and even black, while there are also lichens that resemble dust. Lichens contain many unusual chemical compounds which are responsible for this variety in colours and have been used as sources of natural dyes. Some lichens are so tiny that they are almost invisible to the unaided eye; others form tiny branches and grow in dense clumps reaching few inches tall. The long blue-green, gray or yellow beards that hang from the branches of many trees in many temperate forests of the northern hemisphere are also lichens. They draw moisture from the fog and obtain all the minerals they need, dissolved in the rain. Lichens live mostly on land but one species, Verrucaria serpuloides is a permanently submerged marine lichen living on ocean rocks. Lichens are so versatile that even attach themselves to artificial substances such as glass, concrete, and asbestos. Lichens play a very important role as they are intrepid pioneer species that set the ground for colonization of newly exposed rock surfaces and thus initiate what is called by scientists biological succession in those areas. Thy establish themselves in some of the most hostile environments on earth, where nothing else grows, and when they die, the powder that they are reduced to can provide precisely the necessary food for the establishment of plants. Those lichens that have cyanobacteria as photosynthetic partners are of special importance because they also contribute fixed nitrogen to the soil; such lichens are the primary factor in supplying nitrogen in many regions.
Lichens have been in fact extremely successful. They survive under the most extreme and harsh environments, where no plants or fungi exist in isolation. They grow in the Himalayas at altitudes of about 5480 m and are one of the very few living organisms that can survive permanently in Antarctica, where there are more than 350 species of lichens. They have been found in rocks in the South Pole, where the cold is so intense that growth is only possible for a few days a year. At the other end of the earth, the arctic tundra, lichens grow in a shrub-like shape forming carpets over 0.5 m high, covering extensive areas and growing in such quantity that is the main food of reindeer in winter. For this reason, this species of lichens is called reindeer moss. Lichens are also able to tolerate heat which would dry out and kill most of the plants. Even being parched, they remain alive. When having an opportunity, they collect moisture in large quantities absorbing up to 35 times their won weight within a few minutes. This is imbibition; lichens take up water as blotting paper does.
Simple Yet Efficient Structure
Lichens have long been the subject of biological investigations because of the intriguing nature of the associations between a fungus and its included algae or cyanobacteria. The fungus apparently plays the dominant role in determining the form of the lichen by associating differently with several photosynthetic partners and producing morphologically very different individuals that often led to confusion and error in classifying them. While the algae and cyanobacteria found in lichens can live independently of the fungus as free-living species, the fungus is very rarely found as such in nature. The simplest lichens consist of a crust of fungal hyphae entwining colonies of algae or cyanobacteria. These hyphae penetrate the photosynthetic cells where the exchange of substances between the two organisms occurs. In the more complex lichens, however, the hyphae and the photobiont cells are organized in a tallus with definitive growth form and characteristic internal structure. In these, generally there is an upper and a lower cortex of compressed and heavily gelatinized fungal hyphae sufficiently impermeable to prevent the loss of water. Bellow the upper cortex there is the photosynthetic algal or cyanobacterial layer loosely interwoven with hyphae. Bellow this photosynthetic layer there is the medulla, a thick layer of loosely packed, weakly gelatinized hyphae, making up to two thirds of the lichen tallus and serves as storage area of nutrients and water mostly. The lower fungal cortex, not always present, is thinner than the upper one and is covered with fine anchoring strands of hiphae called rhizinae, that attach the lichen to its substrate from where it obtains its mineral nutrients.
Biology of Lichens
As surprising as it may seem, one of the main factors in lichens survival seems to be the fact that they dry out very rapidly. Lichens are frequently very desiccated in nature, with a water content raging from 3 to 10% of their dry weight. When lichens dry out, photosynthesis ceases. In this “dormant” state lichens can sustain blazing sunlight and great extremes of cold or heat can be endured by many species of lichens. The cessation of photosynthesis depends, in large part, on the fact that the upper cortex becomes thicker and more opaque when dry thus cutting off the light necessary for the algae or cyanobacteria to photosynthesize. A wet lichen may be seriously damaged or even destroyed by light intensities or temperatures that otherwise do not affect a dry lichen. In the presence of water, mostly coming from rain, lichens become rapidly soft and pliable and reach their maximum vitality with maximum photosynthetic rate under light. The photosynthetic rate reaches its peak when the water content is 65 to 90% of the maximum that the lichen can hold; below this level the lichen continues to lose water and the rate of photosynthesis decreases. In many environments, the water content of lichens varies markedly during the day, with most photosynthesis taking place only during few hours, usually in the early morning after wetting by fog or dew. Consequently, lichens have an extremely slow rate of growth with their radius increasing at rates of 0.1 to 10 mm a year. Lichens achieve higher and most luxuriant growth along seacoasts and on fog-shrouded mountains. This very slow and almost uniform growth can thus be used to determine the age of lichens and some specimens have been estimated to be as much as 4,500 years old. Determining the age of lichens, lichenometry, is also used to determine the age of exposed rock surfaces based on the size of lichen thalli. This technique has been introduced by the Austrian botanist Roland Beschel in the 1950s and has found many applications.
Lichens also absorb some minerals from the substrate where they are fixed, as suggested by the fact some species are particularly found on specific rocks, soils or tree trunks only. However, they get most of their mineral supplies from the surrounding water in air and rainfall. Because they have no means of excreting chemicals that they absorb, however, some lichens are particularly sensitive to pollutants and toxic compounds that affect the chlorophyll of their photosynthetic partners. For this reasons, lichens can be used as a good indicative of air pollution, especially around cities. Analysis of lichens can thus trace the amounts of heavy metals and other pollutants around industrial areas. Fortunately, for lichens obviously, many of them have the ability to bind to heavy metals outside their cells and thus escape damage themselves. Therefore, the sight of lichens may not be an indicative of good environmental health; one must have good knowledge of the species in question and its physiology. When nuclear test were being conducted in the atmosphere, lichens were often used to monitor the fallout. Now, lichens provide a useful way to monitor to monitor the contamination by radioactive substances following events such as the explosion of the Chernobyl nuclear power plant, in Ukraine, in 1986.
The association with a fungus profoundly affects the nature of the metabolic output from the included photosynthetic partners. For example, green algae produce large quantities of two particular sugar alcohols, sorbitol and ribitol, only when they are associated with fungi to form lichens. In the same way, lichenized fungi produce many extracellular secondary metabolic products called lichen substances or lichen acids, which can sum up to 10% of the dry weight of the lichen. There have identified more than 350 different lichen substances; they are mostly weak acids and many of them are responsible for the variety in colours observed in lichens. Lichen substances are useful as they can be used to identify and classify as they differ enough between species. The biological role of these substances is still unknown although they may play a role in deterring potential herbivores.
Reproduction of lichens is probably one the more intriguing aspects of this old but efficient association of very different organisms. The fungus belonging to the partnership reproduces, like all fungi, via spores that develop inside special structures which are then released into the atmosphere. A single spore from the many millions freed from these structures is capable of founding a new colony, but to do so, they need to find a new partner, alga or cyanobacterium. It still is unclear how this happens and so far. However, the complexities of coordinating the two sexual cycles of both partners seem to have defeated many isolated lichens. To overcome this, lichens commonly reproduce by simple fragmentation or produce specialized structures called soredia, powdery propagules, or isidia if they are small outgrowths. Fragments, soredia and isidia all have both fungal hyphae and algae or cyanobacteria and act as small dispersal units to establish lichens in new places, transported by wind, running water, rain, or by animals and insects. Travelling as a team, these structures are ready to continue the alliance as soon as they arrive at a new place.
We can separate the algal or cyanobacterial and fungal partners of lichens and grow them separately in culture. Under such conditions, the fungus grows very slowly in compact colonies that look quite different from the lichenized fungus. The fungus requires a large number of complex molecules for growth which only the photosynthetic partners produce. In contrast, the photosynthetic partners seem to prosper more when free-living as they grow more rapidly without the fungus. This illustrates the dominant role played by the lichenized fungus. The dominance of the fungus is such that under extremely difficult circumstances the fungus may kill and digest some of the algal cells. For this reason, in some ways, it is appropriate to think of lichens not as mutualism, in which both parties benefit, but rather as controlled parasitism of the algae or cyanobacteria by the fungus. Nevertheless, a lichen does amount to more than the sum of its components and in that sense it is a distinct “organism”. When grown together in culture, the fungus seems first to bring its photosynthetic partner under control and then to establish the characteristic appearance of the mature lichen.
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