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Symbiotic associations of fungi (mycobionts) and photosynthetic partners (photobionts). These associations always result in a distinct morphological body termed a thallus that may adhere tightly to the substrate or be leafy, stalked, or hanging. A thallus consists of layers, that is, a cortex and medulla made up of the fungus, and a photosynthetic layer of algal or cyanobacterial cells that are closely associated with fungal hyphae. Rhizoids anchor thalli to their substrates. See Cyanobacteria
Lichens are formed from specialized groups of parasitic fungi; this association is one of a controlled parasitism rather than mutualism. Thus, the photobionts that lichen fungi slowly parasitize should be considered victims and not partners. Lichen-forming fungi share two characteristics with fungi that parasitize plants: concentric bodies and specialized branches of hyphae (haustoria) that penetrate host cells and absorb nutrients from them.
Lichens have a worldwide distribution and grow on almost any inanimate object. They are among the hardiest of organisms and thrive in some of the Earth's harshest environments, such as polar regions, deserts, and high mountains.
The name given to a lichen applies only to the mycobiont, while the photobiont has a separate name. Most of the 15,000 lichen-forming fungi are in the fungal class Ascomycotina (ascolichens). Approximately a dozen species of basidiomycetes form lichens. Lichens that do not have sexual reproduction (Lepraria) are placed in the Lichenes Imperfecti.
Photobionts of lichens are either green algae or cyanobacteria. The most common photobiont is Trebouxia. This unicellular green algae has never been found in the free-living state. It is believed that Trebouxia is a lichenized and highly modified form of the filamentous alga Pleurastrum terrestre.
The basic metabolic processes of lichens are photosynthesis, respiration, and nitrogen fixation. Lichens have adapted these processes to different conditions of light, temperature, day length, and water. The mycobiont causes the photobiont to excrete most of the carbon that it fixes during photosynthesis. Only a single type of compound is excreted. The mycobiont absorbs these compounds and converts them to mannitol, its own storage compound. See Plant respiration
Nitrogen-fixing lichens are common and contribute nitrogen to different ecosystems when they decay. In cyanolichens the mycobiont inhibits the nitrogen-assimilating enzymes of the cyanobiont, causing it to release most of the ammonia it produces. The ammonia is absorbed by the mycobiont and used to make proteins and nucleic acids. See Nitrogen fixation
Lichens produce several hundred secondary compounds that accumulate as crystals in the thalli, often at high concentrations. These compounds may protect the slow-growing thalli from harmful bacteria, fungi, and insects and may play a regulatory role in the interactions between bionts. Lichen secondary compounds represent a new class of antibiotics in an age where standard antibiotics such as penicillin are becoming ineffective against antibiotic-resistant microbes. Secondary compounds are used extensively by taxonomists to characterize new taxa of lichens (chemotaxonomy).
a specialized group (Lichenes) of fungi in permanent symbiosis with algae, regarded by some botanists as an independent group of lower plants. The study of lichens is called lichenology.
There are about 26,000 species of lichens, grouped into more than 400 genera. These are grouped into two classes, according to the manner in which the sexual spores are borne (asci or basidia with spores): Ascolichenes, which comprises almost all known lichens, and Basidiolichenes, which numbers only several dozen species. Algae, the constant component of lichens, are blue-green, yellow-green, or green. Usually a particular species of alga is associated with each species of lichen.
Structure. Lichens are distinguished as crustose, foliose, or fruticose. The thallus (the vegetative body) of the crustose lichens, the most primitive, is granular, powdery, or crusty. The foliose lichens, which are more well developed, have the appearance of more or less dissected blades. The fruticose lichens are highly organized and have the appearance of shrubs, pendant filaments, or erect processes.
According to anatomical structure, lichens are classified as homoeomerous (the algae distributed more or less evenly over the entire body) or heteromerous (the algae found only beneath the upper cortical layer). The thallus of the well-developed lichens has upper and lower cortical layers, separated by the algae and a medullary layer.
Reproduction. Lichens reproduce sexually, asexually, and vegetatively. The sexual process results in the formation of spores in the asci of the lichen fungus in Ascolichenes or, very rarely, on the basidia, in Basidiolichenes. The asci with the spores develop in closed fruiting bodies, or perithecia, which have a narrow efferent opening to the outside, or in apothecia, which are open wide on top. Asexual reproduction involves the formation of conidia and pyknospores. Upon encounter, the sprouted spores of the fungus and the corresponding species of alga together form a new thallus. Vegetative reproduction is the regeneration of the thallus from small fragments of its branchlets or lobes or by means of special formations called soredia and isidia. The algae reproduce by division. Green algae often reproduce by the formation of autospores.
Mode of life and distribution. Lichens form numerous chemical (lichenose) substances. Some of these, by coloring the cortical layer of the thallus, protect the algae against strong insolation. Others are deposited on the walls of special “aerial hyphae,” which are not wetted by water and do not swell and thereby ensure the entry of air to the algae. Many acidic lichenose substances change rock and thereby facilitate the attachment of the thallus.
In terms of water regime, lichens are members of the poikilohydrous group of plants, which have no special organs for absorption and release of water and are practically devoid of physiological control over these processes. Water is absorbed by the entire surface of the thallus and is retained by capillary spaces between the hyphae and the algal cells and by the swelling membranes of some of the hyphae. In slime lichens, considerable amounts of water are retained by the slime. The capacity of lichens to absorb water vapor actively from both moisture-saturated and nonsaturated atmosphere gives the lichens an important physiological advantage over other plants. Dissolved inorganic and organic nutrients are absorbed along with the water. The fungus takes a considerable amount of the carbohydrates from the alga. Lichens can tolerate more severe dehydration over longer periods than can other plants.
The intensity of the algal photosynthesis in lichens is closely associated with the water content of the thallus; dehydration decreases or terminates photosynthesis. The slow growth of lichens is explained chiefly by the brevity of the periods of photosynthesis. Crustose lichens usually grow more slowly, and fruticose lichens more rapidly, than do foliose lichens. The minimum growth increase annually is 0.01 mm; the maximum is about 100 mm. Lichens are very tolerant of considerable increases and decreases in temperature.
Lichens mostly grow under good light conditions on a variety of substrates—trees, outcroppings of rock, soil, evergreen leaves, leather, bone, paper, fabrics, glass, and iron—wherever the substrate is motionless for long periods of time. With few exceptions, lichens are very sensitive to air pollution (for example, by smoke or sulfur dioxide). Lichens inhabit all continents, to the extreme boundaries of plant distribution. The greatest species abundance is found in the tropics and subtropics; the most abundant development is found in tundra and high-mountain areas. Deserts are poor in lichens.
Interrelationships of fungus and alga. Of the various notions of the interrelationships in the lichens between fungus and alga, the most widespread until recently was the erroneous concept of symbiosis, according to which the partners help one another: the fungus “supplies” the alga with water and inorganic salts, and the alga “supplies” the fungus with ready organic matter. In reality, the relationship between fungus and alga is based on parasitism—especially on the part of the fungus. The hyphae that pass into the thallus near the algal cells form various microsucker-type absorption organs: haustoria, impressoria, apressoria, and so forth. The nature and degree of parasitism vary in different species of lichens from facultative to obligate and from extreme to moderate. The hyphae embrace only a small proportion of the algal cells and, for a long time, use mostly the reserve nutrients of the latter. Only later do they slowly bring about the death of the algae. In addition, when the joint existence is crowded, the fungus also makes use of dead algal cells. In this stage it is a saprophyte. In 1902, the Russian botanist A. A. Elenkin characterized the relationship between the components as endoparasitosaprophytism.
Origin. The lichens are polyphyletic: they arose at various times among various orders of fungi, as a result of the manifestation from generation to generation of the tendency of fungi to parasitic symbiosis with algae. This explains the discrete character of the basic groups of lichens, which cannot be united in a closely connected system. The earliest fossil lichens are assumed to date from the Upper Cretaceous.
Significance. Because of their undemanding life requirements and their ability to develop on almost infertile sites, lichens are often the pioneers of vegetation. When they die off, they leave organic matter on which other plants can settle. Certain fruticose lichens have great importance as forage for deer (for example, a number of species of Cladonia, or reindeer moss, and Cetraria). Some lichens are used medicinally, as astringents, emollients, peristalsis intensifies, and hypertensors and as sources of vitamins, antibiotics, and antimicrobials. A number of lichens are used in perfumery as fragrance fixatives (for example, Evernia prunastri). Litmus and dyes are manufactured from certain lichens. Although they are not parasites of trees, lichens may cause trees indirect damage, since insect pests often populate the thalli.
REFERENCESGollerbakh, M. M., and A. A. Elenkin. Lishainiki, ikh stroenie, zhizn’ i znachenie. Leningrad, 1938.
Elenkin, A. A. Flora lishainikov Srednei Rossii, parts 1–4. Iur’ev, 1906–11.
Tomin, M. P. Opredelitel’ kustistykh i listovatykh lishainikov SSSR. Minsk, 1937.
Oksner, A. M. Flora lyshaynykiv Ukrainy, vols. 1–2 (issue 1). Kiev, 1956–68.
Opredelitel’ lishainikov SSSR, issue 1. Leningrad, 1971.
Des Abbayes, H. Traité de Lichénologie. Paris, 1951.
Ahmadjian, V. The Lichen Symbiosis. Waltham (Mass.), 1967.
Hale, Mason E. The Biology of Lichens. London, 1967.
Die natürlichen Pflanzenfamilien, 2nd ed., vol. 8. Leipzig, 1926.
A. N. OKSNER