A lichen is an association between a fungus (mycobiont) and a photosynthetic symbiotic (photobiont) that results in a stable thallus, or body, of specific structure. The photobiont is either an alga or a cyanobacterium. A remarkable feature of a lichen is the transformation that the symbionts, in particular the fungus, undergo during the association. A new entity, the thallus, is formed, and unique chemical compounds are synthesized. The physiological behaviour of the symbionts also changes in symbiosis. There are about 15,000 species of lichens, an indication that this type of symbiosis has been highly successful and has involved many species of fungi. Surprisingly, only about 30 different types of algae and cyanobacteria have been reported as photobionts.
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A lichen thallus usually consists of layers such as an upper and lower cortex, algal layer, and medulla. The layers differ in thickness and are better developed in some species than in others. Fungal hyphae make up most of a thallus; the photobiont cells are only a small percentage (about 7%) of the total volume.
There are three main types of thalli: crustose, foliose, and fruticose. A crustose thallus lacks a lower cortex and is generally considered to be the most primitive type. Thalli of Lepraria species do not have layers but consist only of powdery granules. There are more species of crustose lichens than other types, and most of them belong to the genera Lecanora and Lecidea. Many crustose lichens stick tightly to the substratum and appear to be painted on it. Some species grow inside rock crevices and bark and still manage to produce separate layers. Squamules are typical of many species of Cladonia. Squamules are a specialized type of crustose thallus and are attached at only one end to the substratum.
A foliose thallus has an upper and lower cortex, an algal layer, and medulla and is usually loosely attached to the substrate by hairlike structures called rhizines. The thallus has many different sizes and shapes and is often pided into lobes. Common foliose genera include Anaptychia, Cetraria, Parmelia, Physcia, and Xanthoria. Some foliose lichens, such as Umbilicaria (rock tripe), have thalli that are attached to the substrate by only one central point.
Fruticose thalli are upright or hanging, round or flat and often highly branched. Thalli of Usnea are hairlike and can reach a length of 5 m, whereas those of Evernia are shorter and strap-shaped. The layers of a fruticose thallus may surround a central thick cord, as in Usnea, or a hollow space as in some Cladonia species.
Lichens grow practically everywhere - on and within rocks, on soil and tree bark, on almost any inanimate object. They grow in deserts and in tropical rainforests, where they occur on living leaves of plants and ferns. They have been found on the shells of tortoises in the Galapagos Islands and on large weevils in New Guinea. In the dry valleys of Antarctica, endolithic lichens, such as Buellia and Lecidea, grow inside sandstone crevices. Dermatocarpon fluviatile and Hydrothyria venosa grow in freshwater streams, and species of Verrucaria are common in the intertidal zones of rocky, ocean shores. Verrucaria serpuloides is a permanently submerged marine lichen that grows on stones and rocks 4-10 meters below mean low tide off the coast of the Antarctic Peninsula. Douglas Larson has estimated that about 8% of the earth's terrestrial surface is dominated by lichens.
Lichens abound in areas with high annual humidity, such as the fog belt zones of Chile and Baja California. Extensive lichen populations also grow in the cool, northern forests of the world, where hundreds of miles of forest floor are covered with thick carpets of reindeer lichens (Cladonia). Lichens with organized thalli do not grow well in areas that are continuously wet, such as tropical rainforests. Only poorly organized species of Lepraria and leaf-inhabiting lichens are found in these regions. Lecanora conizaeoides and Lecanora dispersa colonize trees and gravestones in industrial cities and towns, but most lichens cannot tolerate the polluted atmosphere and persistent dryness of urban areas.
Lichens are dispersed by thallus fragments and vegetative diaspores such as isidia and soredia. Each diaspore consists of a few algal cells and fungal hyphae. Soredia are powdery granules that originate inside the thallus, as localized overgrowths of algae and hyphae, and break through the upper cortex. Isidia are cylindrical extensions of the thallus. About 39% of all foliose and fruticose species of lichens produce isidia. Diaspores are dispersed by water, wind, insects, and birds.
Lichen fungi produce the same type of reproductive structures as other ascomycetes. Only some aspects of the sexual process have been seen in lichens, such as the fusion of microconidia to the tips of trichogynes. Dispersal of photobionts occurs by means of motile (zoospores) and nonmotile (autospores) spores.
The basic unit of a lichen thallus is one algal cell with enveloping hyphae. Fungal hyphae adhere to the surface of an algal cell by means of a mucilage produced by both symbionts. As the fungus envelops the algal cells, it forms two types of specialized cells, appressoria and haustoria. These structures are common features of pathogenic fungi. The appressorium fastens the mycobiont tightly to the photobiont and gives rise to hyphae that grow into the algal cell and form haustoria. Hyphae penetrate the algal cell by enzymatic and physical means; that is, they partially dissolve their way through the algal wall and also push their way through. The plasma membrane of the algal cell always remains intact, no matter how deeply the hyphae grow inside the cell.
The lichens are a unique example of symbiosis. Most of the studies that have dealt with the physiological interactions between lichen symbionts have focused on the passage of nutrients from the photobiont to the mycobiont. In a lichen thallus the photobiont excretes more than 90% of the carbon that it fixes photosynthetically as a polyol or a sugar such as glucose. The polyol excreted by green symbionts is ribitol, erythritol, or sorbitol; bluegreen photobionts excrete glucose. The fungus may control the rate of polyol excretion by the photobiont.
Carbon dioxide stimulates photosynthesis of the photobiont, while NH 3 increases its respiration and carbohydrate release. When the lichen fungus is actively growing, it can increase the flow of nutrients from the photobiont cells by producing more urease. Lichen acids and certain proteins act as a feedback control because they inactivate urease. Polyols and glucose released by the photobionts are absorbed by the mycobiont and converted to mannitol, which is a fungal storage product. Such a conversion creates a sink to which algal nutrients continue to flow. The fungus uses some of the mannitol for growth and development, but the rest is used to help it withstand the extreme conditions of its habitat.
Lichens contain unique secondary compounds, which are commonly called lichen acids. These compounds were thought to be products of the symbiosis, but studies of isolated mycobionts growing in culture with high concentrations of sucrose have revealed that “the fungus alone produces these compounds”. In lichens secondary metabolism appears to be connected to desiccation and aerial growth of the mycobiont.
Secondary compounds of lichens may have important ecological roles. Many have antibiotic activity and may prevent the microbial decay of lichen thalli, which may live for hundreds and even thousands of years. Lichen substances are also chemical weathering compounds that have a role in soil formation because of their chelating properties.
Lichens - unique life forms that actually are two separate organisms, a fungus and an alga, living in symbiosis - are especially susceptible to air pollution. According to James P. Bennett, a research ecologist with the National Biological Service, “Small plantlike organisms, lichens once were found in every corner of the world. Now, at least a dozen areas in North America - from Los Angeles to tracts of rural Pennsylvania - are classified as lichen deserts.”
Many northern and alpine animals - including moose, elk, muskox, and a number of ground-feeding birds - turn to lichens when the winters are long and other types of food are scarce. For these animals, lichens are "famine foods." In addition, birds and squirrels incorporate lichens in the latticework of their nests to insulate, cushion, and conceal their eggs and young. Humans, too, have benefited from lichens and their products for thousands of years. Lichen extracts were used for dyes in ancient Greece and Rome, as recorded by Pliny and Dioscoridis. Many of the methods for extracting and preparing lichen dyes were perfected in Europe during the Middle Ages.
In the twentieth century, biologists found orcein to be a useful agent for staining chromosomes, enhancing their visibility under a microscope. A closely related dye, litmus, has been widely used to test the pH (acidity or alkalinity) of solutions. The extracts from some lichen species, including oakmoss (Evernia prunastri) and tree moss (Pseudevernia furfuracea), are used by the cosmetics industry for fixative agents in perfumes. In addition, usnic acid - obtained from species belonging to several genera (such as Usnea, Cetraria, Cladonia, and Parmelia) - promises to be useful in antibiotic salves, herbicides, and deodorants.
Today we can find lichens in a wide range of sizes, shapes, and colours. Lichens occur in every type of habitat, promote soil development, reveal air quality, and serve as sources of food, dyes, and medicines. How will lichens be used in the future? Perhaps they will lead us to new nutritional products, important tests for environmental health, or valuable pharmaceutical. Time will tell. In the meantime, let us begin to appreciate this group of organisms that already had contributed so much but are acknowledged so little.
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