Partners for Life.
From the poles to the tropics, in both terrestrial and marine habitats, a group of creatures has colonized much of our planet's surface. They grow on rocks, soil, trees, stone walls, old buildings, and even the backs of some animals. Many occur in bright shades of orange, yellow, and blue; others are dull white, gray, or green; still others are black. Yet we generally overlook them, ignore them, or mistake them for mosses or other plant life. Even Carolus Linnaeus, the great Swedish botanist known as the father of taxonomy, made the error of calling them the most worthless plants on earth.
These creatures are lichens--and no, they are neither plants nor worthless. Rather, each one is a composite of two or three different types of organisms from separate kingdoms: a fungus (Kingdom Fungi), an alga (Kingdom Protista), and sometimes a bacterium (Kingdom Monera). As such, the lichens are a unique example of symbiosis--that is, the constituent organisms live together, performing separate but mutually beneficial functions. While there are many examples of symbiotic relationships in nature, lichens are unusual in that their structure and behavior are markedly different from those of their constituents.
Amazingly, lichens can survive in some of our planet's most inhospitable locations. They are also nature's pioneers in the sense that they are often the first to colonize new habitats, such as those generated by landslides or volcanic activity. Some of them have long been used as sources for food, dyes, and medicines. In addition, many lichens are useful indicators of air pollution.
Meet the lichens
Scientists are still unclear about the origins of this odd combination of partners. They think that lichens first arose about 400 million years ago, when plants were beginning to colonize the terrestrial landscape. The fungi of that time somehow gathered algae or bacteria in a kind of captive embrace that benefited the constituent partners. Whatever the circumstances of that first coupling, the partnership has evidently prospered and endured to the present.
Today we can find lichens in a wide range of sizes, shapes, and colors. Some are as tiny as pinheads and are hard to discern. Others festoon trees with their long, beardlike growth. Some complex lichens look like leafy plants.
Generally speaking, the lichen's body (thallus) consists of a single fungal component--the mycobiont--that engulfs thousands of algal cells or millions of bacterial cells, known as photobionts. The fungal partner takes up about 90 percent of the body mass and determines the lichen's size and shape. It provides housing for the algae or bacteria, protecting them from injury and drought. In most cases, the mycobiont anchors the lichen firmly to the substrate and may absorb water and minerals from the environment.
The mycobionts in most lichens are derived from sac fungi (ascomycetes), a group whose tiny spores have saclike structures. Other mycobionts are related to club fungi (basidiomycetes), which produce spores in little clublike knobs. A few species belong to the group of imperfect fungi (deuteromycetes), that is, fungi that lack a sexual stage in their life cycle.
The photobionts are generally green algae or blue-green bacteria (cyanobacteria, once known as blue-green algae). At least one species of golden brown alga also occurs as a lichen partner. The photobionts are so named because they are equipped with chlorophyll and perform photosynthesis to produce sugars and other carbohydrates from carbon dioxide and water. The cyanobacteria can also use atmospheric nitrogen to produce nitrogen-containing organic compounds. As a result, these tiny partners give each lichen the ability to synthesize its own food.
Some lichens acquire their bright colors from the minerals they absorb from the substrate. Reds, for instance, indicate an iron-rich substrate. In other cases, colors are produced by the interactions of acids and pigments in the surface layers of the fungal tissue. The lichen's coloration helps it endure extremes of temperature and adjust to lighting conditions. In particular, the colors reduce the intensity of bright light and filter out damaging ultraviolet radiation. In weak light, the acids and pigments are reabsorbed, allowing more light to reach the photobionts.
While they are composite organisms, lichens have been traditionally placed in the kingdom Fungi and named on the basis of the larger, more identifiable fungal component. By that approach, about 14,000 species of lichens have been identified. Some scientists, however, think that as many as 6,000 additional species await discovery.
The assignment of scientific names to lichens is complicated by the fact that the shape of the fungal component may vary with the types of algae or bacteria associated with it. In addition, it has been argued that the photobionts are physiologically more important than the mycobiont in sustaining each of the composite creatures. Some scientists have therefore suggested the creation of a new taxonomic term (and status) for these symbiotic complexes. That, however, remains to be done.
Based on recent investigations, some scientists have suggested that the mycobiont is a parasite rather than a symbiotic partner in a lichen. Indeed, in many cases, the mycobiont extends tiny tubes called haustoria deep into the photobiont cells, to obtain a continual sugar supply. Occasionally, when conditions are harsh, the fungal partner may consume some of the algal cells, leaving just enough to repopulate the lichen when conditions improve. Moreover, the algal and bacterial entities are generally capable of living independently of their fungal partner, but the fungus cannot long survive the loss of its associates.
These observations tend to support the view that the photobionts play host to an oversized fungal parasite. Yet it is also true that the fungus absorbs nutrients from the environment and offers protection to the algae and bacteria, allowing the combination of partners to survive in habitats that are too extreme for any of them to colonize in isolation. Thus there is an ongoing debate about whether the relationship between the mycobiont and photobionts should be termed parasitic or symbiotic.
Whatever the relationship between their constituents, lichens have several characteristics that are not exhibited by any of the partners living independently. For instance, the distinctive gnarly and lacy filigrees of lichens differ substantially from the shapes of regular fungi. More fundamentally, lichens manufacture their own food, whereas fungi need to feed on dead or living organic matter. In addition, lichens produce hundreds of chemicals--mostly oils, acids, and pigments--that are not made by other organisms. Some of these chemicals deter herbivores, inhibit competitors, or have antibiotic properties.
Four major groups
Based on the structure of the thallus, lichens have been placed in four major groups: leprose, crustose, foliose, and fruticose. Leprose lichens have the simplest structure, consisting of a loosely woven network of fungal strands (hyphae) within which algal cells are embedded. Lichens in the other groups have more complex structures, in which the fungal hyphae are packed at varying densities to form several layers.
In general, each of the complex lichens has an outer layer of densely packed hyphae, forming a protective cortex. The fungal filaments just beneath the cortex are less densely packed, and the photobionts are housed in that layer. Below that, the hyphae are loosely interwoven to form the medulla. In the case of crustose lichens, the medulla is directly attached to the underlying substrate. Foliose lichens have a second cortex beneath the medulla, while fruticose lichens have a small central core instead of a lower cortex. Some lichens have rootlike strands (rhizines) that emerge from the base to anchor the thallus and absorb water and minerals from the substrate.
Leprose lichens look like powdery patches strewn across the landscape. The crustose variety form a flat, crustlike layer that clings tightly to the surface of rocks, trees, and soils. Being drought resistant, they are the most common type of lichens in dry desert and alpine areas, and in the cold-stressed regions of Antarctica and the Arctic. Foliose lichens have a leafy thallus and grow best in areas receiving frequent rainfall. Several species also occur in the freshwater habitats of North America.
Fruticose lichens resemble miniature shrubs or thickened mosses. Their thallus consists of a filigree of interwoven stalks, cups, and clubs that grow upright. In some cases, the thallus contains pores that resemble the stomata of leaves, regulating the flow of air in and out of the lichen to facilitate photosynthesis. Fruticose lichens predominate in rain forests and cloud forests and along fog-shrouded seacoasts where moisture is plentiful.
Some lichens are not securely attached to any substrate. For instance, some vagrant lichens (genus Lecanora) are common among the desert hills and valleys of the Middle East. They fragment, roll up, and are blown about by wind and rain, sometimes collecting in enormous heaps. Many members of this group can be baked into an edible, breadlike substance. One species, commonly referred to as manna lichen (L. esculenta or Aspicilia esculenta), is thought to be the biblical "manna from heaven" that sustained the Israelites in the desert following their flight from Egypt.
Means of reproduction
The reproduction of lichens is complicated by the need to include cells of both the mycobiont and photobionts in dispersal capsules. The fungal component can undergo sexual reproduction, leading to the release of millions of spores each year. The spores are scattered by the wind and flowing water; if they land at suitable locations, they begin to germinate. Yet, because they lack photobiont components, and germinating spores rarely (if ever) capture new algae or bacteria, these spores are doomed to die.
To circumvent this problem, lichens reproduce asexually. The most common method involves fragmentation of the thallus into smaller pieces, which are then carried over great distances by wind and water. If a fragment contains cells of the mycobiont and photobiont partners and lands on a suitable substrate, it can produce a new lichen.
In addition, many lichen species produce specialized packages that contain both fungal and algal cells. Some species manufacture small packets--called soredia--on their surface. Others produce stalklike capsules--called isidia--that project just above the surface of the thallus. Both soredia and isidia can be easily detached from the thallus and dispersed.
In formidable environments
Given their ability to absorb atmospheric gases (including water vapor) and manufacture their own food, many lichens can dwell in the most formidable environments on earth. They occur on scorching desert sands, cooled lava flows, and higher up on mountains than any other organism. They are also the most common life form--and sometimes the only life form--found near the poles.
Antarctica is home to five species, some of which appear as little black blisters on rocks within 300 miles of the South Pole. In the arctic tundra, lichens often form extensive pastures that are grazed by animals of that region. Some species are equally at home in heat and cold. For instance, a scrubby lichen (genus Ramalina) inhabits the Negev Desert, where temperatures swing from a cool 10_C (50_F) to a near-baking 80_C (176_F).
Under such extreme conditions, lichens grow very slowly--sometimes only a few millimeters per year. Thus it may take several years before a lichen becomes visible to the naked eye, and a one-inch patch may be several hundred years old. Some arctic lichens are thought to be over 4,000 years in age, making them the world's oldest living organisms.
Given their hardiness, lichens are often the first organisms to colonize newly exposed areas of soil and rock following avalanches, volcanic eruptions, and the melting of glaciers. For instance, lichens were among the first to grow on the ash-covered wastelands generated by the cataclysmic explosions of Krakatau in Indonesia and Mount St. Helens in Washington State.
Lichens that colonize barren landscapes are ecologically important because they promote soil development. The acids they produce lead to fragmentation of the underlying rock, generating minute particles. When the lichens die, their organic debris mixes with these fragments. The resultant soil can then be colonized by a succession of mosses, weeds, and grasses, eventually leading to the establishment of a diverse and balanced biological community.
Benefits for animals and humans
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." By contrast, the lichen known as reindeer moss (Cladonia rangiferina) is a dietary staple for reindeer and caribou. Far away, in the rain forests of southern China and Southeast Asia, the endangered Yunnan snub-nosed monkey subsists primarily on Bryoria species, besides consuming a variety of leaves and fruits.
In addition, birds and squirrels incorporate lichens in the latticework of their nests to insulate, cushion, and conceal their eggs and young. Some birds--such as the blue-gray gnatcatcher, hummingbird, and eastern wood peewee--may construct their nests entirely of lichens.
Humans, too, have benefited from lichens and their products for thousands of years. Inhabitants of the deserts of Egypt and Libya, for instance, still gather baskets of manna that they bake into bread. In some parts of the world, certain species are considered delicacies. In Japan, a common lichen (Umbilicaria esculenta) is a favorite ingredient of soups and salads.
Lichen extracts were used for dyes in ancient Greece and Rome, as recorded by Pliny and Dioscoridis. The dye most sought after was a purple known as orchil (archil or orcein), which was used together with purples extracted from shellfish to color the robes of royalty. The royal purple was so jealously guarded that everyone outside the royal circle was forbidden from wearing similarly colored clothing on pain of death.
Many of the methods for extracting and preparing lichen dyes were perfected in Europe during the Middle Ages. In North America, native peoples dyed their blankets and clothing with browns and reds obtained from the wolf lichen (Letharia vulpina). Extracts from the same species were also used for an herbal tea in dilute form and poison arrowheads in concentrated form. Even today, lichens are valued as sources of dyes for Harris tweeds and other fine silk and wool products.
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. These dyes can be prepared from various lichen species, such as Ochrolechia tartarea and Roccella tinctoria.
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.
Given that lichens absorb many nutrients from the air and rainfall, they are affected by various atmospheric pollutants and retain particles of heavy metals, radioactive elements, and sulfur. They are especially sensitive to sulfur dioxide and are killed by prolonged exposure to it. Consequently, lichens are extremely useful as bioindicators of air quality. In the United States, the Park Service and Forest Service examine both lichens and lichen-feeding moths (family Arctiidae, subfamily Lithosiinae) for this purpose. In addition, the Atomic Energy Commission has been checking the health of lichens across the northern landscape to monitor radioactive fallout.
Taking advantage of the extremely slow growth rate and longevity of certain lichens, some geologists have been investigating species such as Rhizocarpon geographicum to estimate the ages of various geological events. This method, called lichenometry, has been successfully used to date the age of moraines and other glacial features in the Northeast and rock slides in the Sierra Nevada. Moreover, archeologists have inspected lichens to estimate the age of ancient artifacts, monuments, and buildings. Examination of lichen growth on the famous giant stone heads on Easter Island has indicated that they were sculpted nearly 300 years ago.
Lichen extracts have long been known to inhibit or destroy molds, viruses, and bacteria that cause various illnesses. Herbal remedies used by the native peoples of China, New Zealand, and North America all included lichen powders and salves to treat wounds and cure infections. Modern herbal medicines still include the use of some lichens such as Iceland moss (Cetraria islandica) as a home remedy for chest ailments, coughs, and colds. The search for lichen medicines continues, especially in China and Japan. Perhaps the most exciting recent news is that rock tripe (Umbilicaria esculenta) seems to inhibit the growth of HIV, the virus that causes AIDS.
How will lichens be used in the future? Perhaps they will lead us to new nutritional products, important tests for environmental health, or valuable pharmaceuticals. Time will tell. In the meantime, let us begin to appreciate this group of organisms that already contribute so much but are acknowledged so little.n
On the Internet
American Bryological and Lichenological Society
Arizona State University Lichen Herbarium
Introduction to Lichens
Lichens of North America
World of Lichenology
Dwight G. Smith is professor and chairman of the biology department at Southern Connecticut State University in New Haven, Connecticut. He is a frequent contributor to The World & I.
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|Author:||Smith, Dwight G.|
|Publication:||World and I|
|Date:||Apr 1, 2003|
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