|"Lichenes" from Ernst Haeckel's Artforms of Nature, 1904|
Lichens are remarkable compound organisms that are an excellent example of symbiotic relationships in nature. Comprised of a photosynthesis-generating alga or cyanobacteria and a protective fungus, these interesting organisms can withstand extreme conditions and are important to humans, plants, insects, and animals. Of the approximately 18,000 to 20,000 species of lichens that have been identified, some have been found to offer medicinal, cloth-dying, and perfume-creating properties. Although there are many species, the classification of these compound organisms is debated. However while some sources claim that lichens are all classified into their own group, common knowledge indicates that lichens are classified according to the scientific name of the fungal component.
A lichen is the symbiotic relationship between a fungus and an alga or cyanobacteria. In a lichen, the fungus is called the mycobiont and the alga or cyanobacteria is called the photobiont.The mycobiont, fungus, creates a safe and stable environment for the alga or cyanobacteria. It does this by growing to create a protective covering for the photobiont in the form of a mass of threadlike fibers called hyphae. The fungus provides the mycobiont with water, protection from too much sunlight, and some minerals and nutrients. In return, the photobiont, which contains chloroplasts, performs photosynthesis for the fungus and thus provides sugars for the fungus to use. In this way, the two organisms cannot survive without one another. Sometimes, a lichen may contain two different photobionts. 
The body of a lichen is called the thallus.Its basic structure is a three-layered one. The outermost layer, often called the cortex, is usually dense and protective. The middle layer contains fungal hyphae and the algae or cyanobacteria. The third layer, which is called the medulla, is the layer of less dense fungal hyphae where nutrients are kept. 
There are many different varieties of lichens. These varieties are generally distinguished by the structures of their growth. Some of the commonly known growth forms are:
Crustose: These lichens form a type of crust on the surface of rocks, trees, or other substrates. The bottom layer is attached to that surface and cannot be removed without damaging the substrate. 
Fruticose: These lichens resemble miniature, hairlike shrubs, because they are connected to the surface at one point and grow from there. They can grow on the ground or hang from branches, often resembling the look of a beard.
Leprous: Not much is known about these lichen species, because they have not been seen to produce any fruiting bodies and lack both an upper and lower cortex.
Squamulose: The appearance of these lichens resembles that of tiny, overlapping scales. These lichens have no third cortex.
Reproduction in such an organism is more complex, because lichens are comprised of two different organisms. In asexual reproduction, a part of the fungus and some of the photobiont's cells break off in a reproductive package called a soredia or isidia. After these break off from the main lichen body, they are transported by wind, water, or animals. Once they reach a resting place, they can grow into a new organism.
Sexual reproduction in a lichen only occurs in the fungus. The fungus produces a fruiting body and has two hyphae of different mating types that fuse and produce spores. These fungal spores get carried away by wind and must find a new photobiont to rely on. The algal cells, on the other hand, can only reproduce on its own by mitosis, thus creating more identical algal cells.
When lichens first begin to develop, they attach themselves to a surface by growing hyphae. Lichens grow very slowly, sometimes only adding a few millimeters to their size in a year. Despite the slow growth, lichens can have very long life spans. One has been found in the Arctic that is more than 4,500 years old.
Because of their symbiotic relationship, lichens have the ability to grow in conditions that other organisms may struggle with. They can grow in harsh conditions such as mountains, deserts, rocky areas, or polar tundra. Because of this, they are often the first to colonize inhospitable areas. Lichens that live in rocky areas can release acids that break down the rock and thus promote soil formation. This encourages other plants and organisms to develop in that area.
Aside from being pioneers in harsh environments, lichens are eaten by many different organisms. Insects and slugs have lichen as a common part of their diet. Reindeer and caribou eat lichens in the winter, when food is scarce. Humans may sometimes eat lichen. It is believed by some scholars that the manna in the biblical story of the Israelites finding food in the desert after fleeing Egypt was made from lichen. 
Lichens produce upwards of 500 biochemical compounds that serve a variety of purposes to the organism itself. These unique biochemical compounds are now used in many ways by humans. In folk medicine, they were used as cures for illnesses such as headaches and asthma. About 50% of lichens have been proven to have antibiotic properties. In the 1940's, certain lichen extracts were proved to produce important antibiotics that could be used to treat tuberculosis and some skin diseases. An antibiotic salve was developed from lichen, because of its effectiveness in its use as a powder to treat wounds in the Soviet Far East. Now, lichens are continued to be investigated by pharmaceutical companies and used in homeopathic medicine.
Lichens and Science
Lichens have been used in the field of science for a variety of purposes. Lichens can be used in the field of geology to estimate the times of geological events such as earthquakes, rock slides, or the retreat of glaciers. Because lichens have such slow growth rates, scientists can use the measurements to infer the dates of when certain geological events occurred.
Lichens have been used in studies that investigate the theory of panspermia, the idea that life could be transported between planets. In 2005, two species of lichen were sent by the European Space Agency into space. The lid on their capsule was open and they were exposed to the space environment for almost 15 days. Despite the extreme temperatures and exposure to radiation from the sun, the lichens remained unchanged when they got back to earth. Their DNA was intact and the lichens resumed normal functions once reaching earth. This experiment does indeed suggest that life has a possibility of moving between planets.
Scientists are starting to use lichen as environmental indicators. In other words, they use lichens to identify the level of pollution in cities and towns. These organisms seem to be very sensitive to sulfur dioxide pollution, because their absorption systems rapidly accumulate the toxin. Sulfur dioxide pollution is mainly caused by the rise of industrialization. Scientists have developed an index based upon the growth of lichens that can determine the amount of pollution in an area. Areas where more lichens are present indicate a healthier environment while areas where less lichens or more hardy lichens are present indicate a more polluted environment.
- Lichens of North America Sylvia and Stephen Sharnoff. Lichen.com
- Hardy Lichen shown to survive in space Kelly Young. New Scientist.com News Service.
- Lichen Richard Matthew McCourt, B.A., M.S., Ph.D. MSN Encarta.
- Lichen Britannica Concise Encyclopedia.
- Natural Perspective: Lichen Ari Kornfeld. Natural Perspective.
- What is a lichen Gordon Ramel. Earthlife Web Productions.
- Lichenland Oregon State University.
- Lichen Wikipedia.com
- Lichens and Pollution Kevin J. Lyman. Milwaukee Public Museum.
- Lichens Encyclopedia of the Atmospheric Environment.