What are Lichens?
The plant-like appearance of lichens hides their true identity. A lichen is not a single organism, but the result of a partnership (mutualistic symbiosis) between a fungus and an alga or cyanobacteria. Some lichens are formed of three or more partners. The body of a lichen consists of fungal filaments (hyphae) surrounding cells of green algae and/or blue-green cyanobacteria. The basis of the mutualistic symbiosis in lichens is similar to the mycorrhizal partnership between some species of fungi and the roots of most plants. The lichen fungus provides its partner(s) a benefit (protection) and gains nutrients in return.
The complexity of lichen partnerships has caused lichens to be described as "small ecosystems". They are classified as members of the Fungus Kingdom by systematists because the fungus partner is always the major partner. After a lichen symbiosis is established, the fungus has the greatest influence on the final form of the lichen body’s shape, and whether it is tough or flexible. The algal and bacterial partner(s) each have their own scientific names, but the lichen symbiosis is known only by the name of its fungus. Cross Section of Lichen The great majority of the 13,500-18,000 species of lichenized fungi are Ascomycetes, the "cup fungi". About 20 species in the tropical and temperate rain forests are Basidiomycetes, the "mushrooms". About 40 genera of algae and cyanobacteria are found in lichen partnerships.
How do Lichens Grow?
The algal and/or cyanobacterial partner(s) possess the green pigment chlorophyll, enabling them to use sunlight’s energy to make their own food from water and carbon dioxide through photosynthesis. They also provide vitamins to the fungus. Cyanobacteria can make amino acids directly from the nitrogen gas in the atmosphere, something neither fungi nor algae can do. The fungus, in turn, protects its partners from drying out and shades them from strong sunlight by enclosing the photosynthesizing partners within the body of the lichen.
This life habit has allowed lichens to successfully colonize many different habitats. Lichens have a truly remarkable resistance to drought. A dry lichen can quickly absorb from 3 to 35 times its weight in water! Lichens can also absorb moisture from dew or fog, even from the air itself if the humidity is very high and the temperature is low. They also dry out slowly, making it possible for the photosynthesizing partner(s) to make food for as long as possible. This ability to quickly absorb and retain water from many sources makes it possible for lichens to live in harsh environments like deserts and polar regions, and on exposed surfaces like bare rocks, roofs and tree branches.
The thallus, or lichen body, comes in four shapes:
- Foliose: flat leaf-like lichens.
- Crustose: crust-like lichens that may be buried in tree bark, or even between the crystals of rocks
- Fruticose: miniature shrub-like lichens.—one lichen of this type is the famous "reindeer moss" of Lapland.
- Squamulose: scaly lichens made of numerous small rounded lobes, intermediate between foliose and crustose lichens.
Most lichens grow slowly, probably because they live in environments where water is available for only short periods. They tend to live for many years, and lichens hundred of years old can be used to date the rock surfaces on which they grow. Lichens spread mostly by small pieces of their body being blown around. All the partners in the original lichen body are present in the fragment, so growth can begin immediately. Some lichens create soredia, balls of tissue made just for dispersal. Although the fungus is the major partner, dispersal by spores is rare.
Uses for Lichens
Lichens have many uses. They differ in their sensitivity to air pollution, and the presence or absence of different lichens in an area has been used to map concentrations of pollutants. Foliose lichens are used to represent trees in model train layouts. Lichens also make about 400 known "secondary products". It is thought that these chemicals are produced by lichens as defenses against disease and parasites, and, in some cases, to make the lichen taste unpleasant to animals. Some of these compounds are now used as anti-viral and anti-bacterial medications.
Other secondary products are used to make everyday life more colorful and pleasant. Some are used to scent Wool Dyed with Lichenssoaps and make perfumes. Others were used in the past to dye woolen cloth. Most colors were some shade of brown or yellow, but blue was produced from a few species. The discovery of synthetic dyes ended the demand for lichen dyes. The synthetic dyes provided many more colors, and did not fade. Lichen dyes are still used by some craft weavers who like their soft, quiet colors. Today, the only commercially important lichen dye is used to make litmus paper, to test the acidity of liquids. The litmus dye turns blue in "basic" (low-acid) solutions like ammonia, and red in acid solutions like vinegar.
Lichens can be an important food source in extreme environments. The Lapp people, who live above the Arctic Circle in Scandinavia and Russia, harvest lichens as winter food for their reindeer, just like farmers in temperate zones stockpile hay. Sheep in the deserts of Libya survive, in part, by eating crustose lichens growing on rocks.
Lichens are also important in making soil. Soil is made up of organic matter, such as decayed plants, and minerals. Species that grow on rocks infiltrate and wedge apart pieces of the rock by both pressure and chemical action. Some of their acidic secondary products dissolve the rock’s surface, freeing mineral grains. This is an extremely slow process, but the resilience and endurance of the lichen fungi puts time on their side.