A lichen is a symbiotic partnership between fungi and either algae or cyanobacteria. While you may not notice lichens in the world around you, they make up a significant part of the biosphere and are important both in ecology and to humans.
- A lichen is a symbiotic relationship between a fungus and a photosynthetic partner (algae, cyanobacteria, or both).
- Lichens grow all over the world and can even survive exposure to space.
- They take many forms and are classified according to the type of fungi they contain as well as their shape.
- Lichens are important because they fix nitrogen, break rocks into soil, and absorb pollution. They have medical and commercial uses, too.
By definition, a lichen is a symbiotic relationship between a fungal partner (the mycobiont) and a photosynthetic partner (the photobiont). Most of the time, a lichen consists of one species of fungus and one species of either algae or cyanobacteria. However, some lichens contain more than one species of fungus, algae, or cyanobacteria. A few lichens contain both algae and cyanobacteria.
The Symbiotic Relationship
The mycobiont and photobiont form a symbiotic relationship. The fungi offers an anchor for the algae, while algae make carbohydrates that the fungi use as food for energy. The relationships lets lichens live in extreme environments that are hostile to the species on their own.
In most cases, the symbiotic relationship is an example of mutualism because both partners gain something from the other. However, lichens also display elements of commensalism or even parasitism. In commensalism, one species benefits, while the other is unharmed. In parasitism, one species benefits at the expense of the other. Some lichens contain algae that are most successful on their own, rather than associated with fungi. The fungi send hyphae into algal cells and obtain their nutrients, killing the cells (although at a rate that matches the growth of new cells). Also, sometimes the fungus depends on the algae to complete its life cycle, much like a parasite. So, how much the photobiont benefits from the relationship is a matter of debate.
Where to Find Lichens
Lichens cover about 7% of the Earth’s surface. They survive in extreme environments, including tundra, deserts, on rocks, and even inside rocks between the grains. So, you can find them from the poles to the equator in places where they have little competition from other species. However, nearly all lichens are terrestrial. While common on land, they rarely colonize water. One two aquatic lichens have been described, with one in fresh water and one in sea water.
Types of Lichens
In the kingdoms of life, lichens get groups in with fungi. They are named and classified according to the fungal species they contain and the form that they take. An ascolichen has an Ascomycete myciobiont. A basidiolichen has a Basidiomycete mycobiont.
A macrolichen is a lichen that appears bushy or leafy. All other lichens are microlichens. But, “macro” and “micro” do not refer to size, only growth habit. So, some macrolichens are small, while some microlichens are large.
Common growth forms (morphologies) are:
- Fruticose: A fruticose lichen looks like a mini-shrub, either upright or upside down.
- Foliose: A foliose lichen resembles flat, lobed leaves.
- Crustose: A crustose lichen forms a crust or coating on a surface.
- Squamulose: A squamulose lichen consists of leaf-like scales over a crustose form.
- Leprose: A leprose lichen appears powdery.
- Gelatinous: A gelatinous lichen has a jelly-like form.
- Filamentous: This type resembles strings or matted fur.
- Byssoid: This form is wispy, like hair or wool.
A lichen generally consists of layers of fungi and algae. Cyanobacteria, when present, form little pockets on the upper layer (cortex) so they get sunlight.
The structures have special names:
- Thallus: The thallus is the nonreproductive “body” of a lichen. This is the part of the lichen that is visually prominent.
- Cortex: The cortex is the outer layer of the lichen. The layer provides some protection from the environment and may be colored.
- Algal Layer: Algae and cyanobacteria form a layer near the surface of the lichen so they can get energy from sunlight and perform gas exchange for photosynthesis. When wet, a lichen displays the vivid colors of its photobiont. When dry, most lichens appear gray or dull brown because the algal layer is dormant.
- Medulla: Most of the thallus consists of loosely-packed fungal filaments. This is the medulla.
- Basal Attachment: The way a lichen attaches to its substrate varies. Some use rhizines, which are fungal filaments that resemble roots. Unlike actual roots, rhizines do not transport water or nutrients. Other fungi attach via a holdfast. A holdfast resembles an umbilical cord or short peg.
Some lichens reproduce asexually when a piece breaks off and starts growing on its own. Some species reproduce sexually, where the fungus produces spores, which disperse and associate with compatible algae or cyanobacteria. Self-fertilization is common, so that a lichen reproduces even when other organisms are distant.
Importance of Lichens
Lichens are important for several reasons:
- They expand the range of algae, enabling photosynthesis in extreme environments, which converts carbon dioxide into oxygen.
- Lichens break down the surfaces of rocks, eventually forming soil that supports other species.
- They fix nitrogen into a form that other species can use.
- Lichens absorb pollutants from the atmosphere.
Is Lichen Edible?
Some types of lichen are edible. However, the polysaccharides they contain are largely undigestible (for humans). Most are mildly toxic and some are poisonous due to high concentrations of usnic or vulpinic acid. Poisonous lichens are usually yellow.
Human Uses of Lichens
People have many uses for lichens:
- Aesthetics: Not only do lichens add value to landscaping, but they serve as miniature trees in models.
- Biodegradation: Lichens capture and degrade various pollutants, prions, and even polyester resin.
- Dyes: While synthetic dyes replace most lichen-based dyes, they are still used for traditional orange, gray, red, and purple dyes and for the pH indicator in the litmus test.
- Lichenometry: Scientists use the slow rate of lichen grow for estimating the age of rock and archeological finds.
- Medical Uses: Lichens and their metabolites find use as antiseptics and antibiotics.
Difference Between Lichen and Moss
Some types of lichens resemble moss and the two organisms often live near one another, but they are different. A lichen lacks roots, stems, or leaves and its chloroplasts only occur in the algae or cyanobacteria on the upper surface. Moss don’t have roots, stems, or leaves, either. But, they have special structures that serve these functions. Also they have chloroplasts throughout their structure. Genetically, every cell of a moss is the same. A cell from a lichen could be fungal, algal, or from cyanobacteria.
Interesting Lichen Facts
- The word “lichen” comes from Greek and Latin words that mean “to lick.”
- In 1867, Swiss botanist Simon Schwendener proposed that lichens consisted of both fungi and algae or cyanobacteria.
- There are around 20,000 known lichen species.
- Some lichens are among the oldest living things. The map lichen (Rhizocarpon geographicum) is an artic species that is 8600 years old.
- When lichens live on plants, they only use them as attachment points. They don’t harm the plants.
- Lichens survive a complete loss of water. This allows them to survive unprotected exposure to space and may make them able to survive on Mars.
- Some people eat the partially digested reindeer lichen (Cladina spp.) from the rumen of freshly killed reindeer or caribou.
- Litmus paper gets its color from a pH-sensitive dye made from lichens.
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