Lichens and People: Uses, Benefits, and Potential Dangers
Interesting and Useful Organisms
Lichens are interesting organisms. They are an important part of nature and are often useful for humans. They currently provide us with dyes and scents for perfumes. Historically, a few species have been used as food after a suitable preparation. In the future, lichens may provide us with antibiotics and sunscreen chemicals. Some species can withstand high levels of radiation. Others can be used as biological sensors that give us information about the environment. Some contain chemicals that may be harmful, however.
Lichens have a wide variety of shapes and body forms. They also have many possible colours, including black, grey, white, green, blue-grey, yellow, orange, red, and brown. Despite their appearance, they aren't plants. Their body contains both a fungus and an alga (or a cyanobacterium). Each organism helps the other in some way, creating a beneficial partnership.
The word "lichens" is generally pronounced "likens". In some places it's pronounced as it's spelled and rhymes with kitchens.
What Are Lichens?
Lichens are beautiful and somewhat mysterious organisms that are classified into three main types, based on the form of their body, or thallus.
- Foliose types have a leaf-like appearance.
- Fruticose types have a highly branched form. They may be erect or hanging.
- Crustose types look like a crust that has formed on a surface.
Intermediate and unusual forms of lichens exist. For example, scaly types look like a cross between a crustose form and a foliose form. Jelly lichens live in moist areas and have a gelatinous appearance when wet.
Unlike a plant, a lichen doesn't have roots, stems, or leaves. It's attached to its substrate by filaments called rhizines or by a single, central extension of the thallus called a holdfast. Most of the water and nutrients that the thallus needs are absorbed from the surrounding air and raindrops instead of through the rhizines or holdfast.
Lichens are scientifically classified according to the fungus that they contain and are placed in the Fungi kingdom. The fungus is often said to be "lichenized" when it's combined with an alga.
Habitats, Substrates, and Ecology
Lichens are found in many different habitats, including temperate and tropical rainforests, deserts, mountains, the tundra, snowy and icy areas, and seashores. In addition, they grow on many different substrates, including apparently smooth ones. Possible substrates include:
- wood and bark
- concrete, metal, and glass
- cloth and leather
- shells of living animals
- other lichens
Lichens perform useful functions in nature. They provide shelter for other organisms. They also provide food for animals and materials that they can use to build their homes or nests. When lichens grow on rocks, the chemicals that they release contribute to the slow process of rock breakdown and soil formation.
A lichen is an example of symbiosis—a relationship in which two organisms live in a close association. The alga in the partnership may be a green alga or an organism that used to be called a blue-green alga but is now known as a cyanobacterium. Occasionally, both an alga and a cyanobacterium are present. The fungus nearly always belongs to a group known as the ascomycetes.
Like most other fungi, the fungal component of a lichen consists of branching, thread-like structures called hyphae. The algal cells are generally located in the middle of the lichen and are surrounded by hyphae. In jelly lichens, the fungal hyphae and algal cells are mixed uniformly.
The algal cells make food for both themselves and the fungus. They contain chlorophyll, which absorbs sunlight. The alga uses the light energy to make carbohydrate from carbon dioxide and water. Fungi don't contain chlorophyll and can't produce their own food. The fungus in a lichen helps the alga by protecting it.
In 2016, scientists made a surprising discovery. They found that many lichens consist of a fungus, an alga or a cyanobacterium, and a yeast belonging to a group known as the basiodiomycetes. The presence of the yeast was unexpected. The researchers think that this organism makes chemicals that protect the lichen from microbes and predators.
Dyes for Wool and Fabric
Many lichens are a grey colour when they're dry. When a lichen is moistened and absorbs water, however, the algal cells give it a deeper hue. The fungus component is often colourless, but in some cases it contains a pigment that gives the lichen a vivid color.
Making wool and fabric dyes from lichens is an ancient process that is still performed today. Suitable specimens are collected, cut into pieces, and added to water. Ammonia is often added to the water. At one time urine was commonly used as the water-ammonia solution. The mixture is left for several weeks in order for the dye to appear.
The dye made from a lichen often has a different colour from the intact organism. Brown, gold, orange, green, purple, blue and red colours are all possible, depending on the species of lichen used and the type of extraction process.
Modern wool and fabric dyers often emphasize conservation as they collect lichens. They tend to gather specimens that have already become detached from their substrate or that are growing in a place from which they are likely to be removed, such as trees that have died. (Lichens don't harm trees.)
British soldiers is a fruticose lichen found in Ontario and the northeastern part of the United States. The name is said to be derived from the red uniforms worn by British troops during the American revolution.
A Useful Dye and an Interesting Pigment
Litmus paper is very commonly used as an acid-base indicator, especially by students who need to know only the approximate pH of a substance. Litmus is a mixture of dyes extracted from specific lichens, especially Rosella tinctoria. Litmus paper is made from filter paper that has been treated with the dye. Neutral litmus paper is purple in color. It turns red when exposed to an acid and blue when exposed to a base (alkali).
Xanthoria parietina is a foliose lichen which contains a yellow pigment called parietin. This pigment absorbs ultraviolet radiation, acting as a sunscreen to protect the algal cells inside the lichen. Some other lichens contain sunscreens, too. It's been suggested that the protective chemicals could be useful in human sunscreens.
Lichens reproduce in various ways. If the fungal component releases spores, as in the apothecium of Xanthoria parietina, the new fungus that is made won't form a lichen unless it meets the correct alga.
Antibiotics, Preservatives, and Toxins
Usnic acid has been found in several lichen species, including members of the Usnea genus. In natural medicine, Usnea is used as an antibiotic and an anti-inflammatory substance. This may not be a safe or an effective practice, however, as explained below. Usnea is also used in some products as a preservative.
Tests in laboratory equipment and lab animals show that usnic acid has antimicrobial properties and kills bacteria, fungi, and viruses. It also decreases inflammation and prevents some types of cancer cells from reproducing. Unfortunately, it may cause serious liver damage in humans. Clinical tests of the effectiveness of usnic acid in the human body are lacking. Substances may not have the same effect in our body as they do on isolated cells and inside lab animals.
The wolf lichen (Letharia vulpina) has a bright yellow-green colour and grows in Europe and western North America. It contains a yellow chemical called vulpinic acid, which is poisonous to mammals. In the past, wolf lichen mixed with ground glass and meat was used as a poison for wolves. It's unknown if the lichen or the glass was most responsible for the death of the animals.
Wolf lichen has also been used for dye extraction and was once used medicinally by native peoples. Lab research shows that vulpinic acid can kill certain kinds of bacteria. As in the case of usnic acid, if vulpinic acid is proven to be helpful as well as harmful for humans, we need to find a way to prevent the chemical from hurting us before we can use it as an antibiotic.
Microcystins in Nostoc
Nostoc is a common cyanobacterium in lichens. The genus produces toxins known as microcystins. Ulla Kaasalainen from the University of Helsinki has investigated lichens containing Nostoc in different countries. He has discovered that some of these lichens also contain microcystins. The toxins can cause liver damage in humans and other animals when they are sufficiently concentrated. As the scientist says, however, the effects of the toxins on livestock eating the lichens is unknown. The same statement probably applies to humans.
Ingredients in Perfumes and Deodorants
Oakmoss (Evernia prunastri) is used to provide scents and fixatives for perfumes. It grows in Europe and North America but is especially valued in France. It lives on oak trees as well as other trees and is a fruticose lichen, not a moss.
Both essential oils and absolutes are extracted from the oakmoss. Essential oils are usually obtained by steam distillation. Absolutes are obtained by solvent extraction and are generally more concentrated than essential oils. The extracts of oakmoss are said to have a lovely earthy scent that resembles the aroma of moss and has an undertone of pine.
Some oakmoss extracts advertise the fact that they are low in atranol. This chemical is allergenic for some people, so it's worth looking for products that have little or no atranol.
Pseudevernia furfuracea is another fruticose lichen used in the perfume industry. The lichen was used to fill the body cavity of Ancient Egyptian mummies. It's unknown whether the lichen was used as a preservative or to provide a pleasant scent. Today lichen components are used in deodorants as well as perfumes because of their pleasing aroma.
Lichens Historically Used as Food
We shouldn't pick up a lichen from a rock or tree and eat it. A few species have been eaten by humans, however. Many species are believed to be mildly toxic, at least a few are poisonous, and most are indigestible in their raw form. Some cultures have learned how to prepare specific lichens in a way that improves their digestibility and even makes them a delicacy. The people's long experience has taught them which of the local lichens are safe to eat when prepared in the right way. Most of us lack this knowledge.
The following uses are historical and may still occur in some indigenous cultures of North America.
- Reindeer moss, or Cladonia rangiferina, is a fruticose lichen that is a staple food of reindeer and caribou. (This is yet another "moss" that is really a lichen.) Some Arctic inhabitants mixed the partly digested lichen from caribou stomachs with raw fish eggs. The result was a concoction known as "stomach ice cream".
- Umbilicaria esculenta is a black foliose lichen that grows on rocks. It has been used in Asian cuisine after being fried. Umbilicaria lichens are often known as rock tripe. In North America, they were used as an emergency food by early explorers after being properly prepared.
- Some groups boiled specific species of lichens and mixed them with berries, fish, or wild onions before eating them.
With a very few exceptions, lichens were generally used as food in famine situations instead of by choice. The vast majority of lichens haven't been tested for edibility or safety or for a preparation technique that makes them safe to eat (if this technique exists). Most people shouldn't eat lichens today because of the possibility of ingesting a toxic one.
Pollution and Dehydration
Some lichens are very tolerant to pollutants such as nitrogen and sulphur compounds, while others are very sensitive to the presence of one or both of these chemicals. People who can identify lichens can learn about local environmental conditions by observing which species are present. The species act as bioindicators. A bioindicator is a species that indicates the health of the environment via its presence, function, or behaviour.
Lichens have a high resistance to damage by dehydration and the ability to quickly absorb a large quantity of water after dehydration ends. This property has enabled them to be used as wound dressings and diapers by people in the past. The organisms stop photosynthesizing when they dry out and start producing food again as they absorb water.
Lichens absorb and store radioactive substances, such as cesium and strontium compounds, without apparent harm. Their thalli can be tested for the presence of radioactive compounds in order to learn about their environment.
At least some species of lichen are very resistant to dangerous radiation. In a 2005 experiment, two species spent sixteen days in space inside an orbiting satellite. Here they were exposed to "massive" doses of ultraviolet and cosmic radiation. When they returned to Earth, they had nearly the same photosynthetic ability as before the flight. In addition, most of the lichens' cells had no observable damage when examined under high magnification.
Searching for Lichens
Just about any walk that I take ends up as a nature walk. Looking for lichens and photographing them is an enjoyable part of my journey. They are sometimes very obvious, as in the photo above. Others may be overlooked by someone who doesn't pause to look at tree bark, twigs, and rocks. The smaller parts of nature often live on these surfaces.
It's fun to examine lichens and other creatures with or without a magnifying glass. It's also interesting to think about the ways in which they are used by humans and the possible ways in which they may help us in the future.
- Life history and ecology of lichens from UCMP Berkeley
- Historical methods of making dyes from lichens from the Australian National Botanic Gardens and the Australian National Herbarium
- Usnic acid: possible benefits and liver toxicity from the Memorial Sloan Kettering Cancer Center
- Antibacterial effect of a Letharia vulpina extract from the National Institutes of Health or NIH
- Cyanobacteria and microcystins in lichens (a PDF document from the University of Helskinki)
- Lichen resistance to radiation from the NIH
- Yeasts in lichens from Purdue University
- Lichens aren't quite what we thought from the CBC (Canadian Broadcasting Corporation)
Questions & Answers
Which acid helps lichens to change rock into soil?
Lichens release a variety of chemicals that can affect their environment. One of these chemicals is oxalic acid. Its formula can be written as HOOCCOOH. Oxalic acid triggers the release of minerals from rock, causing the rock to slowly disintegrate. The process of soil production takes time and requires additional processes.Helpful 1
What are the harmful effects of lichens when eaten?
The safety of lichens when eaten is a topic that researchers still need to explore. They need to not only identify the chemicals present in the different species of lichens but also determine whether the concentration of the chemicals is harmful to humans. They also need to discover how the chemicals affect our body.
As I mentioned in answer to a previous question, lichens that contain vulpinic acid are thought to be harmful to us, though this isn’t certain. If it is true, the toxicity may depend on the quantity of the acid in a particular lichen. Some cyanobacteria produce liver toxins called microcystins when they live alone and sometimes when they are part of lichens. More research is needed to prove that the chemicals hurt us when we eat lichens, however.
Since so much is unknown about lichen safety and toxicity, it’s probably not a good idea to eat them at this time.Helpful 15
Which lichens are harmful to humans when eaten?
The edibility and safety of every lichen that is currently known haven’t been tested. Therefore, people should avoid eating lichens unless they are confident that the type they want to eat is edible and not poisonous and unless they can identify it correctly. It’s best to assume that all lichens are harmful when eaten unless there is no doubt that a particular kind is safe.
Lichens containing significant quantities of vulpinic acid are thought to be toxic to humans. Two lichens in this category are the wolf lichen (Letharia vulpina) and the tortured horsehair lichen (Bryoria tortuosa). There may be many more lichens in the poisonous category. On the other hand, only a few lichens may be harmful when eaten. Researchers don’t yet know which of these scenarios is true.Helpful 13
In which kingdom are Lichens classified?
Lichens are classified according to the fungus that they contain and are placed in the Fungi kingdom. The fungal component of the lichen is referred to as a “lichenized fungus”. The alga in the lichen is classified separately.Helpful 12
I have lived in a cool climate for 40 years, and I think the main lichens on my introduced deciduous trees are usnea and oakmoss. Every year the amount increases and now the trees are almost fully covered. Do you know why there was such a big increase over time?
Lichens are attached to the outermost layer of tree bark, and don’t harm the tree. As a tree ages, its bark frequently develops more crevices, which makes the surface better for lichen attachment. This means that more lichens can grow on the bark. Another factor that helps the bark to get covered with lichens over time is that individual lichens slowly grow and get bigger.Helpful 12
© 2014 Linda Crampton