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 provide us with dyes, scents for perfumes, and sometimes food. In the future, they may also 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.
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, or brown. Despite their appearance, they aren't plants. Their body contains both a fungus and an alga. 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, however, 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.
Exploring Lichens in a Seattle Cemetery
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.
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. It's 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.
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 for humans, we need to find a way to prevent the chemical from hurting us before we can use it as an antibiotic.
It's important to note that even though a chemical is antibacterial in isolated cells and in lab animals it may not have the same effect inside our body.
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 seeking 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 not known if 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 as Food for People
While we shouldn't pick up a lichen from a neighbourhood rock or tree and eat it, some species are eaten by humans. Many species are mildly toxic, a few are poisonous and most are indigestible in their raw form. Some cultures have learned to prepare lichens in a way that improves their digestibility, however, and even makes them a delicacy.
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 mix the partly digested lichen from caribou stomachs with raw fish eggs. The result is a concoction known as "stomach ice cream".
Umbilicaria esculenta is a black foliose lichen that grows on rocks. It's used in Asian cuisine after being fried. Umbilicaria lichens are often knows as rock tripe. The North American version was used as an emergency food by early explorers.
Some lichens are boiled and mixed with fruit and flavouring agents such as onions before being eaten. With a few exceptions, however, lichens are generally used as food in famine situations instead of by choice.
A bioindicator is a species that indicates the health of the environment via its presence, function or behaviour. Lichens can act as bioindicators.
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.
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.
A Lichen Survey in London, England
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.
Questions & Answers
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?
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.
© 2014 Linda Crampton