A lichen is a special group of living things made up of algae or cyanobacteria that live together with fungi in a partnership. This partnership is called mutualism, meaning both parts help each other. Lichens were the first to use the term "symbiosis" in science.

Lichens play an important role in moving nutrients through ecosystems and are food for many animals, such as reindeer, snails, tiny worms, mites, and springtails. Lichens have different shapes, colors, and sizes. They may look like plants but are not plants. They can grow as tiny, branch-like structures, flat, leaf-like shapes, or as a thick, paint-like coating on surfaces. Other forms include a powdery look or a crusty texture.

A macrolichen is a lichen that looks bush-like or leafy. All other lichens are called microlichens. The words "macro" and "micro" describe the way they grow, not their size. Some lichen names include the word "moss," like "reindeer moss" or "Iceland moss," but lichens are not related to mosses or plants. Unlike plants, lichens do not have roots to take in water or nutrients. However, they make their own energy through photosynthesis, like plants. When lichens grow on plants, they do not harm the plants but use their surface as a place to live.

Lichens can be found in many places, from sea level to high mountain areas. They grow on surfaces like tree bark, leaves, mosses, rocks, walls, gravestones, rubber, bones, and even inside rocks. Some lichens live in very harsh places, such as the Arctic, hot deserts, rocky coasts, and polluted areas.

There are about 20,000 known lichen species. Some lichens can no longer reproduce sexually but still form new types. Lichens can be seen as small ecosystems where fungi, algae, or cyanobacteria work with other tiny living things. Some lichens live for a very long time and are among the oldest living things. They are often the first to grow on fresh rock after events like landslides. Scientists use the slow, steady growth of some lichens to estimate the age of surfaces, a method called lichenometry. Lichens are important in many ecosystems and help support trees and birds.

Etymology and pronunciation

The English word "lichen" comes from the Greek word λειχήν, which means "tree moss, lichen, or a lichen-like eruption on skin." This Greek word is related to the Latin word "lichen." The Greek word λειχήν comes from the verb λείχειν, which means "to lick." In American English, the word "lichen" is pronounced the same as the verb "liken" ( /ˈlaɪkən/ ). In British English, the word is usually pronounced like "liken," but it can also be pronounced like "kitchen" ( /ˈlɪtʃən/ ).

Anatomy and morphology

Lichens grow in many different shapes and forms. This appearance is called their morphology. The shape of a lichen is usually determined by how the fungal filaments are arranged. The non-reproductive parts of a lichen are called the thallus. Lichens are grouped based on the type of thallus because the thallus is usually the most visible part. The way the thallus grows often matches a few basic internal structures. Common names for lichens often describe their growth form or color.

Common types of lichen thallus growth include:

• Fruticose – grows like a small, leafless shrub with many branches, either upright or hanging. The branches are three-dimensional and may be round or flat.
• Foliose – grows in flat, leaf-like lobes that are two-dimensional.
• Crustose – looks like a thick layer of paint, sticking tightly to a surface.
• Squamulose – made of small, leaf-like scales that are attached to the surface but free at the tips.
• Leprose – appears powdery.
• Gelatinous – has a jelly-like texture.
• Filamentous – looks stringy or like matted hair.
• Byssoid – appears wispy, like teased wool.
• Structureless – has no clear pattern or structure.

There are variations in growth types within a single lichen species. Some descriptions may overlap, so different authors might use slightly different terms.

When a crustose lichen ages, its center may crack like old paint or dried mud. This is called rimose or areolate. The cracked pieces are called areolas, which remain connected underneath by a layer of tissue. If a crustose lichen grows outward from a center, it is called crustose placodioid. If the edges of the areolas lift from the surface, it is called squamulose.

These growth types are not always clearly defined. Foliose lichens may look fruticose if they branch. Fruticose lichens may appear leafy if their branches are flat. Squamulose lichens may form where edges lift. Gelatinous lichens may look leafy when dry.

The thallus is not always the most noticeable part of a lichen. Some lichens grow inside rocks, with only their reproductive parts visible. These parts may be bright in color. The reproductive parts are often circular, raised, or disc-like, with crinkly edges.

Lichens come in many colors. Their color is usually determined by the photosynthetic partner, such as algae or cyanobacteria. Special pigments, like usnic acid, give lichens colors such as red, orange, yellow, or brown, especially in dry areas. When wet, lichens are usually bright green or olive gray. When dry, they appear gray, grayish-green, or brown. This happens because water makes the outer layer of the lichen more transparent, revealing the green algae inside.

Different colored lichens may grow on different parts of a rock, depending on sunlight exposure. Large areas covered by lichens can look colorful, especially after rain.

Color is used to identify lichens. Dry lichens with cyanobacteria as the photosynthetic partner are often dark gray, brown, or black.

The underside of the leaf-like lobes in foliose lichens is a different color from the top, often brown or black. Fruticose lichens may have flat "branches" that look like foliose lichens, but their underside is the same color as the top. Foliose lichens may branch, resembling fruticose lichens, but their underside is a different color.

The shiny surface of some gelatinous lichens is caused by sticky secretions.

A lichen is made of a photosynthesizing organism, such as algae or cyanobacteria, surrounded by fungal filaments. Most lichens are mostly fungal, but filamentous and gelatinous lichens have more of the photosynthesizing partner. The fungus is called a mycobiont, and the photosynthesizing partner is called a photobiont. Algal photobionts are called phycobionts, and cyanobacterial photobionts are called cyanobionts.

The non-reproductive part of a lichen is called the thallus. The thallus is different from when the fungus or algae grow alone. The thallus is made of fungal filaments called hyphae, which branch and connect to form a mesh. This mesh can be dense or loose.

The fungal mesh often surrounds the algae or cyanobacteria, enclosing them in special tissues. The thallus may have a protective outer layer called a cortex, made of tightly packed fungal filaments. Fruticose lichens have one cortex layer around their branches. Foliose lichens have an upper cortex on the top of their "leaves" and a lower cortex on the bottom. Crustose and squamulose lichens have only an upper cortex, with the inside of the lichen touching the surface they grow on. Filamentous, byssoid, leprose, and gelatinous lichens do not have a cortex.

Fruticose, foliose, crustose, and squamulose lichens may have up to three tissue layers. The top layer, called the cortex, is made of tightly packed fungal filaments. This layer acts as a protective "skin," blocking other organisms and reducing sunlight. The cortex can be up to several hundred micrometers thick. Some lichens have an epicortex, a thin layer of secretions on top of the cortex.

Below the cortex is the photobiontic layer, where the photosynthesizing partner lives. This layer has less dense fungal filaments, allowing air to circulate during photosynthesis. Each cell of the photobiont is usually wrapped by hyphae. In some cases

Physiology

A lichen is a combination of algae or cyanobacteria living with fungi. They form a special partnership where both benefit. The fungi receive carbohydrates made by the algae or cyanobacteria through photosynthesis. The algae or cyanobacteria are protected by the fungi, which also gather moisture and nutrients from the environment and provide an anchor. While some algae or cyanobacteria can survive outside the lichen, the partnership allows both partners to live in more places than they could alone. Both partners mainly get water and nutrients from the air, through rain and dust. The fungi protect the algae by keeping water, acting as a larger area to collect nutrients, and sometimes providing minerals from the ground. If a cyanobacterium is present, it can fix nitrogen from the air, helping the green algae.

In three different groups of fungi, a gene called atp9, which helps make energy in mitochondria, has been lost. This loss makes the fungi fully dependent on their partners.

The algae or cyanobacteria cells use sunlight to turn carbon dioxide into sugars, which feed both partners. Algae make sugar alcohols like ribitol, sorbitol, and erythritol, which the fungi absorb. Cyanobacteria make glucose. Fungal cells in lichens can cause the algae to release photosynthesis products, which the fungi then absorb.

Many lichens also live with a type of yeast called Cyphobasidiales. Without this yeast, growing lichens in labs is hard. The yeast helps form the outer layer of the lichen and may shape it. An example is North American beard-like lichens.

A lichen made of algae or cyanobacteria and a fungus has a different shape, function, and chemistry than the individual parts. The lichen’s body, called a thallus, looks different from the fungus or algae alone. If a lichen fungus is grown without its algal partner, it forms a shapeless mass of filaments. When combined with its partner, it develops its typical form through a process called morphogenesis. In rare cases, one fungus can form two different lichen types when paired with a green alga or a cyanobacterium. These forms were once thought to be separate species until they were found growing together.

Lichens are found almost everywhere on Earth, showing they are successful partnerships. Two types of green algae are in over 35% of all lichens but rarely live alone.

If a fungus has two green algae partners that work best in different climates, this might help the lichen survive in more places.

Algae can make sugars using only water vapor. Cyanobacteria need liquid water to photosynthesize.

Algae sugars are absorbed by the fungus through special filaments called appressoria or haustoria that touch the algal cells. These structures may make the algal walls more permeable or even enter them. Algae can give up to 80% of their sugar to the fungus.

Lichen partnerships can be mutualism, commensalism, or parasitism. If the photosynthetic partner can live without the fungus but not vice versa, the relationship may be parasitic. Algal cells are often destroyed during nutrient exchange, but they are replaced at the same rate they are lost. The fungus surrounds the algae, sometimes growing into their cells with structures like haustoria, similar to those used by harmful fungi. Cyanobacteria grow faster alone than in lichens.

Lichens are so balanced that they are considered self-contained ecosystems. They may also include non-photosynthetic bacteria that help in other ways, forming a larger system called a holobiont.

Lichens are sensitive to pollution and are used to monitor air quality, ozone levels, and metal contamination. They have been used to make dyes, perfumes, and traditional medicines. Some animals, like reindeer, eat lichens. Lichens are important environmental indicators. In polluted areas, only a few algae survive, while clean air supports shrubby or leafy lichens. Crusty lichens are the most pollution-tolerant. Since industrialization, many shrubby or leafy lichens, like Ramalina, Usnea, and Lobaria, have limited ranges, often found only in clean areas.

Some fungi can only live on lichens and are not part of the lichen itself. These are called lichenicolous fungi.

Moisture makes the outer layer of the lichen more transparent, allowing algae to photosynthesize when wet and stay protected when dry. When the cortex is transparent, algae are more visible, making the lichen look greener.

Lichens produce strong antioxidants. Chemicals called secondary metabolites are stored as crystals in the lichen’s outer space. These chemicals may help lichens prefer certain surfaces.

Lichens grow very slowly, less than a millimeter per year. In crustose lichens, growth happens most along the edges, with most growing only 1–2 mm in diameter yearly.

Some lichens live for a very long time, possibly being among the oldest living things. Measuring their lifespan is hard because it’s unclear what defines the same lichen. Lichens grow by breaking off pieces, which may or may

Reproduction and dispersal

Lichens often reproduce without using sex, either by a piece breaking off and growing separately (vegetative reproduction) or by spreading tiny structures called diaspores that contain algal cells wrapped in fungal cells. Because the lichen's thallus (its body) lacks clear divisions, it can be hard to tell if a structure is a diaspore or part of vegetative reproduction. Fruticose lichens can break into pieces, and new lichens may grow from these fragments (vegetative reproduction). Many lichens split into pieces when they dry, allowing wind to carry them until moisture returns and growth resumes. Soredia (singular: soredium) are small groups of algal cells surrounded by fungal filaments that form in structures called soralia. These soredia can be spread by wind. Isidia (singular: isidium) are branched, spiny, or elongated growths that break off from the thallus and are dispersed mechanically. Lichen propagules (diaspores) usually include cells from both the algal and fungal partners, though some "fringe species" rely on algal cells spread by "core species" instead of fungal parts.

Reproductive structures on lichens often look like discs, bumps, or squiggly lines on the thallus. While some argue that sexual reproduction in photobionts (the algal partner) is rare, there is strong evidence that meiosis (a type of cell division involved in sexual reproduction) occurs in Trebouxia. Many lichen fungi reproduce sexually, like other fungi, by creating spores through meiosis and the joining of gametes. After spreading, these fungal spores must find a compatible algal partner to form a functional lichen.

Some lichen fungi belong to the Basidiomycota (basidiolichens) and produce mushroom-like reproductive structures similar to those of non-lichenized fungi. Most lichen fungi are Ascomycetes (ascolichens), which create spores in structures called ascomata. The two most common types of ascomata are apothecia (cups or plate-like discs on the thallus surface) and perithecia (flask-shaped structures inside the thallus with a small opening). When apothecia look like squiggly lines instead of discs, they are called lirellae.

The three most common spore-producing structures are apothecia (raised discs), perithecia (bottle-like cups with a small hole), and pycnidia (similar to perithecia but without asci, which are the spore-containing structures in Ascomycota). Apothecia have a layer of spore-producing cells called asci and are often a different color from the thallus. The outer edge of an apothecium is called the exciple. If the exciple matches the thallus color, the lichen is called lecanorine (like Lecanora). If the exciple is black, it is called lecideine (like Lecidea). If the edge is pale or colorless, it is called biatorine.

A "podetium" (plural: podetia) is a stalk-like structure that rises from the thallus and supports reproductive parts like apothecia or pycnidia. Podetia are not part of the main thallus but may be visually noticeable. They can be branched or cup-shaped. Many lichens have apothecia visible to the naked eye.

Most lichens create many sexual reproductive structures. Some species spread only through sexual spores. For example, the crustose lichens Graphis scripta and Ochrolechia parella do not produce vegetative propagules. Instead, their fungi reproduce sexually through self-fertilization (homothallic), which may help them survive in harsh environments.

Mazaedia (singular: mazaedium) are apothecia shaped like a dressmaker’s pin in pin lichens. These structures consist of a brown or black mass of loose spores enclosed in a cup-shaped exciple that sits on a tiny stalk.

Taxonomy and classification

Lichens are grouped based on the type of fungus they contain. Each lichen species shares the same scientific name (binomial name) as the fungus in the lichen. Scientists are including lichens in the classification systems used for fungi. The alga in a lichen has its own scientific name, which is not related to the lichen or the fungus. About 20,000 lichen species have been identified, and experts estimate that the total number of lichen species, including those not yet discovered, may be as high as 28,000. Nearly 20% of all known fungal species are linked to lichens.

The term "lichenized fungus" can refer to the entire lichen or only the fungus part. This may cause confusion without more context. A single fungus species can form lichens with different algae species, creating what look like different lichen species but are still classified (as of 2014) as the same lichen species.

In the past, some lichen scientists placed lichens in a separate group called the Mycophycophyta. However, this is no longer accepted because the parts of lichens belong to different biological groups. Neither ascolichens nor basidiolichens form a single evolutionary group within their fungal categories, but they do form several major groups that are mainly or entirely lichen-forming. The fungus Geosiphon pyriforme, which belongs to the Glomeromycota, is unique because it contains a cyanobacterial partner inside its cells. Geosiphon is not usually considered a lichen, and its special relationship with the cyanobacterium was not recognized for many years. This fungus is more closely related to fungi that form symbiotic relationships with plants. Fungi in the Verrucariales group form marine lichens with the brown alga Petroderma maculiforme and also have a symbiotic relationship with seaweed (like rockweed) and Blidingia minima. In these relationships, the algae are the dominant part. The fungi may help the seaweed survive when exposed to air. Lichens can also use yellow-green algae (Heterococcus) as their partner.

Lichens formed from fungi and algae or cyanobacteria have appeared multiple times throughout history.

The fungal part of a lichen is called the mycobiont. The mycobiont can be an Ascomycete or a Basidiomycete. Lichens formed with Ascomycetes are called ascolichens, and those formed with Basidiomycetes are called basidiolichens. Living as a symbiont in a lichen seems to be a successful way for fungi to obtain nutrients, as about 20% of all fungal species live this way.

The structures (thalli) made by a single fungus with different partners may look similar and produce the same chemical compounds, suggesting the fungus determines the lichen's shape. However, the same fungus with different algae can create very different growth forms. Some lichens have one fungus paired with multiple algae, such as a species of alga and a cyanobacterium, or multiple types of the same alga.

Even though lichen structures usually look uniform, some evidence suggests the fungal part may include more than one genetic type of the same species.

Two or more fungal species can work together to form a lichen group.

The photosynthetic partner in a lichen is called the photobiont. Photobionts come from various simple prokaryotic and eukaryotic organisms. Most lichens have a green alga (Chlorophyta) or a cyanobacterium as their photobiont. In some lichens, both are present, with the alga usually being the main partner and the cyanobacteria found in hidden pockets. Algal photobionts are called phycobionts, and cyanobacterial photobionts are called cyanobionts. About 90% of known lichens have phycobionts, and about 10% have cyanobionts. Around 100 species of photosynthetic partners from 40 genera and five different groups (prokaryotic: Cyanophyceae; eukaryotic: Trebouxiophyceae, Phaeophyceae, Chlorophyceae) have been found to associate with lichen-forming fungi.

Common algal photobionts come from the genera Trebouxia, Trentepohlia, Pseudotrebouxia, or Myrmecia. Trebouxia is the most common green alga in lichens, found in about 40% of all lichens. "Trebouxioid" refers to a photobiont in the Trebouxia genus or one that resembles it, likely belonging to the Trebouxiophyceae class. The second most common green alga genus is Trentepohlia. Around 100 species of eukaryotes are known to be photobionts in lichens. All these algae can live independently in nature as well as in lichens.

A "cyanolichen" is a lichen where the main photosynthetic partner is a cyanobacterium. Most cyanolichens are also ascolichens, but a few basidiolichens, like Dictyonema and Acantholichen, have cyanobacteria as their partner.

The most common cyanobacterium genus in lichens is Nostoc. Other common cyanobacterial photobionts include those from Scytonema. Many cyanolichens are small and black, growing on limestone. Another group, the jelly lichens in the genera Collema or Leptogium, are gelatinous and grow on moist soil. A third group includes large, leaf-like species like Peltigera, Lobaria, and Degelia, which are grey-blue, especially when wet. These lichens are common in areas with high rainfall, such as the Celtic rain forests in western Britain. Cyanobacteria in these lichens are often closely related to each other but differ from free-living strains.

Ecology and interactions with environment

Lichens grow on and in many different surfaces and places, including some of the most extreme environments on Earth. They are often found on bark, leaves, and hanging from branches of trees in rainforests and temperate woodlands. They also grow on bare rock, walls, gravestones, roofs, and exposed soil. Lichens can survive in places like the Arctic tundra, hot deserts, rocky coasts, and areas with toxic waste. Some lichens live inside solid rock, growing between the rock’s grains, and in soil as part of a biological soil crust in dry areas like deserts. A few lichens do not attach to any surface and instead drift through the air.

When growing on mineral surfaces, some lichens slowly break down the minerals by chemically changing them and physically cracking them apart. This helps turn rocks into soil over time, a process called weathering. While this is usually harmless, it can cause problems for man-made stone structures. For example, lichens growing on Mount Rushmore National Memorial require special workers to clean them regularly.

Lichens are not parasites on the plants they grow on; they only use the plants as a surface. Some lichen fungi can take over the algae of other lichens. Lichens make their own food using photosynthesis and by absorbing minerals from the environment. Even though lichens on leaves may look like parasites, they are not. Some lichens in the Diploschistes group live on other lichens. For example, Diploschistes muscorum starts growing inside the tissue of a host Cladonia lichen.

In the Arctic tundra, lichens, along with mosses and liverworts, cover much of the ground. This helps insulate the soil and may provide food for animals. An example is "reindeer moss," which is a lichen, not a moss.

Only two lichen species are known to live permanently underwater. Hydrothyria venosa grows in freshwater, and Verrucaria serpuloides lives in the ocean.

A crustose lichen that grows on rock is called saxicolous. Crustose lichens on the rock’s surface are epilithic, while those growing inside the rock, with only their fruiting parts exposed, are called endolithic. A crustose lichen on bark is corticolous. A lichen on stripped wood is lignicolous. Lichens inside plant tissues are endophloidic or endophloidal. Lichens that grow on leaves are epiphyllous or foliicolous. A terricolous lichen grows on soil. Many squamulose lichens are terricolous. Umbilicate lichens are foliose lichens attached to their surface at only one point. A vagrant lichen is not attached to any surface and is carried by the wind.

In addition to physically breaking down stone, lichens chemically attack rocks. They release substances that bind metals and create new minerals, like metal oxalates. These processes help turn stone into soil over time.

Lichens may add nitrogen to desert soils when snails eat them and their rock, then excrete the nitrogen into the soil. They also help hold sand in place in dunes. In deserts and dry areas, lichens are part of biological soil crusts that keep the soil stable.

Lichens are pioneer species, among the first to grow on bare rock or areas where life has been destroyed. Though they may compete with plants for sunlight, their small size and slow growth allow them to thrive in places where larger plants cannot. Lichens often grow where there is no soil, such as high mountain areas or the Arctic. Some survive in deserts, and others on frozen Arctic soil.

A major advantage of lichens is that they are poikilohydric, meaning they can survive long periods without water. When dried out, they enter a state called cryptobiosis, where their cells stop most biological activity. This allows them to withstand extreme temperatures, radiation, and drought.

Lichens do not have roots and do not need constant water, so they can grow on bare rock, sterile soil, or artificial structures like walls and monuments. Many lichens grow as epiphytes on plants, especially on tree trunks and branches. They are not parasites and do not harm the plants. Some lichens produce chemicals that stop mosses from growing. Ground lichens like reindeer lichens release chemicals that prevent seeds and plants from growing in the soil. Stability of their surface is important for lichen habitats. Most lichens grow on stable rock or old tree bark, but some grow on soil and sand. In these cases, lichens help hold soil together, especially in deserts where plants cannot grow without lichen crusts.

Some animals eat lichens, like reindeer in the Arctic. The larvae of certain moths, such as the common footman and marbled beauty, feed only on lichens. Lichens are low in protein and high in carbohydrates, making them unsuitable for some animals. The Northern flying squirrel uses lichens for nesting, food, and water.

Lichens exposed to air pollution absorb gases and particles through their entire surface. Without roots, they get most of their nutrients from the air, so their chemical makeup often reflects air quality. Pollution enters lichens through fog, dew, gas absorption, and dry deposition. This makes lichens useful for studying air quality.

Not all lichens react the same way to pollution. Some are more sensitive to specific pollutants than others. A lichen’s sensitivity to pollution is directly related to its ability to survive in polluted environments.

Human use

Lichens are eaten by many cultures around the world. Some lichens are only eaten during times of food shortages, while others are regular food or even considered special treats. Two main challenges exist when eating lichens: the complex sugars in lichens are hard for humans to digest, and lichens often contain mild toxins that must be removed before eating. Very few lichens are poisonous, but those high in vulpinic acid or usnic acid are harmful. Most poisonous lichens are yellow in color.

In the past, Iceland moss (Cetraria islandica) was an important food source in northern Europe. It was cooked as bread, porridge, pudding, soup, or salad and also fed to animals like cattle, pigs, and ponies. In parts of North America, Bryoria fremontii (edible horsehair lichen) was a key food source and was often cooked in pits. In North America and Siberia, people traditionally ate Cladina spp. (partially digested reindeer lichen) after removing it from the stomachs of caribou or reindeer. Umbilicaria spp. and Lasalia spp. (rock tripe) were used as emergency food in North America, and Umbilicaria esculenta (called iwatake in Japanese) is used in traditional Korean and Japanese dishes.

Lichenometry is a method used to estimate the age of exposed rock surfaces by measuring the size of lichen growths. Developed in the 1950s, this technique is used in archaeology, paleontology, and geomorphology. It relies on the slow, steady growth of lichens. The size of the largest lichen on a rock surface indicates how long the rock has been exposed. Lichens can survive on old rock surfaces for up to 10,000 years, but the method is most accurate for surfaces exposed less than 1,000 years ago. Lichenometry is especially useful for dating surfaces less than 500 years old, as other methods like radiocarbon dating are less precise for this time frame. The lichens most often used for lichenometry include species from the genera Rhizocarpon (e.g., Rhizocarpon geographicum, map lichen) and Xanthoria.

Lichens have been shown to break down materials like polyester resins, as seen in archaeological sites in the Roman city of Baelo Claudia, Spain. Lichens can also absorb environmental pollutants such as lead, copper, and radioactive substances. Some lichen species, like Parmelia sulcata (hammered shield lichen) and Lobaria pulmonaria (lung lichen), and many in the Cladonia genus, produce enzymes that can break down harmful prion proteins. These enzymes may help clean up contaminated environments.

Many lichens create secondary compounds, including pigments that protect against too much sunlight and toxins that deter animals or kill bacteria. These compounds help identify lichen species and have been used as dyes, such as cudbear or early antibiotics.

A pH indicator called litmus is made from the lichen Roccella tinctoria ("dyer's weed") by boiling. Litmus is used in the litmus test to determine if a substance is acidic or basic.

Traditional dyes in the Scottish Highlands, like those used for Harris tweed, were made from lichens such as Xanthoria parietina ("common orange lichen") and Parmelia saxatilis ("crottle"), a grey lichen found on rocks. Reports from over 2,000 years ago describe lichens being used to make purple and red dyes. Lichens in the family Roccellaceae (orchella weed or orchil) were historically important for producing dyes like orcein, though these dyes are now mostly replaced by synthetic versions.

In traditional European medicine, Lobaria pulmonaria was collected as "lungwort" because of its lung-like appearance, following the "doctrine of signatures" (the idea that plants resemble the body parts they can treat). Similarly, Peltigera leucophlebia ("ruffled freckled pelt") was used to treat thrush because its structures resembled the disease.

Lichens produce chemicals being studied for possible medical uses. Some lichen chemicals are similar to broad-spectrum antibiotics, while others act like antiseptics. Usnic acid, a chemical found in many lichens, is being researched for its ability to kill bacteria like Escherichia coli and Staphylococcus aureus.

Some lichens, such as those growing on volcanic rocks in Iceland, are reported to be hallucinogenic. Locals in Iceland have consumed them recreationally, often in a preparation called "rock soup." Other lichens, like Dictyonema huaorani, may contain psilocybin, a substance linked to hallucinations.

Lichens can create striking visual displays in nature. In places like Yosemite National Park, Sequoia National Park, and the Bay of Fires, lichens add color to landscapes. Orange and yellow lichens enhance the look of deserts, tundras, and rocky shores. Lichens hanging from trees create a mysterious atmosphere in forests. Fruticose lichens are used in hobbies like model railroading to make miniature trees and shrubs.

In early Midrashic writings, the Hebrew word "vayilafeth" in the Bible’s Book of Ruth is interpreted as Ruth wrapping herself around Boaz like lichen. The 10th-century Arab physician Al-Tamimi wrote about using lichens dissolved in vinegar and rose water to treat skin conditions.

In the science fiction novel Trouble with Lichen by John Wyndham, the story centers on an anti-aging chemical extracted from a lichen.

History

Although people had known about lichens for many years, it was not until 1867 that Swiss botanist Simon Schwendener proposed a new idea. He suggested that lichens are made up of two living things: fungi and algae or cyanobacteria. This helped scientists begin to understand the true relationship between these organisms. Schwendener’s idea was based on his careful study of lichen, algae, and fungi using a light microscope. However, many scientists at the time, such as James Crombie and Nylander, did not agree with him because they believed all living things were independent.

Other scientists, including Heinrich Anton de Bary, Albert Bernhard Frank, Beatrix Potter, Melchior Treub, and Hermann Hellriegel, were more open to Schwendener’s idea. Over time, this concept spread to other areas of study, such as research on microbes, plants, animals, and human diseases. Later, when scientists better understood how disease-causing microbes interact with their hosts, Schwendener’s idea became more widely accepted. In 1939, Eugen Thomas provided more proof by successfully re-creating lichens in an experiment.

In the 2010s, scientists discovered a new part of the lichen partnership. Toby Spribille and his team found that many lichens, once thought to be made of two types of organisms (fungi and algae), are actually made of three: two types of fungi and algae. The third partner is a type of yeast called a basidiomycete.