Lichen

A lichen is a special partnership between two different kinds of tiny plants: algae or cyanobacteria, and a type of fungus. They work together in a way where both benefit, making a new living thing that is different from either of its parts. Lichens come in many shapes and colors, and they can grow on almost anything—rocks, tree bark, leaves, and even metal. Some look like tiny branches, others like flat leaves, and some grow like a thin paint coating a surface.

Lichens are very tough and can live in some of the harshest places on Earth, from cold arctic areas to hot deserts. There are about 20,000 known kinds of lichens, and they play an important role in nature. They help create soil and give food and homes to many small animals. Because they grow very slowly, scientists sometimes use lichens to figure out how long ago certain events happened, like when a new piece of land was exposed after a landslide. Lichens are among the first living things to make a home in new or damaged places, helping other plants and animals to follow.

Etymology and pronunciation

The word lichen comes from an old Greek word meaning 'tree moss'. It then passed into Latin. In American English, we say "lichen" like the word "liken." In British English, people sometimes say it to rhyme with "kitchen" as well.

Anatomy and morphology

Growth forms

Lichens can grow in many different shapes and forms. The way they look is called their morphology. The shape of a lichen is mostly decided by how the fungal threads are organized. The main body of a lichen, called the thallus, is usually the most noticeable part. Lichens are grouped by the type of thallus they have.

Common ways lichens grow include:

1. fruticose – growing like a tiny shrub with branches, either standing up or hanging down
2. foliose – growing flat and leaf-like
3. crustose – growing flat on a surface like a thick paint coating
4. squamulose – made of small, leaf-like pieces that are attached at the edges
5. leprose – powdery
6. gelatinous – jelly-like
7. filamentous – stringy or like tangled hair
8. byssoid – wispy, like loose wool
9. structureless

Sometimes, a lichen can show features of more than one growth type. For example, a crustose lichen might crack as it ages, or a foliose lichen might look branched like a fruticose lichen.

Color

Lichens come in many colors, usually because of the tiny plants or bacteria living with the fungus. Special substances can give lichens colors like red, orange, yellow, or brown. Without these, lichens are usually green or gray when wet, and turn gray or brown when dry. The color can change depending on whether the lichen is wet or dry, and this helps scientists identify different kinds of lichens.

Internal structure

A lichen is made of a tiny plant or bacterium living together with a fungus. Most of the lichen is made of the fungus's threads, called hyphae, which wrap around the plant or bacterium. The main body of the lichen, called the thallus, looks very different from the fungus or plant growing alone. The thallus is made of these fungal threads growing in a special way.

The thallus usually has layers. The top layer, called the cortex, acts like a protective skin. Below this is a layer where the plant or bacterium lives, and under that is a looser layer called the medulla. Some lichens also have special structures to hold on to the surface they grow on. The way these layers are arranged helps scientists tell different lichens apart.

Physiology

Symbiotic relation

Main article: Symbiosis in lichens

A lichen is made up of two or more different living things working together. It is formed when tiny plants called algae or cyanobacteria live among the threads of a fungus. Both the fungus and the algae or cyanobacteria benefit from this partnership. The fungus gets food made by the algae or cyanobacteria through a process called photosynthesis. In return, the algae or cyanobacteria are kept safe by the fungus, which also gathers water and nutrients for them.

Lichens can survive in many different places because this teamwork helps both partners live better. They mainly get water and nutrients from the air, like from rain or dust. The fungus protects the algae by holding onto water and gathering minerals, and sometimes it even gives the algae minerals it finds.

Ecology

Lichens show how different living things can help each other. Some scientists think lichens might also include tiny bacteria that help in other ways. Lichens are good at showing how clean the air is. If the air is very dirty, there might be no lichens around, but if the air is clean, many different kinds of lichens will grow.

Some lichens can even live in very dirty air and are often found in cities on walls and trees. Lichens have been used to make colors, perfumes, and medicines. A few lichens are eaten by animals like reindeer.

Reaction to water

When lichens get wet, their outer layer becomes clearer. This lets the algae inside do their work of making food for the lichen when water is available, and stay safe when it is dry.

Metabolites, metabolite structures and bioactivity

Lichens can be very good at protecting themselves from damage. They make special substances that might help them choose where to grow.

Growth rate

Lichens grow very slowly, often only a tiny bit each year.

Life span

Some lichens can live for a very long time. One lichen in Alaska is thought to be almost 10,000 years old!

Response to environmental stress

Lichens can lose almost all their water and still survive. When water becomes available again, they quickly come back to life. Experiments have even shown that some lichens can survive in conditions like those on Mars, and they have been able to live outside of Earth's atmosphere in space.

Reproduction and dispersal

Many lichens can make new lichens without using seeds. One way is when a small piece breaks off and grows on its own. Another way is by making tiny groups of cells that can grow into new lichens. These tiny groups can be carried away by wind.

Lichens can also reproduce using special structures that look like dots or lines on their surface. These structures help the lichen make tiny seeds called spores. When these spores find the right partner, they can grow into new lichens. Some lichens even make mushroom-like shapes to help them reproduce.

Taxonomy and classification

Lichens are grouped by the type of fungus they contain. Each lichen gets the same name as the fungus it contains. Lichens are now part of how scientists classify fungi. The green or blue-green partner (algae or cyanobacteria) has its own name, but it does not share the lichen's name. There are about 20,000 known lichen species, and scientists think there may be as many as 28,000 in total. Almost 20% of known fungi are part of lichens.

A lichen can sometimes look like different species depending on the type of algae or cyanobacteria it partners with, even though the fungus is the same. Previously, scientists classified these as separate species, but now they are considered the same species if the fungus is identical.

Lichens have developed many times throughout history from fungi partnering with algae or cyanobacteria.

Fungi

The fungus in a lichen is called the mycobiont. It can be one of two types of fungi, leading to lichens being called either ascolichens or basidiolichens. Living in a lichen helps the fungus get important nutrients, and about 20% of all fungi live this way.

The main fungus determines the shape and features of the lichen, but different partners can lead to very different appearances. Sometimes, a single fungus can work with multiple partners, including both algae and cyanobacteria.

Photobionts

The green or blue-green partner in a lichen is called a photobiont. Most lichens have either a green alga or a cyanobacterium. About 90% have green algae, and 10% have cyanobacteria. Common green algae partners include Trebouxia and Trentepohlia. The most common cyanobacterium is Nostoc.

Lichens with cyanobacteria are called cyanolichens. Many are small and black, and some are jelly-like and live in moist areas.

Controversy over classification method and species names

There are about 20,000 known lichen species. However, what counts as a "species" in lichens is different from animals or plants because lichens are made of two or more different living things. By convention, lichens are named after the fungus they contain, not the algae or cyanobacteria. This can be confusing because the same fungus with different partners can look very different. Scientists debate whether these different-looking lichens should be considered separate species or not.

Diversity

Most lichen-forming fungi belong to a group called Ascomycota. Some lichens form in groups where not all members are lichens, while others are found only in groups where every member forms a lichen.

Identification methods

Scientists identify lichens by looking at their shape, using microscopes, and testing how they react to certain chemicals.

Evolution and paleontology

Fossils of lichens are rare because the places lichens live, like tundra or deserts, do not usually preserve fossils well. However, scientists have found lichen fossils in amber and other places. The oldest known lichen fossil is about 400 million years old. Research suggests that lichens may have evolved after plants first appeared on land.

Ecology and interactions with environment

Substrates and habitats

Lichens can grow in many different places, from the bark of trees to rocks, soil, and even walls. They live in places like rain forests, deserts, and cold arctic areas. Some lichens can even grow inside solid rock or blow around freely.

Lichens help break down rocks into soil by changing the minerals. This can be a problem for important stone monuments like Mount Rushmore, which needs special care to keep the lichens from damaging it. Lichens are not parasites; they just use surfaces to live on. In the arctic, lichens cover the ground and help keep the soil warm.

Lichens and soils

Lichens help turn rocks into soil by breaking them down with chemicals. They also help add nutrients to the soil and keep sand in place, especially in deserts. This is important for keeping the land healthy and stable.

Ecological interactions

Lichens are often the first to grow in places where nothing else can, like bare rock or after a disaster. They can survive with very little water by going into a resting state until conditions improve. Lichens can grow on other plants without harming them and help keep the soil stable so other plants can grow.

Effects of air pollution

Lichens can show how clean the air is because they absorb pollutants from the air. Different lichens react differently to pollution, and scientists have used them to monitor air quality for a long time. When lichens are exposed to pollution, it can upset the balance between the parts of the lichen, which may cause them to die off. This helps us understand how polluted the air is.

Human use

Further information: Ethnolichenology

Food

Main article: Edible lichen

Many cultures around the world eat lichens. Some lichens are only eaten when there is not much else to eat, while others are a regular part of meals or even a special treat. Eating lichens can be tricky because some parts are hard to digest and others can be harmful unless prepared properly. Only a few lichens are dangerous, but those with certain acids can make people sick. Most harmful lichens are yellow.

In the past, people in northern Europe relied on a type of lichen called Iceland moss as a food source. It was cooked in many ways, like bread or soup. In North America, another lichen was important for food, often cooked over a fire. People from cold northern regions also eat a special lichen found in the stomachs of reindeer they hunt. A lichen called rock tripe has been used as emergency food in North America, and one kind is even used in traditional dishes in Japan and Korea.

Lichenometry

Lichenometry is a way to find out how old a rock surface is by looking at the size of lichens growing on it. This method was created in the 1950s and is used in studying old places, understanding past life, and looking at how land shapes over time. By measuring the size of the biggest lichens on a rock, we can estimate how long the rock has been exposed. This method works best for rocks that have been uncovered for less than 1,000 years.

Biodegradation

Lichens can break down certain materials, like the resins found at ancient sites in Spain. They can also collect harmful substances from the environment, such as lead and copper. Some lichens can help break down harmful proteins, which might be useful for cleaning up polluted areas.

Dyes

Lichens make special substances that can be used to color fabrics. These substances also help protect the lichen from too much sunlight and can even fight off harmful tiny creatures or bacteria. One important dye comes from a lichen called dyer's weed and is used to test if something is acidic or basic. Traditional clothes from Scotland were colored using lichens, and for thousands of years, lichens have been used to make purple and red dyes. Natural lichen dyes have mostly been replaced by man-made ones.

Traditional medicine and research

In the past, people used certain lichens for treating illnesses because of how they looked. For example, one lichen was used for skin problems because it resembled the affected area. Scientists are studying substances from lichens to see if they can help treat or diagnose diseases. One substance from lichens is being tested to see if it can stop the growth of certain harmful bacteria.

Recreational and entheogenic use

Some reports say that certain lichens, especially those found on volcanic rocks in Iceland, were used by local people for enjoyment. Other lichens have been mentioned as possibly containing substances that change how a person feels.

Aesthetic appeal

Lichens can make landscapes very beautiful. They cover rocks and trees in colorful patterns, making places like national parks and coastal areas look special. Their bright colors add beauty to deserts, cold areas, and rocky shores. Lichens hanging from tree branches give forests a mysterious look. They are also used in hobbies like model railroading to create tiny trees and plants.

In literature

In old stories, lichens were used as a comparison. A famous book titled Trouble with Lichen is about a special chemical from a lichen that can help people live longer.

History

Lichens were known as living things for a long time, but people didn’t understand exactly what they were made of until 1867. That’s when a Swiss botanist named Simon Schwendener suggested that lichens are made from a mix of fungi and algae or cyanobacteria. At first, many experts didn’t agree with this idea because they believed all living things worked alone.

Later, more scientists began to see the value in Schwendener’s idea, especially as they learned more about how tiny living things interact with each other. In 1939, an experiment finally proved that lichens really are a mix of different partners. Then, in the 2010s, scientists found something even more interesting: many lichens actually have three partners — fungi, another type of fungi called basidiomycete, and algae.