Updated on January 19th, 2022
Lichens are interesting organisms with some unique features. They are easy to find at any time in my part of the world, but I especially appreciate seeing them in winter. Though nature walks are still enjoyable at this time of year, there is less variety in the scenery than in other seasons. I’m happy to discover the beauty in the little things.
What Is a Lichen?
A lichen is an impressive example of symbiosis. Its body contains multiple organisms living in what is generally believed to be a mutualistic relationship. In mutualism, all of the associated organisms benefit from the relationship. The lichen is formed from a fungus living in partnership with a green alga and/or a cyanobacterium, which was once known as a blue-green alga. According to a relatively recent report, some lichens contain a yeast as well.
Functions of the Different Components in a Lichen
The fungus is the dominant member of the community in terms of the material forming the thallus (the lichen’s body). It relies on the other members for survival in the thallus form, however. Lichens are usually named and classified according to their fungal component. This practice is sometimes controversial. Some puzzles still exist in relation to why a fungus sometimes joins with other species to form a lichen.
The alga or cyanobacterium in the lichen carries out photosynthesis. It uses light energy to make carbohydrate from carbon dioxide and water. The carbohydrate is food for all of the components of the lichen. The fungus can’t perform photosynthesis, but it protects and helps the other organisms, perhaps by chemicals that it produces as well as by physical protection.
The role of the yeast is unclear. It may produce chemicals that repel microbes and predators. The researchers who discovered that a yeast was present in certain lichens also found evidence that yeast cells exist in many others. Yeasts are actually a single-celled type of fungus. They aren’t photosynthetic.
Types of Lichens
Three main types of lichen thalli exist: fruticose, foliose, and crustose.
- A fruticose lichen is highly branched and looks like a little bush. The “bush” may be erect or hanging.
- A foliose lichen has a shape resembling a collection of leaves. It often has a flattened appearance.
- A crustose lichen forms crusts that adhere tightly to its substrate. Some forms have vibrant colours, including green, yellow, orange, and red.
Some people add other types of lichens to the list.
- Squamulose lichens are somewhat like crustose ones but look as though they’re made of little pebbles in a close arrangement.
- Jelly lichens are unusual forms that have a gelatinous appearance.
Fungal and algal cells in lichens are arranged in separate layers. A layer of fungal cells covers the thallus and layers of fungal and algal cells are found inside. (The algal cells underneath the fungal ones give the thallus a green colour.) In jelly lichens, the different types of cells are mixed together.
Attachment and Growth
The lichen thallus is attached to its substrate by a holdfast or rhizines. A holdfast is a single extension at the bottom of the lichen. Rhizines are more numerous and are thinner. The substrate is often natural, such as a rock, tree trunks, branches, or soil, but is sometimes synthetic, such as metal, plastic, or glass. The holdfast and rhizines are not the equivalent of roots in vascular plants.
Lichens grow slowly, especially the crustose type. At least some of them live for a long time. Some grow in very cold climates and have estimated ages of hundreds or in some cases thousands of years. Lichens are found in many different parts of the world and habitats and can be seen in the Arctic, Antarctica, temperate areas, and tropical ones.
Reproduction of the Organisms
Reproduction in lichens is quite complex. Several variations of the process exist. Tiny pieces of a lichen containing fungal and alga cells sometimes break off from its body and then form a new lichen. This is a type of asexual or vegetative reproduction. The pieces are known as isidea. Lichens also reproduce asexually by producing even tinier spores called soredia, which contain fungal cells surrounding algal ones.
The fungus in the organism also reproduces sexually and produces spores. When the spores are released and germinate, they must join with algal cells in order to form a lichen. If they don’t do this, only a fungus will form.
Some Uses of Lichens
Some lichens have additional uses besides adding enjoyment to a walk. Certain species are an important food source for caribou and reindeer. Though some species have been eaten by humans, it’s important that a casual observer of the organisms doesn’t do this. In general, the organisms aren’t edible for us. Even the ones that have been eaten by people generally require special preparation before ingestion.
Some lichens are a good source of dye for human crafts, which may be worth exploring for artistic people (as long as an area isn’t entirely stripped of its lichens in order to produce dye). Oak moss is a fruticose lichen and not a moss, despite its name. It has a pleasant scent that is used in the perfume industry.
The lack of specific lichen species that would normally grow in an area can be an indicator of air pollution. The organisms absorb nutrients from the air around them, which sometimes contains pollutants. As mentioned above, unlike vascular plants, lichens don’t have roots.
An Interesting and Still Puzzling Component of Nature
I enjoy examining lichens. They add interest to my nature walks. The colour of lichens is often richer after rain, so a wet day can be a good time to see and photograph them as long as the camera can be kept safe in the rain. The organisms are interesting to study and photograph and are an important part of the natural world.
There is still a lot to discover about lichens. The species of Usnea have been described as both medicinal and toxic, for example. I’m looking forward to seeing what else scientists discover in their research. For the general public, lichens are worth observing in their natural habitat. There is definitely beauty in the smaller components of nature.