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.
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 2016 report, some lichens contain a yeast as well.
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 named and classified according to their fungal component.
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. Although the researchers who discovered the yeast found evidence that yeast cells exist in many lichens, as far as I know their presence hasn’t yet been confirmed by other scientists. Yeasts belong to the fungal kingdom and aren’t photosynthetic.
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.
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 colour of lichens is often richer after rain.
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.
Lichen reproduction is quite complex. I’ll discuss it in a future post once I have some reasonably good photos of lichen reproductive structures. The fungus in the organism produces spores, but these grow into new and free-living fungi, not lichens. Lichens also have ways to distribute packages of fungal and alga cells into the environment in order to grow new lichens, however. Though some people may not think much about the organisms, I think they are interesting to observe and explore.