Unfortunately, lichens are alien to most people. Have you ever wondered what those colorful splotches are that you can find on the bark of plants or even on the surface of stone? They don’t seem to be plants, a lot of people identify them simply as “a fungus” or “moss,” and sometimes they are even confused with trail blazes and have led many hikers in the wrong direction, but more on that later. So what are lichen? They are a little bit of everything.
Lichens come in many forms and sizes. Some are crustose and can look like splotches of color, some fruticose, or shrub-like, and still others are foliose, resembling dried leaves. They can be found in almost every part of the world and enable lush forests to grow out of volcanic islands by creating the medium for plants to grow in, they feed reindeer in the Arctic tundra, trap carbon dioxide and delay climate change, and many birds use them as nesting material in trees that were named after the lichens that grow on them. Lichens deserve a little more attention, so let’s try familiarize ourselves with them.
Lichens are highly organized symbiotic relationships between fungi and algae and/or a cyanobacteria as a photosynthetic partner. Together they create a common body, also known as the thallus, in which the fungus protects the algae and stabilizes the body, and the algae provides nutrients to the fungus by performing photosynthesis (the process that plants, including algae, and cyanobacteria use to convert light into chemical energy in the form of carbohydrates.) Neither the so called photobiont (the algae or cyanobacteria), nor the mycobiont (the fungus) can naturally exist without each other outside of the common thallus for a longer period of time. So even though lichens are in reality relationships between at least two different life forms, they are, in a way, scientifically treated as single entities with a scientific name for each species of lichen.
But how are there different kinds of lichens if they always consist of a fungal and an algal partner?
To answer this question we should go a little more into detail. Every species of lichen consists of a different set of partners. Many species of lichenized algae, for example, belong to the genus Trentepholia, which is responsible for the orange-yellow color of some lichen, most lichenized cyanobacteria belong to the genus Nostoc, and by far the most lichenized fungi can be found in the phylum of the Ascomycetes, which are also responsible for the sexual reproduction of the lichen.
Sexual reproduction in lichens occurs when the fungal partner produces spores that fuse as haploid gametes (with only one set of chromosomes) into diploid zygotes (cells with two sets of chromosomes.) The zygote then has to get together with a suitable photobiont (the algae or cyanobacteria) to form a lichen with a common body.
Characteristic for ascomycete fungi is that they produce these spores (or ascospores) in special cells (or asci) that are located in the sexually reproductive organs, called ascocarps, which can have many different beautiful shapes and colors, for example in the form of apothecia (see photo of a Ramalina with clearly visible cups, Cladonia leporina with bright red apothecia, and Graphis scripta with curved or worm-shaped apothecia.). Everybody should be excited about lichens!
