Have you ever wondered what’s up with those splashes of color you see on rocks in the mountains or in the desert?
In many cases, what you’re seeing are lichens. Lichens are living organisms that aren’t plants and they aren’t fungi. They’re actually made up of two types of organisms living together in a symbiotic relationship.
The main body of a lichen is formed by a fungus. This structure is called a thallus. Nestled inside the thallus is a thin layer of single-celled algae. The arrangement works well for both organisms because the fungus provides protection and some nutrients to the algae while the algae shares with the fungus some of the sugars (i.e. carbohydrates) that it makes through photosynthesis.
The lichens you see growing as a crusty layer on rocks are called crustose lichens. This growth form contrasts with the more leafy foliose lichens and the stringy or shrubby fruticose lichens. I wrote about beard lichens, which have the fruticose form, in a previous post.
Not all crustose lichens grow on rock; some species prefer to grow on soil or tree bark. And not all lichens growing on rocks are crustose. Complicated, I know.
As in the photo above, multiple species of crustose lichens may be seen growing close together on the same rock face (the palest species in that photo might be a type of foliose lichen– not sure). They come in an assortment of colors: yellow, green, orange, brown, red, black, and more. Color is one way to identify different species of lichens. But telling species apart can be tricky, in general.
A crustose lichen firmly attaches itself to its rock substrate. It grows into the surface of the rock and extracts minute amounts of material from it.
The ability to live on solid rock, combined with a tolerance for extreme conditions, make crustose lichens pioneer organisms. They are often the first living things to colonize newly exposed or newly formed rocks (e.g. lava that has recently cooled).
And they are the first organisms to begin the long process of breaking big rocks down into soil. Microscopic fungal fibers called hyphae penetrate the rock surface, sort of like the roots of a plant. At a very tiny scale, this breaks apart the rock. Crustose lichens also break rock down chemically by secreting acids, such as our friend oxalic acid. By starting this process of making soil, lichens pave the way for later waves of plants and animals.
How do these lichens show up in barren places? They are blown in on the wind. Microscopic propagules that contain cells of both the fungal and algal components of the lichen land on bare rock and start growing. They need moisture and can get this from the atmosphere.
Crustose lichens grow very slowly in the harsh places that many of them live– places that are dry and either really hot or really cold. A thallus may add less than a millimeter of growth per year in such places.
Lichenometry is a clever technique of dating geologic and man-made features that capitalizes on the slow but predictable growth rate of crustose lichens. These lichens often grow outward from a central starting point in a ring pattern. If you know the general growth rate of the species you’re looking at, you can estimate the amount of time a rock has been exposed by measuring the diameter of the largest single lichen ring on that rock. Pretty cool, huh?
I think crustose lichens are quite lovely, with their assorted colors and textures. To me, they look like miniature coral reefs. And their natural history is just fascinating. I still have a lot to learn about them.