Lichens

Perhaps the first thing to say is that it is not just one species but, in fact, there are some 20,000 known species of lichen and it is estimated that 6% of the earth's land surface is covered by them.

Put in its simplest form, lichens are made up of two living things: a fungus and an alga or bacteria. The fungus and the alga benefit from living together, as the alga produces food in the form of simple sugars through photosynthesis, while the fungus gathers water. In this way a lichen can survive harsh weather that would kill a fungus or an alga growing alone.

When lichens grow on living things such as trees, they are not parasitic on these plants, but only use them as a substrate to grow on. Indeed, they do not even have roots or require constant hydration, which is why lichens are able to grow in places that other plants cannot, such as deserts, mountaintops and frozen landscapes.

(Linh Anh Cat
I am a scientist interested in how tiny microbes make big impacts in ecosystems. My research has brought me to scenic environments from deserts to boreal forests. I earned my Ph.D. in biological sciences studying airborne microbes, particularly those that cause disease. Now, I am a biologist with the National Park Service in San Diego, CA.)