What is a lichen?
Often unknown to the general public, lichens are good bioindicator present all around us. There are more than 20,000 species worldwide. Lichens have the particularity of being made up of two organisms: a heterotrophic fungus and an autotrophic alga or cyanobacterium.
In this association, the fungus constitutes the dominant part of the lichen body, also called the thallus. It provides water and mineral nutrition captured from the atmosphere (rain, fog, etc.). In return, the alga or cyanobacterium carries out photosynthesis, producing the energy necessary for the lichen growth. This type of mutually beneficial association is called symbiosis.
Lichens and air pollution
Unlike plants, lichens do not have roots. They uptake all the nutrients from the atmosphere, which makes them sensitive to air pollution. However, not all lichens are affected in the same way by pollution: the same pollutant can be harmful for one species and beneficial for another.

By studying both diversity and abundance of lichens that grow on trees, it is possible to assess air quality without employing chemical or physical sensors. Due to their slow growth, lichens integrate pollution over several years, making them good indicators of air quality.

FAQ about lichens
Are both partners equal in the symbiotic relationship?
No, the fungus is dominant, its biomass represents more than 99%. Moreover, the fungus cannot live alone while the algae can.

How long can a lichen live?
Lichens can live several dozen to several hundred years. Some species even reach several millennia.

Can we eat lichens?
Some species are edible but others are toxic.