Describe how lichen are used to indicate air quality

Lichens are composite organisms in which a single species of fungus (mycobiont) lives symbiotically with one or more algal species (phycobionts), some of which may be nitrogen (N) fixing (cyanolichens), e.g. members of the blue green algae. The fungus provides structure and protection for the algae which reciprocates, providing energy and assimilates, via photosynthesis. Many secondary metabolites are synthesised by the fungus, which are unique to lichen symbioses.

In Europe lichens have been used as sensitive bioindicators of air quality for more than a century. As with most vegetation, lichens show a range of sensitivities to pollutants. Not all lichens are sensitive to a particular pollutant, some can be remarkably tolerant. Among lichens are species that are sensitive to sulphur, nitrogen, acidity, halogens (e.g. fluoride), heavy metals and ozone.