Finding red lichens – in this case probably Herpothallon rubrocinctumon – on trees is a good indicator of air quality. Pink or red lichens are extremely sensitive to sulphur dioxide (SO2), which is released by the burning of fossil fuels. As a result, red lichens like this one usually only grow in places with low levels of air pollution. Did you know that lichens are made up of two separate organisms, a fungus and an alga, in a symbiotic partnership? Algae are photosynthetic and responsible for producing nutrients, while fungi offer protection and prevent dehydration.