Written on the walls: what lichens tell us about our environment

Lichens have no roots, no waxy leaves and no way to filter what they absorb. Everything they take in comes directly from the atmosphere so they are extraordinarily sensitive to air quality. For more than a century, scientists have used lichens to track pollution

Lichens are not parasites, and they are not a single organism. A lichen is best understood as a composite life form: a fungus living in intimate association with a photosynthetic partner, usually a green alga or a cyanobacterium. Lichen growing on the bark of trees in Marina Park, Cork. Picture: Caroline Delaney

They cling to stone walls, creep across tree bark, and wash rooftops in greys, greens and soft golds. Most of us pass them every day without a second glance, mistaking them for stains, weathering or signs of age. Yet these quiet markings are alive, and they belong to some of the most remarkable organisms on Earth.

Lichens grow where few others can survive. They endure intense cold, prolonged drought and relentless exposure to wind and rain. Some can lose almost all their water, shut down completely, and revive hours (or even weeks) later when rain returns.