Chlorine Dust Lichen

The Chlorine Dust Lichen (Chrysothrix chlorina) is a crustose lichen in the family Chrysotrichaceae, historically known from calcareous and siliceous rock surfaces and bark in parts of Europe, including the United Kingdom. It belongs to the genus Chrysothrix, a group of powdery, granular lichens that form bright yellow to greenish-yellow leprose thalli — lacking the organised cortex layers of most lichens — composed largely of granular soredia. Chrysothrix species produce unique secondary metabolites including rhizocarpic acid, which contributes to their intense yellow colouration and may serve as protection against grazing invertebrates. The Chlorine Dust Lichen takes its name from the sharp, bleach-like or chlorine odour emitted when the thallus is moistened — a distinctive diagnostic feature. The IUCN has assessed this species as Extinct in Great Britain, where it was historically recorded from ancient woodland and shaded rock sites but has not been confirmed in recent decades. Its apparent extinction in parts of its former range is attributed primarily to severe air quality degradation during the industrial era, as many crustose lichens are highly sensitive to sulphur dioxide and other atmospheric pollutants. It may persist in small areas of cleaner air, and improved air quality since deindustrialisation offers some hope for natural recolonisation.