Lichens

This group gathers Fungi subcategory members that are lichens, not a single kind of organism but a stable, self-sustaining partnership between a fungus, which provides structure and protection, and a photosynthetic partner, usually an alga or a cyanobacterium, which supplies sugars produced through photosynthesis in return. Lichens grow in an unusually wide range of forms, from flat, crust-like growths fused directly to bare rock or bark to leafy, lobed forms and upright, branching, shrub-like forms, and they are found from bare Arctic tundra and windswept mountaintops to tropical rainforest canopies, tolerating extremes of cold, drought and ultraviolet exposure that would kill most other complex organisms. Because many species absorb both water and airborne pollutants directly through their surface with no protective outer layer such as bark or cuticle, several are used by scientists as sensitive, low-cost indicators of air quality, disappearing from otherwise healthy-looking landscapes long before more visible pollution damage appears.