Marine lichen. Close-up of the lichen, Leconora sordida, on a rock on a beach above the high tide mark. The lichen appears as a pale grey, cracked crust. The spore-producing apothecia are embedded in the crust and form small wart-like lumps each with a central depression. Lichens are a composite organism made up of a fungus and an alga living together. The fungus provides a structure that prevents the alga from drying out. The photosynthetic alga provides food for the fungus in the form of carbohydrate. Photographed in North West Scotland.