Lichens produce small amounts of carbonic acids from their “roots” and these slowly dissolve the rock, releasing nutrients. Other nutrients are obtained from water-borne or air-borne particles of both organic and inorganic materials. The body of a lichen contains an alga; this is able to photosynthesise sugars, some of which are passed on to the fungal component of the lichen. In return the alga gets a tiny but significant amount of shelter within the tissues of the lichen.