A lichen is not a single organism; it is a stable symbiotic association between a fungus and algae and/or cyanobacteria.
Like all fungi, lichen fungi require carbon as a food source; this is provided by their symbiotic algae and/or cyanobacteria, that are photosynthetic.
The lichen symbiosis is thought to be a mutualism, since both the fungi and the photosynthetic partners, called photobionts, benefit.
The British Lichen Society
The Smell of Water 