The relationship between a fungus and an alga in a lichen is categorized as mutualism because both organisms derive significant benefits from their association. Alga Performs photosynthesis to produce organic food (carbohydrates/sugars), which it shares with the fungus. Fungus,on the other hand, provides the structural framework (thallus), which protects the alga from harsh environmental conditions (like desiccation and UV radiation), and absorbs water and essential minerals from the environment to provide to the alga.