Lichen
-The mutualistic relationship of a fungus with an alga or a cyanobacterium, or both. The fungal part of a lichen absorbs water and nutrients from the surroundings and provides a suitable environment for the alga or cyanobacterium. These produce carbohydrates for the fungus by photosynthesis. Lichens can thrive in harsh conditions and are found in many forms. The relationship between the different organisms is so close that lichens are often referred to as a single organism.