Fungi are vital decomposers in the ecosystem, breaking down dead organisms and biological waste, freeing nutrients for use by other organisms and clearing away their remains. Fungi also act in partnership with some plants and algae, and are often vital to the survival of these organisms. Some species are parasites.
Fungi all receive energy and nutrition from their environments, and are incapable of generating food for themselves as plants do. Fungi grow as masses of thread-like structures known as hyphae. These have a very high surface area for their volume, and allow the fungi to absorb nutrients easily. They are generally buried deep in the soil and in decaying organisms, such as rotting wood. Parasitic fungi have specialized hyphae for penetrating living organisms, usually plants.
Fungi in mutualistic relationships with algae are called lichens. The associations between the fungal species and the species of algae are often so complete that each type is given a species name as a whole, despite containing two different organisms. Because of this association, lichens can survive where no other photosynthetic organism can, and they are a vital food source in some very cold environments. Other fungi grow in association with plant roots, where they provide vital nutrients in exchange for sugars and amino acids. It is estimated that 90 percent of vascular plants have associated species of fungi in mutualistic relationships with them.