Mycelium

Mycelium is the vegetative part of a fungus, consisting of a mass of branching, thread-like hyphae. The mass of hyphae is sometimes called shiro, especially within the fairy ring fungi. Fungal colonies composed of mycelia are found in soil and on or in many other substrates. Mycelia may form fruiting bodies such as mushrooms. A mycelium may be minute, forming a colony that is too small to see, or it may be extensive. "Is this the largest organism in the world? This 2,400-acre site in eastern Oregon had a contiguous growth of mycelium before logging roads cut through it. Estimated at 1,665 football fields in size and 2,200 years old, this one fungus has killed the forest above it several times over, and in so doing has built deeper soil layers that allow the growth of ever-larger stands of trees. Mushroom-forming forest fungi are unique in that their mycelial mats can achieve such massive proportions." (Stamets)

It is through the mycelium that a fungus absorbs nutrients from its environment. It does this in a two stage process. Firstly the hyphae secrete enzymes onto the food source, which breaks down polymers into monomers. These monomers are then absorbed into the mycelium by facilitated diffusion and active transport.

Mycelium is vital in terrestrial and aquatic ecosystems for its role in the decomposition of plant material. It contributes to the organic fraction of soil and its growth releases carbon dioxide back into the atmosphere. The mycelium of mycorrhizal fungi increases the efficiency of water and nutrient absorption of most plants and confers resistance to some plant pathogens. Mycelium is an important food source for many soil invertebrates.

Sclerotia are compact or hard masses of mycelium.

Uses
One of the primary roles of fungi in an ecosystem is to decompose organic compounds. Petroleum products and pesticides that can be contaminants of soil are organic molecules. Fungi therefore should have potential to remove such pollutants from the soil environment, a process known as bioremediation.

Mycelial mats have been suggested (see Paul Stamets) as having potential as biological filters, removing chemicals and microorganisms from soil and water. The use of fungal mycelia to accomplish this has been termed "mycofiltration", although there is no reason to suspect that the process is any different from that of bioremediation using fungi.