The mycelium is the network of fine, thread-like filaments called hyphae that forms the body of a fungus. This vast, often unseen network serves a foundational purpose in nearly every terrestrial ecosystem as a primary decomposer. Fungi do not ingest their food like animals, nor do they produce it through photosynthesis like plants. Instead, the mycelium secures all its necessary nutrients from the environment outside its structure, allowing it to break down and recycle the most stubborn organic materials on Earth.
The Mechanism of Mycelial Feeding
Mycelial feeding is a process known as external or extracellular digestion, which is fundamentally different from the internal digestion of animals. The hyphae grow directly into or onto the food source, be it a log, leaf litter, or living tissue. Once contact is made, the mycelium releases a powerful cocktail of enzymes directly into the surrounding environment.
These enzymes act like molecular scissors, breaking down large, complex organic molecules into much smaller, soluble compounds. For example, cellulase breaks down cellulose, and ligninase targets the tough polymer lignin. The resulting simple sugars, amino acids, and other small molecules are then absorbed across the thin cell walls of the hyphae.
The structure of the mycelium, with its immense network of hyphal threads, maximizes the surface area exposed to the food source. This high surface-area-to-volume ratio makes the absorption of the digested nutrients extremely efficient. The speed and reach of its enzyme secretion directly determine its ability to grow and colonize new territory.
The Primary Menu: Complex Organic Matter
The most significant carbon sources are cellulose and lignin, which form the structural components of wood and plant cell walls. Cellulose is a chain of glucose molecules, while lignin is a highly complex, woody substance that provides rigidity and protection to plants.
Few other organisms possess the necessary enzymatic machinery to fully dismantle lignin, making certain fungi the main recyclers of woody debris. Fungi that target lignin, often called white-rot fungi, must first break apart this polymer to access the more readily digestible cellulose and hemicellulose.
Beyond carbon, mycelium requires nitrogen for synthesizing proteins, enzymes, and chitin, the structural material of its cell walls. Nitrogen is often obtained from organic sources like proteins and amino acids present in the substrate. The ratio of available carbon to nitrogen (C:N ratio) in the food source is a major determinant of how efficiently the mycelium can degrade the material and grow. Mycelium also extracts various minerals, such as phosphorus, potassium, and trace elements, that are dissolved in the water within the substrate.
Dietary Specialization: The Three Main Types
This specialization leads to three primary classifications: saprotrophic, parasitic, and mycorrhizal fungi, each with a unique food source. Saprotrophic fungi are the most common group and specialize in consuming non-living organic matter, such as fallen trees, dead leaves, and animal waste. They are the ecosystem’s primary agents of decomposition, ensuring that nutrients are returned to the soil.
Parasitic fungi, conversely, derive their nutrition from a living host, which can be a plant, animal, or insect. Their diet consists of the host’s stored energy and living tissues. The mycelium of a parasitic fungus will actively grow into the host and release enzymes to break down the living cells, absorbing the resulting sugars and proteins directly from the host’s body.
Mycorrhizal fungi engage in a mutualistic symbiosis, typically with the roots of plants. Their specialized diet consists of simple sugars and carbohydrates, such as glucose, which they receive directly from the host plant’s roots. In exchange for this carbohydrate diet, the mycelium extends the plant’s root system, providing the plant with increased access to water and scarce soil nutrients like nitrogen and phosphorus.