Mycelium is the primary vegetative body of a fungus, analogous to the root system of a plant. It exists as a vast, often hidden network of threads that grows within a food source, such as soil or wood, to absorb nutrients. While mushrooms are the more familiar part of a fungus, they are merely the reproductive fruit. The mycelium is the main organism, constantly growing and expanding beneath the surface.
The Mechanics of Mycelial Expansion
Mycelial networks are composed of countless microscopic, thread-like filaments known as hyphae. The expansion of the network is driven by a process called apical growth, where each hypha extends exclusively from its extreme tip. This focused growth allows the fungus to efficiently explore its surroundings and penetrate new areas for nutrients. The rate of this extension can be remarkably fast, reaching up to 40 micrometers per minute in some species.
The growth process begins when a single fungal spore germinates under favorable conditions, sending out the first hyphal strand. This hypha then branches repeatedly, with each new tip also growing outwards. This branching action results in a complex, three-dimensional web that colonizes large areas.
At the cellular level, apical growth is a highly organized process. An organelle called the Spitzenkörper, which is an aggregation of vesicles, directs and organizes growth at the hyphal tip. These vesicles travel along the cytoskeleton to the apex. Through exocytosis, the vesicles fuse with the cell membrane at the tip, releasing enzymes and cell wall components into the environment. This continuous delivery of materials builds the new cell wall and membrane, propelling the hypha forward.
Internal turgor pressure acts as the driving force for this extension, pushing against the flexible, newly formed cell wall at the apex. Further back from the tip, the cell wall becomes rigid, ensuring that growth is directed forward. This mechanism of polarized growth enables fungi to invade substrates, from soft soil to hard wood, and establish extensive networks.
Environmental Conditions for Growth
The expansion of a mycelial network is heavily dependent on specific environmental factors. The substrate, or the material the mycelium grows in, serves as its primary food source. Common substrates include organic materials like wood, grain, straw, and coffee grounds, which provide the necessary carbon and nutrients for the fungus to grow. The mycelium secretes enzymes that break down these complex materials into absorbable nutrients.
Moisture is another requirement for mycelial development. High humidity, between 80% and 95%, is needed to keep the substrate and growing hyphae from drying out. While the environment must be moist, overwatering can be detrimental, as it can lead to a lack of oxygen or encourage the growth of competing molds.
Temperature directly influences the metabolic rate of the fungus. Most mushroom-forming species have an optimal temperature range for mycelial growth, between 20°C and 30°C (68°F and 86°F). Temperatures outside of this ideal range can slow down or even halt growth. For example, many oyster mushroom species see optimal mycelial expansion between 28°C and 30°C.
Like animals, fungi respire by taking in oxygen and releasing carbon dioxide. Proper gas exchange is necessary for a healthy, growing mycelial network. While some CO2 is tolerated, high concentrations can inhibit growth and prevent the eventual formation of mushrooms. A constant supply of fresh air is needed to ensure oxygen levels are sufficient and CO2 does not accumulate to harmful levels.
Ecological Functions of Mycelial Networks
In natural ecosystems, mycelial networks act as decomposers. Fungi are the primary agents breaking down tough organic matter like wood and dead leaves. By secreting powerful enzymes, the mycelium decomposes polymers such as cellulose and lignin, recycling nutrients back into the soil. This process is part of the carbon cycle, releasing carbon dioxide as the fungus metabolizes its food.
Beyond decomposition, mycelium forms symbiotic relationships with plants. Through mycorrhizal associations, fungal hyphae connect with the root systems of most plant species. In this arrangement, the fungus extends the plant’s reach for water and nutrients like phosphorus and nitrogen, while the plant provides the fungus with sugars from photosynthesis. These networks can link many plants together across vast areas, and a single gram of forest soil can contain up to 200 meters of hyphae.
Mycelium in Human Applications
Humans harness the growth of mycelium for various purposes, most notably in mushroom cultivation. By providing a suitable substrate and controlling environmental conditions, growers encourage mycelial colonization before inducing the fungus to fruit. This process produces edible and medicinal mushrooms.
In biotechnology and material science, mycelium is a sustainable resource. It can be grown on agricultural waste to create eco-friendly materials. These mycelium-based composites can be formed into shapes for biodegradable packaging, as an alternative to polystyrene. Researchers are also developing mycelium “leather” as a sustainable textile and using its binding properties to create materials for construction and insulation.