Mycelium

Did you know the largest organism on earth is a mushroom? The Armillaria ostoyae is a type of fungus that covers 2,384 acres (965 hectares) of soil in Oregon's Blue Mountains. Based on its current growth rate, the fungus is estimated to be 2,400 years old but could be as ancient as 8,650 years, which would earn it a place among the oldest living organisms as well.

We learnt in the page on Earthly Beginnings why without mushrooms, we probably wouldn’t all be here. Due to their synergistic relationship with plants they allowed terrestrial ecosystems to thrive and likely be the reason why the earth is so biodiverse.

What is Mycelium

The above-ground part of the mushroom is called the fruiting body, while the root system of the mushroom is called the mycelium. Composed of a dense mass of fine, thread-like filaments of tissue called hyphae, the mycelium is an essential part of the life support system for the fungus. These threads naturally extend into the soil, plant matter, wood and other materials to pick up water and nutrients and bring them back to the “mothership.” To do so, the mycelium (the spelling of the plural form of the word is the same as the singular form) first excrete enzymes to break down the substrate around them — whether it’s a fallen log, plant matter on the forest floor or nutritious substrates used in indoor cultivation, such as organic oats or rice — into a digestible size so that they can be absorbed through the mycelium’s cell wall. These bite-sized nutrients can then travel up to the fruiting body and be exchanged for important sugars made via photosynthesis.

The long, microscopic fibers of the mycelium are the most bioactive stage of the mushroom’s lifecycle (without the mycelium, there would be no mushroom) and form an underground network that doesn’t just serve to feed the mushrooms on the surface. Though it may sound like it’s out of an amazing science-fiction movie, research has shown that they also form a communication network with other plants and work symbiotically to facilitate the exchange of nutrients and sugars all over the forest. This beautiful interconnectivity is often referred to as the “wood wide web,” and ensures the health not only of the mushrooms but of the entire ecosystem.

While, clearly, the distribution of resources all over the forest is an important function of mycelium, it’s not the only benefit. Research has also shown that mycelial networks can prove effective for reducing competition of invasive species as well as for supporting the host plant’s immune system via bioactive compounds found only in the mycelium. Put simply, the mycelium faces challenges as it extends and grows into its substrate surroundings and must find ways to defend against competitors, such as pathogens. The mycelium develops enzymes and metabolites that it introduces into the substrate and these compounds create protection for the mycelium and the entire mushroom.

Mycorrhizae in latin means; "myco"-fungi and "rhiza"-root.

Making the connection

The mycelium also plays a crucial role in fungal reproduction.

The mycelia of two compatible fungi can fuse together, allowing the cells of each fungus to combine and their DNA to mix.

After fusing, the cells end up in new spores held inside (like in truffles) or exposed outside (like in mushrooms and crusts) the fungus' reproductive structures. This is essentially how some fungi have sex when the environmental conditions are just right.

Many fungi reproduce and form spores — these tiny, seed-like structures are transported by the wind or by animals.

When they land on the ground, they germinate and can grow forming new mycelium.

Fungi can also split their mycelium off into smaller fragments, which then go on to form independent mycelial networks.

The mycelium can even act as a carbon storage facility and will reinvest the carbon into plants in tumultuous times. This relationship is so prevalent that scientists believe 92% of all plants form a mycorrhizal relationship in the soil. This exchange of nutrients is no minor task for a plant to undertake either. It is thought that 40-60% of total photosynthates produced is actually given to the mycelium of the mycorrhizal fungi. This relationship between mycelium and plant life grows even deeper. The mycorrhizal fungi do not simply grow in accordance with one individual but instead connect many individuals throughout the soil. Often one species of mycorrhizal will actually connect species of different plants as well. This type of mycelial network is essential to the health of ecosystems because the mycorrhizae aids in plant communication and even transfers nutrients amongst species. It has been found that plants will actually give nutrients to weaker trees in the community which effectively helps the entire forest. It has been proven that nutrients being recycled from older trees do not always go to older-established trees but instead the mycelium can help the nutrients go to the saplings in the area to help them grow.

Fungal mycelium and carbon sequestration

Mycorrhizal fungi, in symbiosis with plant roots, also play a crucial role in removing carbon from the atmosphere.

Kew Scientists are developing a method to measure fungal diversity and carbon storage below ground, with the aim of scaling this up to habitats across the UK to understand the carbon capture capabilities of different mycorrhizal fungi.