Essentially, the term mycelium is used to refer to the thread-like structures of fungi. Mycelium (plural mycelia) develops from the fungal hyphae. While mycelia plays an important role in reproduction (vegetative parts of fungi), they are also involved in the decomposition of organic matter, which makes them very important in nature.
The life cycle of most fungi starts with the production of spores that germinate to form hyphal threads. Given that most of these fungi are sessile, apical extension/growth of the hyphae ultimately results in the formation and growth of the mycelia (mycelial network).
Given that mycelia are a collection/bundle of hyphae, they are more visible to the naked eye compared to hyphae that may not be as visible.
To the naked eye, fungal mycelia appear like a ball of cotton. Here, a network of hyphae bundle together to form mycelia, which are found on substrates, soil or underground where they obtain nutrients.
Unlike fungal hyphae, mycelia are highly branched, which makes it visible to the naked eye. In addition to being highly branched, they have also been shown to be highly septate, which means that the tubular structures are divided into compartmental cells by septa.
Some of the organelles that can be found in these cells include:
Each of the cells is surrounded by a plasma membrane, which is in turn surrounded by the cell wall. This strengthens the mycelia as they rapidly continue growing and branching.
While mycelia can be found on the surface of the substrate or on the soil surface where there might be substrates and nutrients, they also have large masses of hyphae that reside under the ground.
These masses, known as shiro, can also be found in the substrate on which the fungi are growing. These masses play an important role of obtaining nutrients beneath the soil or from the substrate to support fungus growth.
For instance, such fungi as mushrooms are connected to each other by the masses and network of these mycelia. As they obtain nutrients from beneath the soil or in the substrate, these nutrients are used to ensure that the fungus continues to grow and develop further.
The mycelium is therefore well capable of spreading as far as the substrate of the nutrients is available for continued growth. As a result, they have been shown to spread many kilometers away.
* While parts of the fungi, such as the visible mushroom, tend to grow and die on soil surface, mycelium remains beneath the soil in all conditions. During favorable conditions, it gives rise to the different forms of fungi that can grow on the surface of the soil while providing it with the nutrients it requires.
Even as they continue to grow and spread, it is worth understanding how this takes place. Whereas hyphae growth may be guided by external stimuli (in response to chemicals or light etc), mycelium growth tends to expand outwards from the initial source of nutrients or the substrate.
They grow out in a circular manner while expanding further outwards (thus forming a larger circle). As the mycelia deplete available nutrients in the inner part of the circle, the inner mycelia is no longer maintained given that there are no more nutrients. This therefore leaves an empty central part as the mycelia continue to spread outwards more.
This is showed in the diagram below:
To be able to take up nutrients more efficiently, mycelia also release different types of enzymes in their environment so as to break down the complex materials into simpler material that they can easily absorb.
For instance, complex sugars and proteins are broken down to their simpler forms (amino acids, glucose) etc. In these forms, they are easily absorbed through the pores present on their wall. On the other hand, they also grow towards water or areas with high moisture concentration so as to absorb water required for development.
Mycelia are involved in reproduction when a spore germinates to form a type known as homokaryotic mycelia. Also referred to as homokaryon, the homokaryotic are mycelia that have nuclei of the same genotype.
For instance, when such spores as basidiospore germinate, they grow and produce homokaryotic mycelium consisting of monokaryon or uninucleate cells. When two monokaryons come in contact with each other, the hyphal walls break open in a process known as hyphal anastomoses. This allows for the nuclei of vegetatively compatible monokaryons to move into the mycelia of the other monokaryon.
Ultimately, this results in the formation of binoculeate cells and consequently in the formation of dikaryon mycelium. Depending on whether the external conditions (moisture, temperature and pH among others), the dikaryotic mycelium can produce fruiting bodies that are involved in sexual reproduction.
* As the mycelium continue growing and spreading inside or on the surface of the substrate, it absorbs nutrients that are then transported to support the grown and reproduction in the fruiting bodies.
Both hyphae and mycelium are involved in the decomposition of substrates.
Because they grow and spread far and wide, mycelium play an important role in the decomposition of various complex organic compounds. This converts biomass to compost.
They can enhance crop yields through their symbiotic relationship with plants.
They can cause food spoilage
Islam, M. R. et al. (2017) Morphology and Mechanics of Fungal Mycelium.