Mycelium: What You Need To Know

Understanding the basics of mycelium is important in helping us to understand the reproduction of fungi, as well as the decomposition of all organic matter – these are structures that play a hugely important role in nature.

To help you get your head around mycelium, we have put together everything you need to know.

What Is Mycelium?

The term mycelium is used as a description for all vegetative parts of fungi that grow on solid surfaces and consist of filamentous cells.

The term mycelium can be applied to both unicellular and multicellular organisms, but it is most often associated with the growth of filamentous fungi, such as mushrooms or molds.

In this context, mycelium refers to the mass of interconnected filaments of individual cells that form the body of the fungus.

Fungal mycelium consists of two types of cells: hypha (filament) and conidia (spore).

• Hyphal cells are long, thin, thread-like cells that produce enzymes, which break down organic material into smaller molecules. They also provide nutrients to other fungal cells through their production of extracellular enzymes and by secreting small molecules called exudates.

• Conidia are short, round cells that contain DNA and protein. These spores are produced in specialized cells called sporangia, which are found at the tips of hyphae. Spores are released from the sporangia when conditions become favorable.

There are three basic stages involved in the life cycle of a fungus: spore germination, hyphal extension, and fruiting body formation.

Spore Germination

Spores are produced in specialized cells called sporangia.

When the right environmental conditions are present, the sporangiospores are released from the spore sacs.

Once they reach the surface of the substrate, they begin to germinate.

This process involves breaking open the outer wall of the spore and releasing the inner contents.

As the spore begins to germinate, it produces an enzyme known as chitinase – this chitinase then helps break down the protective layer surrounding the spore, allowing it to absorb water and nutrients.

Once the spore has been fully hydrated, the germ tube emerges from the spore’s tip.

The germ tube grows rapidly until it reaches the surface of the substrate whereupon it becomes branched.

At this point, the hypha will continue to grow and branch outwards until it forms a network of interconnecting hyphae.

Hyphal Extension

Once the hypha has reached the surface of the substrate and branches out, it continues to grow.

It does so by producing a new cytoplasmic membrane, which allows it to take up more nutrients.

The hypha then extends itself further away from the parent hypha.

If the growing hyphae encounter another hyphae, it may either extend towards it, or it may simply attach itself to it.

In some cases, the hypha will not encounter another hypha and instead will grow directly upwards. This is known as aerial hyphal growth.

Aerial hyphal growth occurs because there is no physical contact between hyphae.

Instead, the hypha is able to sense chemical signals emitted by the substrate. These chemicals cause the hypha to move towards them.

When the hypha encounters another hyphal structure, it may either extend toward it, or it may attach itself to it.

Attachment is very common in many species of fungi.

For example, if a hypha encounters another hyphae, it may either extend itself towards it, or it attaches to it.

Attachment can be temporary or permanent. In the case of temporary attachment, the hypha detaches after a certain period of time.

However, if the hypha permanently attaches to another hypha, it will remain attached for the rest of its life.

Fruiting Body Formation

After the hypha has grown sufficiently, it will form a fruiting body. A fruiting body is a complex structure formed by several different types of cells.

Fungal fruiting bodies have two main parts: the cap (or head) and stipe (or stem).

The cap is made up of sterile tissue, while the stipe is made up of fertile tissue.

Sterile tissue is non-reproductive tissue that contains enzymes needed to break down organic matter.

Stipes are also used to transport nutrients throughout the mycelium.

• The cap is usually shaped like a dome and is composed of layers of dead tissues. The cap is covered by a thin skin called the pellicle. The pellicle protects the cap from damage and prevents contamination. The stipe is usually cylindrical and hollow. It is often covered by a thick cuticle. The stipe is responsible for transporting nutrients throughout the mycelial mass. The cap is typically colored white, yellow, orange, red, brown, black, gray, blue, purple, purple, pink, green, or other colors depending on the type of fungus.

• The stipe has an opening at one end called the ostiole. The ostiole is surrounded by a ring of cells called the peridium. The peridium is important because it provides protection against pathogens. When a pathogen attacks the mycelium, the peridium ruptures and releases spores into the air.

Spore Release

As mentioned earlier, the spore release process begins when the hypha comes in contact with another hypha.

Once this happens, the two hyphae become connected via a bridge of cytoplasm.

At this point, the hypha is said to be “sporulating”. Sporulation is the process where the hypha produces a large number of spores.

The spores are then released through pores located near the base of the sporangia. The spores are then carried away by wind currents.

Sporulation is a highly regulated process. During sporulation, the hypha must first produce enough energy to carry out the process.

If the amount of energy produced during sporulation is too low, the spores will not be released.

Once the spores are released, they travel through the air until they land somewhere else.

They can land anywhere in the environment; however, most fungal spores fall to the ground within a few days of being released.

When the spores land, they germinate. Germination is the process whereby the spores come back to life.

Germination involves changes in the spore’s shape and metabolism. After germination, the spore becomes a new hypha.

This new hyphae is called a basidiospore. Basidiospores are the reproductive structures of the fungus.

How Does Mycelium Reproduce?

Fungi reproduce sexually as well as asexually. Sexual reproduction occurs when two haploid nuclei fuse together to form a diploid nucleus.

In fungi, sexual reproduction occurs when two mating hyphae fuse together. Mating hyphae contain both male and female gametes.

Fungi reproduce asexually when there is no fusion between mating hyphae. Instead, each individual hyphae reproduce independently.

Each individual hyphae contains only one nucleus. Asexual reproduction results in more than one offspring.

Sexual Reproduction

During sexual reproduction, the two mating hyphae produce a zygote (a single cell containing both male and female nuclei).

The zygote undergoes meiosis (the division of chromosomes) and forms four haploid nuclei. These haploid nuclei combine to form a diploid nucleus.

Asexual Reproduction

During asexual reproduction, the hyphal tip grows and divides repeatedly.

Every time the tip divides, it creates a daughter hypha that looks identical to its mother.

As long as the hyphal tips keep dividing, the fungus keeps growing.

In some cases, the hyphal tips stop dividing after creating a certain number of daughter hyphae. This stops the growth of the organism.

In other cases, however, the hyphal tip continues to divide without producing any daughter hyphae. In these cases, the fungus grows indefinitely.

The number of times the hyphal tip divides before stopping depends on many factors.

For example, if the temperature is high or the nutrients are scarce, the hyphal tip may stop dividing sooner.

Reproductive Cycle

As we have seen, the mycelial cycle consists of three phases: vegetative growth, the resting phase, and sporulation.

Vegetative Growth

During vegetative growth, the mycelium is actively growing. It is constantly absorbing food, water, and oxygen.

When the mycelium absorbs food, it produces glucose. Glucose is stored inside the cells for later use.

Resting Phase

After the mycelium has absorbed all the food it needs, it enters a rest period. During this period, the mycelium does not grow at all.

It just sits around waiting for something to happen.

Sporulation

After the mycelium rests for a while, it begins to make spores. Sporulation is the process by which the mycelium makes new hyphae.

Spores are produced from special cells called conidia. Conidia are like tiny seeds and look similar to pollen grains.

However, unlike pollen grains, conidia do not need pollination to be fertilized.

Conidia

Condida are produced during the resting phase. As soon as the mycelium starts making conidia, it goes into the next stage of the mycelial cycle.

Spores are released from the conidia through an opening called a pore. Pores can be found on the surface of the conidium.

They also appear on the outer wall of the hyphae. When the spore is ready to be released, it pushes out through the pore.

Then, the spore gets caught by the wind and falls to the ground.

If you see a mushroom covered with white powder, it means that the fungus was recently fruiting, while mushrooms that are still greenish-yellow will probably not fruit for several days.

Fruiting

Fruiting occurs when the mushroom releases spores. After a few days, the spores germinate and start growing into new hyphae.

New mushrooms begin to sprout from the old ones.

Final Thoughts

Understanding the structure and reproductive processes of mycelium is important for developing a more thorough understanding of mushrooms and fungi in general, and this is a fairly broad topic with plenty to get your head around.

Our brief overview is a great introduction and will set you on a path to developing a stronger understanding of this area of microbiology.

https://www.youtube.com/watch?v=eh08I5_Q_t8