7 October 2022

What in earth? Understanding what fungi really are

They’re a common sight all over the world, but what actually is a fungus?

By Eddie Johnston

An illustration of several different kinds of fungi

Take a walk through a forest, a field or a park during the autumn and you’ll almost definitely spot them.

Clusters of toadstools popping up out of the ground, growing in rings on the grass or even out of the side of trees.

You might know toadstools as a type of fungus, a group that’s often linked with mould and rot. But fungi are in fact a huge diverse range of organisms that are crucial for life on earth.

So… what actually is a fungus?

A brown cluster of mushrooms growing in a green forest dell
Armillaria ostoyae mushrooms (fungal reproductive structures), growing in southern Germany. The mycelium is hidden belowground. Holger Krisp on Wikimedia Commons CC BY SA 3.0
Several colonies of green, white and brown microfungi
Microfungi, Andrei Trigubovich on Wikimedia Commons CC BY-SA 4.0

The family tree and fungi

For hundreds of years, all fungi were considered to be plants. Given that they grow in similar places, and appear to have a similar lifestyle, it’s easy to see why scientists of the time believed this.

But as scientific equipment improved, we began to discover more about the secrets of fungi that showed they were actually very different to plants.

It wasn’t until the mid-20th century that fungi were moved from the Plant Kingdom to their own new Kingdom by ecologist Robert Whittaker, who based it on how fungi obtained their food amongst other differences.

Did you know?

Humans and fungi are actually more closely related to each other than they are to plants, as they share a common ancestor around 1.5 billion years ago.

A pair of gloved hands use a pair of forceps on a small plastic test tube
Inspecting fungi samples © RBG Kew

Spot the difference

So what are some of the differences between plants and fungi?

One of the major differences lies in their cells. Plant cells maintain their shape with a structure known as a cell wall, which is primarily made of cellulose. Fungi also have a cell wall, but instead it’s made out of a material called chitin.

Chitin is also the material that makes up the exoskeletons of beetles, the wings of butterflies and even the beaks of giant squid!

Fungal cells also differ from plants in that they lack chlorophyll (the green pigment in most plants), which leads to another difference between fungi and plants; how they get their energy.

Chlorophyll allows plants to use the power of the sun for photosynthesis, to create sugars from water and carbon dioxide. Fungi, on the other hand, need to get their fuel from the world around them, similar to how animals do. But their approach is a little different.

Whilst we take food from the environment and put it into our bodies to be digested and absorbed, fungi work in reverse: they produce chemicals to digest food in the environment, then absorb it into their bodies.

A large brown and beige beak from a giant squid
A giant squid (Architeuthis dux) beak, which is made from chitin, the same substance found in the cell walls of fungi, The Trustees of the Natural History Museum, London CC BY SA 4.0
A black and white microscope image of fungal cells
Microscopic view of cells of Rhodotorula taiwanensis © RBG Kew

More than just mushrooms

For a lot of us, fungi are synonymous with mushrooms. But mushrooms are only one part of the lifecycle of some fungi.

A fungus will begin its life as a spore, which can be carried away by the wind or an unwitting animal courier, like an insect.

When the spore finds a suitable environment, it will germinate (sprout) and begin growing into long thin strands known as hyphae. These hyphae will stretch out through soil or plants to begin gathering nutrients.

As these hyphae grow, they form a network known as the mycelium. As these mycelial networks grow larger, they’ll sometimes bump into other mycelial networks. That’s when one of two things can happen.

If the fungi are incompatible, each of them will do their best to destroy the other whilst defending themselves by creating anti-fungal compounds. Fungi are highly competitive, so will battle to ensure they have the resources they need to survive.

Wood from trees that have played host to these fungal battlegrounds is known as spalted wood and is highly prized for its unique patterns.

A close up on wood showing areas with lighter colouring and black lines running through it
Spalted wood showing past fungal combat, Bernhard Hoffman on Wikimedia Commons

All’s fair in love and spore

But if the fungi are compatible, they avoid fighting. Instead, they begin to merge their mycelia together, and share cells between each other.

This mixing of mycelia leads to the production of spores, which end up in the fruiting bodies of the fungus. In many species, the fruiting bodies are what we would recognise as mushrooms, but in some species the fruiting bodies remain underground, like truffles.

Mushrooms release their spores through various means into the air, to be carried away to their new homes by the wind. Often, mushrooms will sprout after the rain as the humidity helps distribute the spores further afield.

Fruiting bodies underground are instead reliant on animals for their distribution. Truffles are commonly rooted up by pigs and boars, that distribute the spores when they eat them.

What makes fungi compatible? While they don’t have sexes in the way that we might recognise them, each fungus has a mating type which needs to be compatible. In some species of fungi, there can thousands of different mating types, meaning finding a match can be tough!

Close up of brown bumpy false truffles covered in soil
Belowground fruitbodies of false truffles (Elaphomyces granulatus) © Caroline Hobart
A small wild boar standing in golden brown foliage
A young wild boar (Sus scrofa), who frequently root for truffles in the ground, Sander van der Wel on Wikimedia Commons, CC BY SA 2.0

Don't forget the little guys

But some fungi don't make fruiting bodies at all. A group of around 13,000 species of fungi exist almost exclusively as a mycelium, and are known as microfungi. These include the white powdery mildew that grows on plants, as well as the mold that grows on rotting food. Instead of releasing spores through a fruiting body, they come directly from the hyphae itself.

Microfungi are just as varied as their macro relatives. Along with causing plant diseases like mildew, they are behind skin conditions such as athlete's foot. 

But they aren't all bad. One species, Penicillium chrysogenum is used to produce pencillin, a medicine which has saved the lives of millions of people.

Want to learn more about the incredible kingdom of fungi? Check out some of the weird and wonderful fungi found all over the world, including reliable rotters, and glow in the dark surprises.

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