You know what a fungus is.

Chances are you’ve enjoyed some on a pizza or seen them growing in your refrigerator after leaving that cheese in there just a little too long. But some fungi are a lot harder to spot than those you eat, or even the ones you see on a stroll through the woods.

New research suggests that most of the world’s fungi aren’t just rarely seen or found solely underground. They’re flat out invisible - and that’s becoming a big problem. [♪ INTRO] The mysterious fungi we’re talking about are part of something called the “dark fungal taxa”, or dark fungi. Other than being a pretty cool name, these fungi were named after dark matter, since, like dark matter, we know they’re out there but we can’t see them.

And this isn’t as out there as it sounds, because not every part of even the fungi we do know about can be seen with the naked eye. The mushrooms we know and love are just one part of a fungus - the fruiting body of a vast network called the mycelium, that’s usually hidden underground. And there are many types of fungi living en masse in the soil beneath our feet, many of which will never grow fruiting bodies above ground.

So it isn’t all that surprising that there are invisible fungi in the soil. The weird part is that dark fungi don’t have any life stage or structure that we can see, at any point, even after digging around for them. In fact, dark fungi are really only detectable when we look for traces of their DNA.

Thanks to advances in DNA sequencing technology, dark fungi account for most of the fungal species that have been discovered in the past five years. And these are just the fungi found in soil. Researchers have barely scratched the surface of the dark fungi found in water, air, wood, and plant material.

It’s estimated that there are between 11 and 13 million fungal species out there, but only 150,000 have been described. So for all we know, dark fungi could be the most common variety. Since we know fungi tend to have certain roles in ecosystems, like fixing nitrogen, decomposing dead matter, and recycling carbon, we have to assume that the incredible mass of invisible dark fungi are also playing important roles.

For example, Mycorrhizal fungi which grow in a symbiotic relationship with plant roots, are estimated to help move an incredible 13 billion metric tons of carbon dioxide underground every year. It really makes you wonder what kind of heavy lifting dark fungi are doing in the ecosystems where they live. But if we can’t document and study them, we can’t figure out their potentially crucial jobs.

So, we need to find them. The problem is, they’re very good at hiding from scientists, and this might be because many of them are thought to have a parasitic life cycle. Dark fungal species are likely found throughout the fungal family tree, but mycologists think they’re particularly common in the very early evolutionary branches.

Previously described fungi in these early evolutionary branches are often parasites. They lack the essential genes to live independently, so they rely on other organisms to keep them going. And it’s thought that many dark fungi fall into this category, too.

The trouble here is that because parasitic fungi rely on a host to survive, it makes them especially difficult to culture in labs for further study. So researchers can find unique DNA from the species in the environment, but can’t match that DNA with anything they observe in that soil or culture in a lab. Just to hit home how difficult this makes things, let’s think about bacteria.

Bacteria are tiny, with a single bacterial cell of E. coli being just 1-2 micrometers long. That’s 1000 times smaller than a millimeter. Despite being itty bitty, we can culture them, pop them onto slides, and observe them under microscopes.

But culturing dark fungi is seemingly impossible. We only know they exist from their DNA. This leaves mycologists struggling with the bizarre problem of how to name these invisible fungi.

See, in order to name and classify any kind of new species, you need to follow a pretty rigid set of internationally agreed upon rules. This includes having something called a type specimen. That specimen is the first one that the researcher found, and is used to describe the physical features of that thing.

So the type specimen of, say, an African elephant, would be described as big, gray, long trunk, etc. The same is true of fungi - when you find a new one, it needs a description. Like the type specimen of iconic toadstool mushroom, with its bright red cap and white warts.

And the first one discovered is what researchers would use as the reference for all others of its species. That’s true of that E. coli bacteria we talked about before, too. They were discovered, described visually, and through that description, other researchers could also learn to identify E. coli.

But we can’t even culture these dark fungi, so there’s no way to describe them, leaving them unnamed and largely ignored. The research community is now looking for a way forward, and a group of mycologists have suggested that we shake up the way we name fungi. Rather than using visible samples, they want to name the most common types of dark fungi based on strictly defined aspects of their DNA sequences, something that the current rules on naming fungi species doesn’t allow.

This suggestion has sparked a lot of debate. Some mycologists are ready to go back to the drawing board with this new DNA-based list of criteria to describe the thousands of overlooked fungi. Others are wary of overthrowing the rules, since information could get lost in the transfer.

But everyone seems to agree that we need to find the best way to account for our hidden friends. For now, researchers are using a huge database to catalog their DNA for research purposes, meaning that they can start collaborating on studies of dark fungi. Naming them will help science and be essential for communicating about them to the wider world to inspire conservation efforts.

But until the international mycologist meeting next year, when hundreds of scientists will discuss how to name them, it looks like the dark fungi will remain where they have spent millennia - in the dark. And learning about how there’s all these missing fungi out there might have you wanting to get outside and look for them too. If you want to start hunting for the mysteries that are invisible to our naked eyes, we’ve got just the thing to help you get started.

Our sibling channel, Journey to the Microcosmos, has a microscope you can order today! If you’d like a microscope to call your own, head over to microcosmos.store. And if you find any brand-new fungi, maybe you can name them after us.

You know, after the scientists figure out the whole naming thing Thanks for watching! [♪ OUTRO]