Hundreds of millions of years ago, plants started to colonize the land. But amazingly, they did this without roots. So how on Earth did these early plants feed themselves? It looks like they found some helpful friends: A group of fungi provided them with nutrients from the ground, fossils suggest. Today, the vast majority of plants team up with these so-called mycorrhizal fungi, which live in their roots. In return for nutrients, the plant provides the fungi with carbon in the form of sugar and fats. It’s an age-old symbiosis, and one that continues to fascinate soil scientists. How does the plant-fungi relationship work? Is it love or just a deal? In this episode, Anja Krieger learns more from Katie Field, Toby Kiers, Bala Chaudhary and podcast co-producer Matthias Rillig, and explores the world of fungi: How do they travel the world, and what would the world look like from a fungal perspective?

---

TRANSCRIPT

Theme “Deep in the Soil” by Sunfish Moon Light

Bala Chaudhary
They’re connectors. Networks. That’s actually something that I really enjoy doing, I love connecting people. I love connecting research ideas.

Katie Field
I think I would quite like to be a mycorrhizal fungus. Because you would form these sort of very intimate relationships with other organisms.

Toby Kiers
It’s this huge open pipe system underground, where the nutrients are flowing between plant roots. And to be able to experience that kind of massive underground flow in these fine, fine tubes, is something I definitely want to experience.

Anja Krieger
Welcome to Life in the Soil, a podcast by the plant, fungal and soil ecology lab at Freie Universität Berlin.

Toby Kiers
I’d want to be a mycorrhizal fungus just because the vastness of it, right? It can grow for kilometers and be this single individual.

Katie Field
You’re not limited to forming a partnership with just one plant for the rest of your life. You can chop and change and you can connect with other fungi and you’re part of this living network below the soil.

Matthias Rillig
Imagine you live inside of your favorite food, whatever that is, and you’re not just eating it, but you’re inside of that food and you absorb it via your entire body. I think that’s a pretty cool thought.
­
Vocoder sings “Life in the Soil”

Anja Krieger
Thanks for joining us for the second episode of this podcast on soil ecology, supported by the BiodivERsA research network. My name is Anja Krieger, and I’m your host. In the first episode, we explored the special architecture and strange conditions in soil. Now, we want to take a closer look at the fascinating organisms that live in this hidden habitat. And we’ll start with a kingdom of life that often gets confused with plants: Fungi. They can help decompose dead leaves or animals. They can be pathogens, making other organisms sick. And then there are the “mutualists” which team up with plants and other organisms. So fungi can have many different lifestyles, and all are equally important.

Music – Eagle Bugs by Sunfish Moonlight

Katie Field
I think they’re one of those groups that sort of fall between the cracks a bit because they’re not immediately obvious, when looking around you when thinking about or, or classifying types of organisms.

Anja
Katie Field is a professor of plant-soil processes at the University of Sheffield in the UK.

Katie Field
My main area of interest and expertise is in the interactions between plant roots and soil fungi, which together we call mycorrhizas.

Anja
I used to think of fungi as mushrooms – delicious, mind-expanding, some even poisonous. But in fact, the little stems and umbrellas you see on a hike or eat for lunch, those are just the fruiting bodies of a much larger organism. Like the apple of a tree – but a tree hidden in the ground that you don’t see.

Katie Field
If you pick it, sometimes you get what’s left of the mycelia, attached to the bottom, it’s coming out of the soil, your instincts probably are sort of – well, this is a bit like a plant, so I’ll put it in with plants.

Anja
But really, that is only the very tip of the fungus. Its body is formed by fine filaments called hyphae, tiny living lines that spread out invisibly below the ground. They form intricate networks, the mycelium, and belong to an ancient kingdom of life. It first emerged even before plants and animals colonized the land.

Music – Enlichenment by Sunfish Moon Light

­­Katie Field
If we had a time machine and went back 500, 550 million years, we got out the time machine we looked around ourselves. We wouldn’t see any plants on Earth’ landmasses, it would be mostly barren. However, if you’d looked a bit closer at the land surfaces, you probably would have been able to see, actually, there were some sorts of filamentous fungi present on Earth. In fact, recently, scientists have found that actually filamentous fungi could well have been on land up to a billion years ago, which is kind of mind blowing.

Music – Enlichenment by Sunfish Moon Light

Anja
Around half a billion years ago, something equally amazing happened. Aquatic plants moved out of the water onto land. These first land plants probably resembled the simple mosses we know today. There’s strong evidence that it was the fungi that actually helped these plants make their first steps on dry land – often in a fascinating way. The body of fungus got intertwined with the body of the plant at a cellular level.

Katie Field
Mycorrhizal fungi form these really incredibly intricate and beautiful structures within plant cells called arbuscules. These little arbuscules are finally branched fungal hyphae that look like little trees growing inside of cells which sort of leads to the name arbuscule, “tree-like” and you can make out arbuscules in fossils that are sort of 410, 440 million years old. And in sort of modern day plants, and they look almost identical.

Anja
It looks like the early plants and fungi started a partnership, a symbiosis – connected and merged by finely branched fungal hyphae deep in the cells of the plants. And across these structures, they were able to share their resources.

Katie Field
Carbon is transferred from plant to fungus in the form of fatty acids, or sugars. And then nutrients are transferred from fungus to plants in the forms of ions, so phosphates, nitrates, things like that.

Anja
This became a hugely successful evolutionary partnership. Fungi cannot photosynthesize, so they cannot get their carbon like plants do. Instead they have to access it in organic form, after it has been fixed by plants. And plants need mineral nutrients. So their symbiosis made it possible to share and trade.

Katie Field
So if you can imagine 500 million years ago, if you are an alga or a very tiny little land plant, just making your first steps onto the land, and you haven’t got any roots, right, and there’s no real soil – so you can’t feasibly, there’s no real way for you to get nutrients. And so these fungal partners, we think were really important in helping them get access to nutrients that are kind of bound up in minerals and rocks that were present.

Theme (Interlude)

Matthias
Fungi are just so different from us. I guess that’s the most amazing thing. I’m not sure I mean, there’s many organisms that I guess they’re quite different from us, but nothing quite as extreme as a fungus because there’s two things about them that make them really different from us. One is they’re a connection of tubes. That’s what the fungus is. So just imagine you exist not as this body with your two arms, two legs, one head. This is what’s called a unitary plan for an organism – fungi are the exact opposite, they are an example of modular organisms – so they consist of just modules, in this case, a module is a tube.

Anja
This is Matthias Rillig, my co-producer on the podcast. He’s a soil biologist at Freie Universität in Berlin, and runs the soil lab here. Many of the 50 scientists working in his lab are doing research on fungi. Matthias often asks himself: How would a fungus experience the world?

Matthias
Your entire existence is a connection of a bunch of tubes. And that to me is just way out there in terms of imagining that, because you experience just a wide range of environments that are totally different simultaneously, such a… you could imagine, you know, you’re eating a pizza on one end and an apple on the other end and you dip into some cool refreshing beer, and then you have another end of you in french fries. And one end of you is in the sun, the other end is in the shade and one part is hot, one part is cold and all of that basically you experience at the same time. So you are at many places at the same time because you just exist as this network. That’s way out there, it’s very difficult to comprehend what that really means. And the other aspect is that you are invasive. A fungus is basically made to be a tunneling machine, as somebody once called it, so you live in the interior of solid substrates. So if I imagine my favorite food, let’s say vanilla ice cream. So, we as animals, we eat it. Which means we have a digestive system and then we digest it and get nutrition out of it. And fungi don’t do that, fungi don’t eat. Fungi have what’s called absorptive nutrition. Which means they absorb nutrients via their entire body surface. So you like vanilla ice cream, and what you would do as a human, you would eat it. If you’re a fungus, you grow inside of it and you absorb it via your entire body. I think that’s pretty cool thought.

Anja
Yeah, for me, it would be chocolate ice cream. Aaah! So I would be like basically swimming through a pool of chocolate ice cream and eating it at the same time. I would be like immersed, like all of me would be chocolate ice cream?

Matthias
Yes. Well, swimming is maybe not exactly the right word, because it’s more like living inside of a solid substrate. If a fungus finds itself in a liquid medium, then it doesn’t make these hyphae. Because imagine that, you know, if you’re in a liquid and you make these little lines, you would just get, you know (laughs) knotted up, I mean, it would be almost silly, you know. So, when a fungus finds itself in an aquatic environment, then it makes yeasts. So yeast is not a particular type of fungus, it’s a form of a fungus, namely, the single-celled form of a fungus, like baker’s yeast, that makes beer and wine and dough. That’s one manifestation of a fungus. But the more common manifestation is that exists as these lines, but these lines only makes sense when you grow in a solid substrate. So, I wouldn’t say you swim in ice cream, I would say you invade the ice cream as a solid substrate, that is what fungi are made for. That’s the entire idea, is that they are tunneling machines. So you give them something solid to invade, this is what they’re made for.

Anja
What we heard from Katie is that, basically, without fungi, all the plants, all the green life in the terrestrial ecosystems might not even exist, right? They’ve basically engineered the world for all of us.

Matthias
I think that’s a valid view of what may have happened back then – by favoring land plants, and to allow them to colonize the terrestrial surface, they may have changed forever the face of the Earth. And actually, it’s a fascinating idea of why wasn’t it them who took over, right? I mean, but there is fossils of these amazing structures that I would pay a lot of money to see those in real life, but that were like trees of basically fungus or maybe more like a lichen, I think it’s not exactly clear what that really was. But you know, you may ask yourself, why wasn’t the fungi that took over and really formed this planet? Why was it plants that give it basically its face?

Anja
Yeah. Like, why did they retreat into the ground?

Matthias
Yeah. Well, you know, one, trivial answer is that they are not photosynthetic, so you know, they are like you and I, chemoorganoheterotrophs, which means they need organic carbon for making more of their own cells and for deriving energy, and that’s got to come from somewhere. And from someone, and that someone is plants. So, I mean, there is no ecosystem possible with just fungi there needs to be autotrophs.

Music – Theme Interlude

Toby Kiers
These fungal networks are really everywhere. Roughly 80% of all land plant species have these fungi in their roots.

Anja
Toby Kiers is a professor of evolutionary biology at the Free University of Amsterdam. She explores the underground markets of arbuscular mycorrhizal fungal networks, and the evolution of symbiosis.

Toby Kiers
So at first, I think that historically, people had thought of these fungi more as parasites, because they really penetrate into the host cell. And, and this is kind of a growth attribute that you would expect of a parasite to penetrate into a cell. But when I first started this research, scientists were realizing that – no, this was actually a very beneficial fungus, and that it was providing nutrients. And that was a trade symbiosis and a trade mutualism where both partners would benefit. And so there was this big push to acknowledge that these mycorrhizal fungi were positive for plant growth and could actually help plants grow. And my research sort of came in at a point where, you know, this echoing of them being such positive symbionts had really started to be well recognized. And our research kind of began to turn that view a tiny bit by saying that, yes, of course, these fungi are in a cooperative partnership, and there’s this trade mutualism where both partners benefit. The fungus is getting these carbon… sugars and fats from the plant roots. And in return, the fungus is giving nitrogen and phosphorus. And it all seems very harmonious. But actually, if we look even more closely, there are these fungi that enact strategies that are not necessarily as harmonious as people wanted to think at the time.

Anja
It seems this romantic vision of fungi is still very much alive. Toby cautions to take a step back. Yes, it is a cooperative partnership, she says – but at the same time both partners are exploiting each other.

Toby Kiers
Our lab has started to take really high resolution videos of what these nutrient flows look like inside of fungi. And they’re incredibly dynamic, it’s almost like surfing a wave inside a very, very small tube, you know, a tube that’s thinner than a cotton thread. But if you really look down inside these tubes, they’re these dynamic flows that go different directions. Not only do they change, like they reverse – the whole flow goes one direction and then switches to another – but you have simultaneous flow in both directions at the same time. So you have some of the flow going left and some of the flow going right. So they’ve developed this way of moving resources simultaneously in two directions in a single tube. So, I sort of think of it as like a traffic system. But there’d be no barrier between the traffic going North and South. And yet somehow, the highway is still functioning.

Music – Eagle Bugs by Sunfish Moonlight

Anja
It’s clear that fungi are able to allocate resources strategically. They can slow down the flow, they can reverse directions, and guide the nutrients into different parts of their network, to or away from different plant partners.

Toby Kiers
They’re helping plant roots but in the same time, they really doing this to benefit themselves. And I think that a lot of people forget that, that the fungus itself is really trying to maximize its growth and its reproduction. And so they do this by interacting with plants in very interesting ways.

Music – Eagle Bugs by Sunfish Moonlight

Toby Kiers
For example, they can be used as a pipeline for communication. There’s some evidence that information between plants is sent across these networks. Now, whether that’s a passive diffusion of molecules that are diffusing from the plant roots and moving across the network to other plants, or whether it’s a real active process is still unknown. But they’re still acting as this physical network. Bacteria, for example, use the network to move across between different plants. So they can be used as a highway, not only for plant chemicals and molecules, but also as a physical highway for bacterial motility, for them to move across.

Music – Eagle Bugs by Sunfish Moon Light

Anja
Toby is especially interested in the strategies fungi have evolved to trade with their plant hosts.

Toby Kiers
When does it benefit to be a good partner? And when does it benefit to, you know, skim the cream off the milk and just take a little bit more for yourself?

Anja
One of the fungi Toby studies seems especially cunning and sophisticated: It’s name is Rhizophagus aggregatum.

Toby Kiers
Probably of all of the strains that we work with, it shows sort of the most extreme strategies in terms of its ability to cheat the plant. And this fungus in particular is really interesting, because what it’s good at, is taking up nutrients from the soil, and rather than automatically trading them with the host plant, it actually stores these nutrients, for example phosphorus, in its network, until it’s able to get a better price for that phosphorus from the plant host.

Anja
So this fungus can keep resources until they’re worth more!

Sound of a coin

Anja
And that’s just one of the strategies found in fungi-plant economies.

Sound of a fake cash register

Anja
It’s really quite intriguing, considering this is an organism that doesn’t even have a brain. To Toby, this has fascinating implications.

Toby Kiers
I think for so many years, we’ve seen microbes as sort of dull a-social organisms that, you know, simply divide and move resources around. But now we’re really seeing them more as behavioral creatures, right? We can study them the way that you would study animals in nature. They exhibit behaviors, just like organisms that you would study, like birds or monkeys, you know. They do it, they share information in a totally different way. But they’re still showing behaviors. And I think that’s sort of where the field is going now – trying to actually capture and track these types of behaviors.

Music – Theme Interlude

Anja
It seems like fungi really open up a window to the role and the importance of symbiosis, right?

Matthias
This is true, I mean, fungi are excellent symbiosis partners for a wide range of organisms. At the coarsest level, they’re excellent symbiosis partners with plants. But fungi are also excellent symbiosis partners with animals, you know, like the fungus growing ants, the attine ants, where they basically they’re farming their own fungus in their nests. And actually, they don’t eat plants at all. They only eat the fungus that they farm. That’s I mean, it’s also a fascinating association and…

Anja
Ah, you mean those little ants that, that walk around with like leaves? Yeah, they cut… these leaf cutter ants. And you see like whole streets of them, like full with their little leaves, and they carry them to their nest. And then they have this fungus that they feed there with the leaves. And then the fungus grows and they – whatever, they… eat the fungus?

Matthias
They only eat the fungus, they cannot digest the leaves. That’s the whole point. So the whole point of that symbiosis, and why fungi are such excellent symbiosis candidates for animals is their just unparalleled enzymatic capabilities, so they can basically crack just about anything out there. Just some things are more difficult for them to digest, I mean, that’s why… it’s difficult to say that’s “that’s why” but that’s, (laughs) that’s why plants have like, you know, trees with stems, they’re made out of lignin, because that happens to be a molecule that is really difficult to crack. But fungi can do it. I mean, the white rot fungi, as they’re called, they can completely dissolve lignin – it just takes a while. Because it’s expensive for them to do. It’s difficult to do, but they can they can get it done.

Music – Black Sea Nettle by Sunfish Moon Light

Anja
Fungi can chew up lots of stuff. Depending on the kind, they can attack wood, certain plastics, and even stone. Their diverse appetites can also make them incredibly destructive, from a human point of view. Fungal infections kill large amounts of food crops each year, and also cause diseases in humans. So, there is this dark side to fungi which you can’t sugarcoat. But they also have all these important roles that they play. As we heard, they feed the plants. And we haven’t even covered all the things that different kinds of fungi do. For example, there’s the saprotrophs which are free-living so they don’t team up with plants, so they don’t connect with plants. But what they do is, they turn nutrients into the form that plants can actually use. So, if it wasn’t for these decomposers, our planet would basically look like a heap of trash. And then of course, fungi bring us all these wonderful things like cheese, wine, beer, and mushrooms. And to some scientists, fungi just mean pure joy!

Bala Chaudhary
It’s true. In graduate school, we had this thing called “Spore Fridays”, and everybody would come with a spore that they had found, you know, mounted onto a microscope, and we would hang out around the microscopes and look at each others’ spores and work together to identify them to species – but also just wonder in their beauty.

Anja
Bala Chaudhary is an assistant professor in the Department of Environmental Science and Studies at DePaul University in Chicago. She’s especially interested in the spores that mycorrhizal fungi produce.

Bala Chaudhary
Some form, you know, bright yellowish green, some can be bright red to orange, they can be clear, they can be black, they also vary with respect to really intricate ornamentation. And so, some have knobs on the surface of them, they have spikes, they can have long projections, they can be smooth – and they also vary with respect to just a whole suite of different characteristics.

Anja
Some fungi make mushrooms aboveground, as you know, which then release the spores. And the green stuff that sometimes grows on a loaf of bread or a lemon in the fridge? Yeah, that’s also fungal mycelium with millions of spores as well. But these special fungi, the arbuscular mycorrhizal fungi that Bala studies, they exist entirely below ground.

Bala Chaudhary
They also form comparatively large spores. So compared to all of the other fungal groups, the AM fungi form spores that are much larger. And so it was kind of typically thought that these large spores that are just produced below ground have no way of getting into the air and so their aerial dispersal is not terribly common. But that didn’t really jive with the information that showed that, you know, you see these species all over the world, and some are incredibly cosmopolitan. So you know, if they’re below ground and limited with respect to dispersal, but then you see them all over, you know, how does that work?

Anja
The scientists didn’t really understand how these beneficial fungi were able to disperse so widely. With their big spores, could they even travel through the air?

Bala Chaudhary
And I remember actually, in my PhD defense, my advisor asking me, well, what could be driving these mechanisms besides deterministic processes? What do you think about stochastic processes and dispersal? And I said, well, yes, I think they’re important. And she said, well, how would you test it? And I was so exhausted at that point, you know, three hours into this examination. I said, I don’t know, put traps on skyscrapers and look for spores in the air? And then little did I know, several years later, I would move to Chicago where there are ample skyscrapers to put spore traps on, and we just started doing it.

Sound of Chicago rooftop
Sound of person walking up stairs

So as a new professor working in the city, it was an experiment that I could do by just walking to the roof of my building!

Anja
Bala and her students put dust collectors on rooftops in Chicago. They wanted to try to track the AM fungi in the air. When would they find them? And which ones?

Bala Chaudhary
The first surprising result was that we actually found AM fungal spores in the air in this highly urbanized environment during all months of the year, even winter. And that was a real shock to be out, you know, in the middle of a snowstorm and see AM fungal spores flying through the air in the city.

Anja
The scientists also found a surprising temporal pattern. Between August and November, the abundance of spores in the air was especially high. At the beginning of the harvest season, could it be that humans were playing a role in sending the spores into the air?

Bala Chaudhary
Right, so in the harvest season, there’s a lot of heavy machinery that’s used. if you’re standing nearby a field during harvest season, you know, you see the harvester or the tractor, and behind it, you’ll­­­ often see this huge plume of soil, of dust that’s being lifted up. Now, whenever I am driving, and I see farm machinery spewing soil into the air, I pull over and I take a picture! (laughs) Because it’s such a striking example of, in this region, how humans are primarily moving soil, creating dust – and then with that all of the organisms that live in the soil, as well.­­­­

Theme (Interlude)

Anja
Fungi can grow into huge networks underground. There’s a report of a fungus that weighs 10 tons and covers many 10,000s of square meters. Presumably its spore landed there about 1500 years ago. Today, we humans assist fungi in traveling the world. For example, by the way we farm our land. Around Chicago, there are lots of monocultures of soybean and corn. And when the harvest season comes, their fungal friends start swarming through the air. This is just one of the recent discoveries. And there’s still a lot to uncover. I asked Matthias about the big questions his lab is working on right now.

Matthias
Yeah, I mean, we’re working on a number of different issues. I mean, we’re very interested in the forces that basically form communities of fungi. So no fungus ever lives alone they live in, in a community of different species in the soil, for example, and we are interested in all the various parameters that structure these communities through time and space. Global change factors is a major topic in our lab right now. So we are changing this planet in many different ways. And we are trying to figure out what does this actually mean for the fungi, their communities? What does it mean for how they function? And then we also have a bunch of other projects that try to sort of get to the heart of what it means to be a fungus, and this is called a trait-based approach. So what we are doing is we are looking at a very large set – very large, in our case is 30 different fungi isolated from the same soil – and they’re all different species. But we are trying to get away from viewing them just the species, but we view them as basically a collection of different traits or properties really, if you will. And then by measuring just a lot of things on this big set of fungi, we’re trying to figure out some fundamental things about fungi, like how do they branch? How do they grow? Can they grow fast, and at the same time produce a lot of enzymes? Or are those fundamental trade-offs that we can discover? And we’re discovering some of these fundamental trade-offs, what it means to be a fungus.

Anja
Would you like to be a fungus?

Matthias
I would certainly like to be a fungus for a while just to see what it’s like (laughs). I like the idea of absorbing food across my entire body surface that is strangely appealing – and invasive growth? Yeah. But I also don’t like dark. So yeah, I think it’s a give and take, but I’d sure like to experience it!

Theme Music

Anja
This was episode 2 of Life in the Soil, with Katie Field, Toby Kiers, Bala Chaudhary and Matthias Rillig. My name is Anja Krieger and I’ve produced the podcast together with the Rillig Lab. Our story consultants are Joana Bergmann, Milos Bielcik, Stefan Hempel, Tessa Camenzind, and Moisés Sosa Hernández. If you enjoyed the show, share it with your friends, and subscribe, rate and review our podcast. To get in touch, you can find Matthias as mrillig on YouTube, Twitter and Instagram, that’s m-r-i-l-l-i-g and I’m anjakrieger on Twitter. The music is by Sunfish Moon Light and cover design by Maren von Stockhausen.

In the next episode, we’ll explore more of the amazing organisms living in soil – and I can tell you, there are many! We’ll talk about one of the most complex food chains in the world – a food chain that is super important for all of us. So if you wanna know more, see you next time!

Vocoder sings “The Life in the Soil”­­­, end of audio.

FULL CREDITS

Produced by: Anja Krieger and the Rillig Lab
Funded by: Digging Deeper / BiodivERsA
Story consultants: Joana Bergmann, Milos Bielcik, Stefan Hempel, Tessa Camenzind, Moisés Sosa Hernández
Thanks to feedback: Mendel Skulski, Florian Hintz, Julie Comfort, Gerhard Richter, Lena Ehlers

Cover art: Maren von Stockhausen
Music: Sunfish Moon Light / Future Ecologies
Sounds: Freesound.org, CC-Zero

The Digging Deeper project was funded through the 2015-2016 BiodivERsA COFUND call for research proposals, with the national funders Swiss National Science Foundation, Deutsche Forschungsgemeinschaft, Swedish Research Council Formas, Ministerio de Economía y Competitividad and Agence Nationale de la Recherche.