The Exchanges

Q Um, what, what is this type of fungus?

A Well, first of all, it's, it's amazing that you, that somebody says they're excited about A, soil, and B, mycorrhizal fungi, because that doesn't happen a lot, and C, that you read the paper, so well done. Um, but to your question, so mycorrhizal fungi, the word comes from the Greek, so myco means fungus, and rhiza means root, so it literally means fungus root. And so they're fungus root fungi, which sounds odd, but what it means is that hardly a root on the planet is actually just a root. It's, it's very often this association. And, so what it is, is a mutualistic partnership, um, where the plants, where, where the fungi, the threads of the fungi form this really close association with the root cells. They go, they either go into the cells, or they go around them, and then from there, they spread out into the soil. So I don't know if you've ever looked at your bread mold closely, but it would generally radiate out, and that's exactly what These fungi do. And they've got really small thread-like bits of their bodies, so they can go into soil pores where roots can't access.

AI assessment note: “so what it is, is a mutualistic partnership, um, where the plants”

Q so these fungi have been around for at least four hundred fifty million years, ah, having this symbiotic relationship, generally a symbiotic relationship with, with plants. Um, how prevalent are they today? Like, do we have a sense of, is it, is it everywhere? Is it in certain ecosystems? Is it some plants and not others? Is it everywhere? Like how, yeah, how, how ubiquitous is this type of fungus?

A Well, ubiquitous was actually the word I was going to use because there is, they're on all continents of the globe. And then with those plants, they're, they're with 90% of plants on the globe. So it's, it's really prevalent and everywhere. It's, it was a successful partnership in the past and it seems to continue to be a successful one. Um, but you do get different types being more prevalent in different places, so one type of mycorrhizal fungus is called ectomycorrhizal, so ecto meaning outside. They, they don't go into the root, but they tend to sort of sit around the root in a sheath, and that's really prevalent in forests. So if you've got, um, you know, pine and beech and Birch, um, all sorts of conifers, conifers, but also other types of trees. They're really prevalent then in forests, and actually really don't occur with so many species, but they really occur, um, um, In the northern hemisphere, in, in forests really, um, intensively. And then you get other types called arbuscular, which are with almost every other type of plant, including crops. And I don't know how long you want me to go on for, but you get ericoid mycorrhizae. They occur with plants in heathlands, including your blueberries and cranberries and crowberries, and you get orchid mycorrhizae, and they've really evolved quite a A strange relationship with, uh, orchids in that some orchids actually depend o…

AI assessment note: “they're on all continents of the globe. And then with those plants, they're with 90%”

Q Has it always been this way? Like, did, did, uh, these mycorrhizal fungi, did they evolve with, co-evolve with the plants upon which they depend, or did they develop later? Like, what do we know about the history here?

A Well, we know from the fossil record that by the time plants moved out of an aquatic environment, so the sea, onto land, we know that this partnership already existed. So the fungi are really, really old. How old exactly? We don't know. But we know by about four hundred and fifty million years ago that these fungi were definitely around. They were definitely colonizing plants, and then over time, that symbiosis, that mutualism has evolved and re-evolved and modified many, many different times, so that you've ended up with quite a few different types of these mycorrhizal fungi, um, and, and we think, we know why this happened, because as you can imagine, if you're a An aquatic plant, and you're in, in the sea, and nutrients, water, nutrients and water are literally swimming around you. You don't really need a root system. But then, Getting onto land, you'd suddenly be faced with this quite harsh environment of a soil, which is quite dry. The nutrients are somewhere in patches here and there. And, um, the roots of the plants at that time would have been something like moss. I don't know if you've ever grabbed a handful of moss and turned it over. Of course you have. But those root-like things that moss have Are pretty similar to what those early plants would have had, which are really just hold fast. They weren't really very good at getting nutrients or water. So that's where the…

AI assessment note: “by the time plants moved out of an aquatic environment... this partnership already existed.”

Q Like, how, how do we get to such a big number?

A This is something that's actually tripped people up in, in, um, you know, Twitter. I see lots of comments about, oh, you know, that, that sounds like really a lot, and then, um, It gives the impression that somehow mycorrhizae are this massive, ah, carbon reduction mechanism. So I think if you take, you know, all the carbon that is in the soil, um, I did a little calculation a little bit earlier, and we, we come up with a massive amount, if we express it in CO₂ equivalents, it's nearly 9000. And our mycorrhizal pool is about 13 gigatons of CO₂ equivalents. So you can see, if you express it as a, as a function of the whole soil carbon sink, Uh, then it's really like .1%. And if we look at the forest sink, which is about nearly 1500 gigatons of CO two equivalents, then compared to that, our mycorrhizal pool is about one percent. Uh, so I hope that sort of puts it in perspective. Whereas if we look at our emissions, Yeah, it is 32, 36% of that. It, it, it, I think maybe what's, you know, what is difficult is thinking about additionality. This process is going on all the time, um, regardless. It's not like we can somehow use mycorrhizae To additionally now draw down more CO two than they are already. The, the use of the energy related emissions was just a way to give people a reference point, like how much, you know, CO two is this? Because it's, you can say a number like 13 gigato…

AI assessment note: “if we express it in CO₂ equivalents, it's nearly 9000. And our mycorrhizal pool is about 13”

Q And, and do we have any sense of, on the question of permanence, and you've said a couple times, sort of, we don't, we don't quite know, but relative to the rest of soil organic carbon, is there any reason to think that this, that the mycorrhizal fungi would be any different in terms of permanence?

A Possibly, because there's some evidence, it's just a few papers, that the exudates from mycorrhizal fungi, so that's the small molecular weight fluid that's given off, like sugars, amino acids, and, and plants do it too, but there's some indication that the exudates from mycorrhizal fungi might be even more important than root exudates In eventually ending up being in quite a stable carbon form. I, I was actually listening to one of your other podcasts about soil carbon, and, and the guy there was speaking about how our understanding of soil carbon has changed, and how we now recognize that soil microbes, um, can quite efficiently use these really small molecular weight Um, organic carbon and quite rapidly fix them onto, or via the microbes, they can end up being bound to mineral particles, which is quite a stable form. And then there's, there's other fractions in the soil as well, like leaf litter and dead roots and, and that may be somewhat more vulnerable to loss. Um, So I think we're beginning to realize that microbes as a whole, of which mycorrhizae are part, are really important in the, the process of taking soil carbon that has just entered the system, say quite new carbon, and then fixing it onto soil minerals, which are then relatively stable. So it's not just about how much, but the quality Of the carbon, and then, um, their involvement in it. So I think that's quite …

AI assessment note: “Possibly, because there's some evidence, it's just a few papers, that the exudates”

Q how big a deal it is, like, what is the mechanism? So now we understand that we have these, these mycorrhizal fungi, they, they're extensions of a root network essentially, and they have the symbiotic relationship wherein they trade nutrients basically with, with a plant or a tree. Um, what happens with CO two? What is the role? What is the mechanism through which CO two uptake goes through these

A Right. So, just to come back to what is known, it, it's been known for a long time that, um, how the CO₂ reaches, you know, into the fungus. It's just that we're the first to make a global estimate of the extent of this carbon pool. So, but to come back to your question, it's really all happening At the level of the root cells, but where it begins is with CO₂ fixation by green plants. So they fix the CO₂ into their, ah, leaves, and then that gets converted together with sunlight and water into sugars, and then that gets sent down to the roots and to the leaves and elsewhere in the plant, but where you've got this Symbiosis, where you've got, it's almost like a handshake at the cellular level, where you've got the fungus Either in or around the root cell, and then you've got the root cell, and you've got this interface. And depending on the type of, um, mycorrhizae, you'll have different layers there, but the point is you can have a, an exchange across that interface of the nutrients and water coming from the fungus, and then the sugars coming from the plant. Into the fungus.

AI assessment note: “exchange across that interface of the nutrients and water coming from the fungus, and then the sugars”

Q So what's the impact of that on CO two? I get that the CO two is fixed by the plant and then converted into sugars that are, that extend down into the root network and, and throughout the plant itself. What, what, what's the impact on the CO two uptake when there is that exchange with the mycorrhizal fungi?

A Right. Well, the CO two drawdown is Is happening due to the plant, but because you've got this, uh, added sink, um, and I use the, the word sink, uh, in terms of carbohydrate physiology, we talk about sources and sinks, but you've got this whole other extension of the root system, which is, uh, an additional sink, you can call it for the CO two. So it's not just going down into the plant, Leaves and roots, but it's also additionally being, um, going into the fungal body, which can extend out quite a far way, and being built into the structure of the fungus. So, what would happen, you know, if there was no fungus? Well, you would still have CO₂ drawdown, but it presumably wouldn't be as much. And, well, we know that it wouldn't be as much because, you know, Under experimental conditions, you can grow plants without these fungi, and you can, or with them, and then you can measure what the cost is in terms of CO₂ that's now, instead of being used in the plant, is being sent to the fungus. So you can have a, you can even have, um, Less. You can have that the plant has got less carbohydrate available, but it still may grow better because the symbiosis is providing it with these other benefits, if that makes sense. So there's a cost, but there's also enough, usually enough of a benefit. So that, that's where the drawdown would come. That The carbon will be built into the fungal body …

AI assessment note: “you've got this whole other extension of the root system, which is... an additional sink”