The giant chestnut tree, growing in place for hundreds of years, would have been impossible to miss. Its leaves were glossy and dark green, its bark riven like a mountain range seen from above. The fungi it relies on were harder to see.
A fungi-hunter is not looking for an object so much as a system, brushing aside a layer of damp leaves to find the gossamer filaments that hold up the world. These multitudes of hairlike fungal threads—individually called hyphae, and collectively, mycelium—are the true body of fungi, shuttling nutrients to and fro across the forest floor. The blackness of soil is also a tell: A layer of loamy, shiitake-smelling richness, two or three inches deep, is a sign that fungi are making more life out of old life, digesting the dead to feed back into the system, keeping the whole scene alive.
Fungi-hunting is what I found Toby Kiers and her team of mycologists doing one morning, when I reached them via video call in Corsica, the French island in the Mediterranean best described as a mountain in the sea. It’s where some of the oldest trees in the Mediterranean still stand, gnarled and huge around their base. It had begun to lightly rain. “The first rain in months!” Kiers said. The team of six was rushing to collect samples while the parched ground changed around them. Dry fungi would have told them a little about how these organisms act when they’re drought-stressed; wet ones would tell them something different. Water activates the fungi’s inner workings, and genes that lie quiet in the dust turn on with a sprinkling of moisture.
And Kiers and her crew were there for the genes. They’d gone to Corsica to ask how fungi helped old-growth trees respond to climate change. Record-high temperatures and wildfires are the island’s new reality. But some of these trees are still there. Could this be the fungi’s doing? Kiers, an evolutionary biologist at Vrije Universiteit Amsterdam, thinks it’s likely. In a world where one-third of tree species are at significant risk of extinction, and where climate change is already disturbing the networks of fungi on which trees depend, understanding exactly how fungi shore up this system could show just how crucial fungal health is to our collective survival.
Mycorrhizal fungi—the kind that colonize tree roots—help forests, and the ones found around these healthy old-growth trees, Kiers supposed, might be particularly good at what they do. If so, perhaps such star fungi could be conscripted to help other beleaguered trees on the island recover from climatic extremes. But even the most fungus-obsessed scientists are still working to understand the basics of these organisms. In this regard, we’re a bit like society pre–germ theory. An invisible force is working on the health of our systems, but science has yet to fully define it. In fact, it has hardly begun to look.
The biologist Toby Kiers holds up forest soil colonized by white fungal threads, called mycelium, in Bocca di Larone, Corsica. (Quentin van den Bossche)
At least 90 percent of fungal species likely out there are as of yet undiscovered, even though mycologists identify about 2,500 new ones each year. Kiers’s team was collecting fungal DNA simply to “see who’s here,” Kiers said, her hands in the dirt. But the trip’s primary goal was finding RNA, which has even more to say: It could tell scientists what the fungi were doing at the base of the chestnut tree. Were they decomposing leaf litter? Were they siphoning up water, piping it through their network to plants? Maybe they were transporting phosphorus and nitrogen that they had isolated out of the soil, in exchange for carbon the tree had made from sunlight. All of this assistance is, remarkably, the domain of fungi. Any one of these fungal actions, or all of them together, could have made the tree more resilient to the stresses of drought and fire. And if that’s true, it also matters exactly which fungi are doing that work.
What this team was doing had never really been done. Scientists extract RNA from fungi grown in the calm sterility of labs, but not typically from wild soil. “Soil has so many contaminants,” Francis Martin, a molecular biologist at the French National Institute for Agriculture, Food, and Environment who studies tree-microbe interactions, told me while crouched in the dirt, the chestnut’s emerald leaves dangling behind him. Doing science outdoors is always more messy. Life in the real world is densely layered and hard to separate. All of it, the aphids, the mites, the probably 10,000 species of bacteria, the viruses—“We don’t know anything about the viruses,” Kiers said—counts as “contamination,” from which your true subject must be isolated. And then those subjects, the 200 or 300 fungal species that Martin estimated were in the top four millimeters of soil in this spot, must be teased apart from one another too.
Soil RNA is extraordinarily delicate. As with a comb jelly pulled from the ocean, there may not be much to see once the air hits it. Some RNA degrades in minutes. Other RNA takes longer, maybe an hour. But the team had a white box of dry ice, flown from the mainland that morning and steaming like a cauldron, to help keep it intact. I watched as Aurelie Deveau, a microbial ecologist at the French National Institute, and Nicolas Suberbielle, a mycologist from the National Botanical Conservatory of Corsica, took turns hammering a short metal tube into the ground and pulling it back out, extracting a cylinder of soil each time. Martin sifted and poured that dark powder into clear vials with blue caps. They then ran their vials to the car, to the steaming white box, and shoved them between stones of dry ice as fast as they could. The vials, on ice, would be flown to mainland France, where Martin and his lab would carefully extract the RNA and compare it with the full genomes of the fungi they’ve sequenced thus far. From there, answers about what these organisms were and what they were doing, at least in this spot, would begin to come into view. All of this information would be added to an online fungal atlas, the first globally interconnected one of its kind.
Trees feature prominently in conversations about sequestering the carbon dioxide warming our planet, but what is most missing from those conversations is fungi. The carbon we think of as sequestered in plants may actually be, in large part, stored in their fungal collaborators. A recent paper on which Kiers is an author found that 36 percent of current annual CO2 emissions from fossil fuels are sequestered, at least temporarily, in fungi. Mycelium mats may be major pools of carbon. Understand that, and suddenly our climatic future hinges on not only what trees we can save, but what soil—what fungi.
This idea has yet to seep through to popular understanding. Just the day before my call, Kiers’s team sampled beneath a 1,300-year-old tree, an absolutely huge specimen, its trunk covered in mosses and ferns. “It was almost like a place of worship,” Kiers said. Locals on motorcycles rolled through amiably to ask about the mycologists’ work, driving right over the trees’ roots, wheels marking the bark and compressing the loose soil at the tree’s base. This tree was a landmark in the area, but no one seemed to think about its immediate underground vicinity, Kiers told me.
To collect fungal samples, the team hammers steel cores into the soil. In a forest in Asco, Corsica, the biologist Merlin Sheldrake sieves a sample to remove stones before getting the fungal-soil mixture on dry ice. (Quentin van den Bossche)
Institutional awareness is not much better. Fungi are largely ignored in conservation efforts. A recent survey of more than 100 management plans at U.S. natural areas found that only 8 percent mentioned mycorrhizal fungi at all, though they frequently discussed the ecosystem services the fungi provided. The United Nations has recently begun to acknowledge soil’s colossal role as a carbon sink and the ways in which global soil losses are accelerating climate change, but fungi are still scarcely portrayed as a vital part of the picture. Kiers and her team are trying to change that too. In 2021, Kiers co-founded SPUN, the Society for the Protection of Underground Networks, which sends teams of mycologists to places as far flung as Argentina, Guatemala, Northeast India, Armenia, Colombia, Panama, Pakistan, Ivory Coast, Mongolia, Patagonia, Poland, and Nepal in an effort to simply inventory what fungi exist—something else that has never been done before.
Back on Corsica the mycologists, now quite damp, packed up. They’d return tomorrow, to some other spot on the island, to see what’s there and try to understand how this age-old partnership between trees and fungi is reacting to new stresses. By the time the first roots evolved (perhaps explicitly to house beneficial fungi), the two groups had already been associating with each other for some 50 million years, if not more. Their partnership is so tight for a reason: Fungi can’t photosynthesize, and they receive much of, if not all the carbon they need—some five billion tons a year, by one estimate—from their plant associates. In exchange, fungi mine minerals from rock and decomposing material, delivering to plants nutrients such as nitrogen and phosphorus, which they may not get enough of on their own. But the exchange is not always 1:1; both parties are incredibly opportunistic, sometimes shortchanging one another or outright stealing what they need. As Kiers once put it, it’s the purest free market—unconstrained by morality—and it’s completely ruthless.
And yet, without it, we may have very little life at all. Whether we notice them or not, fungi hold up the world. Through the work of mycologists such as Kiers and her colleagues, that invisible kingdom will slowly begin to show itself. We can’t save, it is often said, what we can’t name. Preserving some version of the planet we know, then, might depend on this most basic of tasks: finding more of the many fungi on which all of Earth’s biological life rests, and understanding what their daily lives look like as they busy themselves with the work of stitching the world together.
www.theatlantic.com
https://www.theatlantic.com/science/archive/2023/10/tree-survival-fungi-corsica-climate-change/675739/
