Merlin Sheldrake is a biologist and bestselling author of Entangled Life: How Fungi Make Our Worlds, Change Our Minds, and Shape Our Futures.
An extract from Entangled Life: How Fungi Make Our Worlds, Change Our Minds, and Shape Our Futures by Merlin Sheldrake, merlinsheldrake.com
More than 90 per cent of all plant species depend on mycorrhizal fungi. They are the rule, not the exception: a more fundamental part of planthood than fruit, flowers, leaves, wood or even roots. Out of this intimate partnership – complete with co-operation, conflict and competition – plants and mycorrhizal fungi enact a collective flourishing that underpins our past, present and future.
For the relationship to thrive, both plant and fungus must make a good metabolic match. In photosynthesis, plants harvest carbon from the atmosphere and forge the energy-rich carbon compounds – sugars and lipids – on which much of the rest of life depends. By growing within plant roots, mycorrhizal fungi acquire privileged access to these sources of energy: they get fed. However, photosynthesis isn’t enough to support life. Plants and fungi need more than a source of energy. Water and minerals must be scavenged from the ground – full of textures and micropores, electrically charged cavities and labyrinthine rot-scapes. Fungi are deft rangers in this wilderness and can forage in a way that plants can’t. By hosting fungi within their roots, plants gain hugely improved access to these sources of nutrients. They, too, get fed. By partnering, plants gain a prosthetic fungus, and fungi gain a prosthetic plant. Both use the other to extend their reach. It is an example of Lynn Margulis’ ‘long-lasting intimacy of strangers’. Except that they’re hardly strangers any more.
It isn’t clear how mycorrhizal relationships first arose. Some venture that the earliest encounters were soggy, disorganised affairs: fungi seeking food and refuge within algae that washed up onto the muddy shores of lakes and rivers. Some propose instead that the algae arrived on land with their fungal partners already in tow. Either way, explained Katie Field, a professor at the University of Leeds, ‘they soon became dependent on each other’.
The earliest plants were little more than puddles of green tissue, with no roots or other specialised structures. Over time, they evolved coarse fleshy organs to house their fungal associates, who scavenged the soil for nutrients and water. By the time the first roots evolved, the mycorrhizal association was already some 50 million years old. Mycorrhizal fungi are the roots of all subsequent life on land. The word ‘mycorrhiza’ has it right. Roots (rhiza) followed fungi (mykes) into being.
Today, hundreds of millions of years later, plants have evolved thinner, faster-growing, opportunistic roots that behave more like fungi. But even these roots can’t outmanoeuvre fungi when it comes to exploring the soil. Mycorrhizal hyphae are fifty times finer than the finest roots and can exceed the length of a plant’s roots by as much as a hundred times.
Their mycelium makes up between a third and a half of the living mass of soils. The numbers are astronomical. Globally, the total length of mycorrhizal hyphae in the top ten centimetres of soil is around half the width of our galaxy (4.5 × 10 17 kilometres versus 9.5 × 10 17 kilometres). If these hyphae were ironed into a flat sheet, their combined surface area would cover every inch of dry land on Earth two and a half times over. However, fungi don’t stay still. Mycorrhizal hyphae die back and regrow so rapidly – between ten and sixty times per year – that over a million years their cumulative length would exceed the diameter of the known universe (4.8 × 10 10 light years of hyphae, versus 9.1 × 10 9 light years in the known universe). Given that mycorrhizal fungi have been around for some 500 million years and aren’t restricted to the top ten centimetres of soil, these figures are certainly underestimates.
In their relationship, plants and mycorrhizal fungi enact a polarity: plant shoots engage with the light and air, while the fungi and plant roots engage with the solid ground. Plants pack up light and carbon dioxide into sugars and lipids. Mycorrhizal fungi unpack nutrients bound up in rock and decomposing material. These are fungi with a dual niche: part of their life happens within the plant, part in the soil. They are stationed at the entry point of carbon into terrestrial life cycles and stitch the atmosphere into relation with the ground. To this day, mycorrhizal fungi help plants cope with drought, heat and the many other stresses life on land has presented from the very beginning, as do the symbiotic fungi that crowd into plant leaves and stems. What we call ‘plants’ are in fact fungi that have evolved to farm algae, and algae that have evolved to farm fungi.
Mycorrhizal fungi do more than feed plants. Some describe them as keystone organisms; others prefer the term ‘ecosystem engineers’. Mycorrhizal mycelium is a sticky living seam that holds soil together; remove the fungi, and the ground washes away. Mycorrhizal fungi increase the volume of water that the soil can absorb, reducing the quantity of nutrients leached out of the soil by rainfall by as much as 50 per cent. Of the carbon that is found in soils – which, remarkably, amounts to twice the amount of carbon found in plants and the atmosphere combined – a substantial proportion is bound up in tough organic compounds produced by mycorrhizal fungi. The carbon that floods into the soil through mycorrhizal channels supports intricate food webs. Besides the hundreds or thousands of metres of fungal mycelium in a teaspoon of healthy soil, there are more bacteria, protists, insects and arthropods than the number of humans who have ever lived on Earth.
Mycorrhizal fungi can increase the quality of a harvest. They can also increase the ability of crops to compete with weeds and enhance their resistance to diseases by priming plants’ immune systems. They can make crops less susceptible to drought and heat, and more resistant to salinity and heavy metals. They even boost the ability of plants to fight off attacks from insect pests by stimulating the production of defensive chemicals. The list goes on: the literature is awash with examples of the benefits that mycorrhizal relationships provide to plants.
In 1940, Albert Howard professed that we lacked a ‘complete scientific explanation’ of mycorrhizal relationships. Scientific explanations remain far from complete, but prospects for working with mycorrhizal fungi to transform agriculture and forestry and to restore barren environments have only increased as environmental crises have worsened. Mycorrhizal relationships evolved to deal with the challenges of a desolate and windswept world in the earliest days of life on land. Together, they evolved a form of agriculture, although it is not possible to say whether plants learned to farm fungi, or fungi learned to farm plants. Either way, we’re faced with the challenge of altering our behaviour so that plants and fungi might better cultivate one another.