Into the great unknown of terrestrial dark matter: fungi

Five hundred years ago, Leonardo da Vinci wrote, “We know more about the movement of celestial bodies than about the soil underfoot.”

Leonardo’s statement is not only still true, but the knowledge gap he identified seems more perverse with every passing year. Human knowledge of the soil is eclipsed many times over, not only by astronomy, but by almost every other field of natural history too.

“We understand water and air beautifully because we can see through them,” says Martin Bidartondo, professor of Molecular Ecology at Imperial College London. “In soil, it’s much harder to assess what’s there, what it’s doing and what’s changing.”

The single most important component of soil – in fact, the only reason we have soil in the first place, are fungi. Fungi are the difficult third kingdom – difficult to study, difficult even to define. They’re surely not an animal but are genetically closer to human beings than to any plant. A lot of fungi never even produce mushrooms, which are the fruiting bodies of the organism that’s otherwise flourishing in the extensive, microscopic networks known as mycelium that remain hidden in the soil. To give you an idea of how hard it is to study fungi, Bidartondo likes to say: “Imagine being a plant ecologist, going into the forest and only ever seeing your plant when it flowers. That’s what it’s like for a mycologist all the time.”

The 94 percent

It is impossible to say exactly how much we do not know about fungi, but mycologist David L. Hawksworth and botanist Robert Lücking estimate that there are between 2.2 and 3.8 million different species. Only 120,000 of these have been described.

In other words, we know little or nothing about at least 94 percent of fungi species.

Think about that for a moment: we know nothing about the overwhelming majority of the kingdom that has given us beer, bread, penicillin and other antibiotics, organ transplants, cancer treatments, citric acid, psychedelic substances, biological detergent, truffles, soy sauce, Marmite, meat substitutes, Stilton cheese, stonewashed jeans, Harris tweed and the litmus test.

If that list briefly summarizes what we do know, imagine what we do not. The simile mycologist Merlin Sheldrake likes to use is that fungi are to life on Earth what dark matter is to the universe.

And yet, this astonishing ignorance is not why da Vinci’s comment five centuries ago is growing ever more perverse. According to a 2007 study by soil scientist Rattan Lal, there is nearly four-and-a-half times as much carbon sequestered in the soil than in all the planet’s plant life, including trees. Climate scientists ignoring the soil is like a web developer ignoring not only Google, but Apple, Amazon, Facebook and Microsoft as well.

If we are going to talk about soil carbon, then we have to talk about the star of the show, the fungi, the organism that has been doing the hard work of pumping all that carbon into the soil for hundreds of millions of years.

Colonizing the planet

The carbon stored in the soil started as carbon dioxide in the air. Plants photosynthesize that carbon dioxide and create carbohydrates: sugars. Most of those sugars go into building up plant biomass, but as much as 17 percent go to the plant’s fungi.

We commonly imagine that a plant’s roots reach into the ground to extract water and minerals for the above-ground lifeform. That idea is wrong. “Trees don’t have roots,” says Bidartondo. “They have mycorrhizal fungi.”

Mycorrhizal fungi entwine with a plant’s roots, which are, in truth, too crude and cumbersome to do the fine work of mining minerals from the soil. The filaments and threads of fungi are far better adapted to this hostile environment, their efficiencies showcased by the fact that mychorrhizal fungi are not only used by an estimated 90 percent of plant families but also paid handsomely for their work.

“The fungi use the photosynthesized sugars to mine for minerals that the plant needs, in particular nitrogen and phosphorus,” Bidartondo explains. “As mycorrhizal fungi, they become a mining machine, and the plant pays them in sugars.”

This collaboration or mutualistic symbiosis began around 480 million years ago. Without the mineral-mining mycorrhizal fungi, how else could a plant become established on bare volcanic rock? “This mutualism was really successful and generated the greening of the land,” says Bidartondo.

The symbiosis between fungi and plants also created the atmosphere as we know and breathe it today.

“ ‘Decarbonizing’ is one of today’s buzzwords,” says Bidartondo. “People say things like, ‘We need to decarbonize the economy,’ but the original decarbonization was by fungi and plants.”

Four hundred eighty million years ago, the atmosphere contained about 4,500 parts-per-million (ppm) carbon dioxide. After 100 million years, by fixing the carbon dioxide into the soil, fungi and plants had brought that concentration down to just 350 ppm. Our high-oxygen, human-friendly atmosphere had arrived.

“For the past 200 years we’ve been reversing that process and trying to put carbon back into the air,” says Bidartondo says. “Now we are really messing with the system.”

Languorous ways

“We are always demanding more and more from agriculture,” says Bidartondo. “So, to make things go faster, we started pumping nitrogen, phosphorus and carbon into the atmosphere and the soil.”

Unfortunately, it seems that all organisms, including plants and fungi, tend toward laziness. If nitrogen and phosphorus are easily accessible, then why would plants ‘pay’ fungi with more carbohydrates than absolutely necessary?

This ‘laziness’ was exactly what Bidartondo and his co-authors found in a 2018 study published in Nature.

“As we pump more nitrogen and phosphorus into the system,” Bidartondo explains, “the system will pump less carbon into the soil because the ‘lazy’ fungi can get those minerals with less investment.”

“We are capturing less carbon and releasing more,” he adds. “It’s a double whammy.”

Branching out

The fact that we know so little about fungi and the soil is not just a harmless piece of trivia. The skew of our knowledge risks luring us into less effective remedies to global heating.

“People are becoming obsessed with planting more trees,” says Bidartondo. “But that assumes that these trees are going to behave in the same way that they evolved for. The problem is that, by enriching the biosphere as a whole, we have created ‘lazy’ systems that don’t need to pump a lot of carbon into the soil.”

Politicians will make impressive commitments to reforestation, volunteers will proudly plant millions of seedlings and the trees will indeed grow into mighty forests – but this doesn’t guarantee the soil becoming a carbon sink.

One alternative to obsessive tree-planting, particularly in lowland areas of Europe, might be the conversion of unproductive land to heath shrublands.

“Heathlands are amazing places as far as mycorrhizals go,” says Bidartondo says. “Heathlands are characterized by extremely low levels of minerals, so the fungi have to work hard to extract them, and that means they pump loads of carbon into the soil.”

But the first step toward using fungi to help global needs is, still, figuring out what lies underfoot. “It’s not all gone to hell yet,” Bidartondo says with a reassuring smile. “This is the time to figure out what’s going on.”