What Are Fungi? Neither Plant Nor Animal, and Closer to Us Than to Trees

Fungi were separated from plants once it was clear how differently they live. Plants make their food from sunlight; fungi, like animals, must get theirs from other organisms. Animals eat and then digest; fungi digest and then eat, secreting enzymes into whatever they are growing on and absorbing the broken-down result through their walls. Their cell walls are made of chitin, the material of insect shells, not the cellulose of plants, and their genes place them with animals on a branch of life called the opisthokonts, which split from the plants over a billion years ago. There are around 150,000 named species and an estimated two to four million in total, most of them unseen, so that fungi may be second only to animals as the most diverse kingdom of complex life.

Most fungi are made of hyphae, tubular threads a few thousandths of a millimetre wide that grow at their tips and branch, and a mass of hyphae is a mycelium, which can be a tangle in a petri dish or a network threading through hectares of forest floor. The network explores, finds food, and moves nutrients along itself, and when conditions are right it produces the structures that release spores: the mushroom, the bracket, the puffball, the mould's dusty surface. A single mushroom can release billions of spores, each a cell that can start a new mycelium, and fungal spores are in every breath of air. Yeasts are the exception to the thread rule, single cells that bud, and they are what makes bread rise and grape juice ferment.

The kingdom lives by a few strategies, each of which shapes the world:

• •Decomposers: fungi are almost the only organisms that can break down lignin, the tough polymer of wood, and one popular though disputed idea is that before they evolved that ability, around 300 million years ago, dead trees piled up unrotted and became coal
• •Partners: about ninety percent of plants have fungi wrapped around or inside their roots, mycorrhizas that trade phosphorus and nitrogen mined from the soil for sugar from the plant, and that link trees to each other in what has been called the wood wide web
• •Lichens: a fungus housing algae or cyanobacteria, together able to live on bare rock, which is how the first soil was made
• •Parasites and pathogens: rusts, smuts and blights that destroy crops, the chytrid that has wiped out amphibians on every continent, and the moulds and yeasts that infect people
• •Food and drink: yeast in bread, beer and wine; moulds ripening cheese and soy sauce; mushrooms themselves

The first antibiotic was a mould's weapon against bacteria, and fungi have supplied much of the pharmacy since: penicillin and the cephalosporins, the cholesterol-lowering statins, first found in a mould in 1976, the ciclosporin that made organ transplants possible, and the ergot alkaloids used in migraine and childbirth. They also cause disease. Around a billion people have a fungal infection at any time, mostly of skin and nails, and the serious ones, in the lungs and blood of people whose immunity is weakened by illness or treatment, kill over a million a year, with few drugs to treat them, because fungi are so close to animals that a chemical which harms one tends to harm the other. Warming is bringing new ones: Candida auris, a yeast unknown before 2009 and resistant to most antifungals, has spread through hospitals on five continents.

Fungi are the least studied of the great kingdoms. Perhaps five percent of species are described; the extent and meaning of the mycorrhizal networks that carry nutrients and possibly signals between trees is argued over; the largest and oldest organisms on Earth may be fungi nobody has mapped; and the question of how a network with no brain decides where to grow has drawn the attention of computer scientists, who have used slime moulds and mycelia to solve routing problems. They are also being put to work: mycelium grown on agricultural waste is being sold as packaging and as leather, fungi that eat plastics and oil are being tested for cleaning up spills, and the mushrooms that were a foraged food for most of human history are now a global crop worth tens of billions a year.

Folk tests for telling edible mushrooms from poisonous ones are widespread and all unreliable. Claims that poisonous mushrooms blacken a silver spoon, that anything which peels easily is safe, that cooking destroys the poison or that a mushroom eaten by slugs or squirrels will not harm you have each led to deaths. Animals have different digestion from yours, and many of the most dangerous toxins are unaffected by heat. The only safe approach is to identify the species with certainty, which for many groups takes expert knowledge and sometimes a microscope to examine the spores.

Most fatal mushroom poisonings worldwide are caused by the death cap and its close relatives, which contain amatoxins. These compounds block a key enzyme cells need to make proteins, and the liver, which processes them, is damaged most. What makes them so dangerous is the delay. Symptoms usually begin six hours or more after the meal, often with severe vomiting and diarrhoea, followed by a period in which the person seems to recover while the liver is failing. By then the toxin has been absorbed. Death caps look fairly plain and have been mistaken for edible species by foragers from other countries, where similar-looking mushrooms are safe.

A fairy ring shows hyphal growth on a scale you can see. A fungus that starts from a single spore in a lawn or meadow spreads outward underground in all directions, feeding at its growing edge. Behind that edge the food is used up and the old mycelium dies, so the living part becomes an expanding circle, and when it fruits the mushrooms come up in a ring. The grass often shows it too, greener where the fungus releases nitrogen and sometimes browner where the dense mycelium stops water soaking in. Because rings widen slowly and steadily, the largest are thought to be centuries old.

Some fungi go further and steer their hosts. In tropical forests a fungus of the genus Ophiocordyceps infects carpenter ants and, over days, changes their behaviour: an infected worker leaves its colony's trails, climbs a plant and clamps its jaws onto the underside of a leaf or twig, a position that suits the fungus. The ant dies there, and a stalk grows from its head and showers spores onto the forest floor below, where other ants forage. Researchers have found that the fungus surrounds and invades the ant's muscles, though exactly how it controls the behaviour so precisely is still being studied.

In the Blue Mountains of eastern Oregon, a single honey fungus has spread underground through an area of forest of around nine square kilometres. Most of it is invisible: a network of thread-like filaments and tough black strands running through the soil and under bark, feeding on the roots of trees. Genetic tests showed that samples taken far apart all belonged to the same individual.

Estimates of its age run to thousands of years, based on how fast such fungi grow. It is often described as one of the largest living organisms on the planet by area. For the forest, it is a serious problem, because honey fungus kills the trees it infects. For biologists, it is a reminder that a mushroom on the surface is only a tiny visible part of a much larger organism.

Fungi are a kingdom of organisms closer to animals than to plants, built of hyphal threads that digest their food outside their bodies and fruit as mushrooms and moulds to release spores. They are the world's main decomposers, almost the only things that can digest wood, partners to nine tenths of all plants through their roots, makers of bread, beer, cheese and penicillin, and the cause of crop blights and infections that kill a million people a year.