What Are Fungi? Neither Plant Nor Animal, and Closer to Us Than to Trees
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A mushroom is not the organism; it is the fruit. The organism is a network of threads, each finer than a hair, spreading through the soil or the log or the loaf, and in one Oregon forest a single individual of honey fungus covers nine square kilometres and is thought to be several thousand years old. Fungi were classed as plants until the 1960s and are in fact closer to animals, and they run the recycling of the living world: without them there would be no soil, no bread, no beer, no penicillin, and the forests would be buried under their own dead wood.
A kingdom of their own
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 are probably the second most diverse kingdom of complex life after the insects.
How they are built
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.
What they do
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 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
Fungi and medicine
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.
What is still unknown
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.
The takeaway
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, 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.