The Hidden World of Fungi

For most people the word fungus brings to mind a mushroom pushing up through damp soil, or perhaps the mould that spoils a forgotten loaf of bread. Yet these familiar sights are only small clues to a vast and largely hidden kingdom of life. Fungi are neither plants nor animals but form a separate group of their own, distinct from both. Unlike plants, they cannot make their own food from sunlight; instead they feed on other living or dead matter, absorbing nutrients directly through their surfaces. In this respect their way of life is closer to that of animals than of the green plants among which they so often grow. This single difference, the way they obtain their food, sets fungi firmly apart and shapes almost everything else about how they live and where they are found.

The mushroom that appears above ground is not the whole organism but merely the part that produces and releases spores, the tiny cells from which new fungi grow. The main body of a fungus lies out of sight, spreading through soil, wood or leaf litter as a network of exceedingly fine threads. Each individual thread is called a hypha, and together the mass of threads forms what is known as the mycelium. This hidden web can be astonishingly extensive, and a single fungus growing through a forest floor may stretch across a remarkably wide area while remaining almost entirely invisible from above.

Fungi obtain their food by releasing chemicals, known as enzymes, into the material around them. These enzymes break down complex substances into simpler ones that the fungus can then absorb. This ability makes fungi some of the most important recyclers in the natural world. When a tree dies, it is largely fungi that break down the tough woody material and return its nutrients to the soil, where plants can use them again. Without this constant work of decay, dead plant matter would pile up and the nutrients locked inside it would remain unavailable to living things. In a very real sense, the growth of forests depends on the quiet activity of the fungi that dismantle what has died.

Not all fungi are content to feed only on the dead. Many form close partnerships with living plants, and one of the most widespread of these is the relationship between fungi and plant roots. The fungal threads wrap around and even grow inside the roots, extending far out into the soil and gathering water and minerals that the plant could not reach on its own. In return, the plant supplies the fungus with sugars made through photosynthesis. This mutually beneficial arrangement, called a mycorrhiza, is thought to occur in the great majority of land plants, and many species grow poorly, if at all, without their fungal partners.

The reach of this underground network has led some scientists to describe it in striking terms. Because the threads of different fungi can connect the roots of many separate plants, nutrients and even chemical signals may pass from one plant to another through the fungal links. Some researchers have suggested that trees in a forest may share resources or send warnings of danger along these threads, a picture that has captured the public imagination. It should be said that the details remain a subject of active debate, and not every claim made about such networks is firmly established.

Fungi matter to human beings in countless practical ways. They make possible the raising of bread, the brewing of beer and the ripening of many cheeses. They are also the source of some of the most important medicines ever discovered; the first widely used antibiotic was produced by a mould, and fungi continue to provide compounds used in treatment today. At the same time, fungi can be destructive. They cause diseases in crops that can ruin harvests, and some attack the human body directly. A number of fungi are also dangerously poisonous, and telling an edible mushroom from a deadly one is notoriously difficult even for the experienced.

Despite their importance, fungi remain comparatively poorly understood. The vast majority of fungal species are thought never to have been formally described by scientists, and new ones are found regularly. Because so much of their life is spent hidden underground, studying them is far harder than studying plants and animals that can be seen and counted. What is already clear, however, is that fungi are not a minor curiosity at the edge of nature but one of its central pillars, holding together the cycles of growth and decay on which almost all other life depends.