Science & Technology: When mushrooms go wild

The apparently sedentary life of forest-floor fungi is not what it seems, says Simon Hadlington - there are countless battles raging underfoot

A quiet stroll in the woods can be the perfect antidote to a noisy, fume-filled, jostling urban jungle. But while on the surface everything may appear calm and tranquil, beneath the feet, in the rotting leaves and in the dead branches, silent battles are being fought with a terrible ferocity. This is the dark, mysterious and surprisingly violent world of fungi. These organisms spend their lives searching for new sources of food, and, when two fungi come across the same piece of dead wood or pile of leaves, a battle for supremacy can ensue.

At Cardiff University, Professor Lynne Boddy has been studying fungus wars for more than 20 years. Boddy's motivation is to try to develop a fuller understanding of these interactions, of the role fungi play in the wider ecosystem of the woodland, and of the 3.9 billion hectares of the Earth that is covered by forest. "After all," she says of her work, "before we can save the planet we need to know how it works."

Fungi are supreme waste-recyclers. They release nutrients locked up in dead tissues and make them available again. Because of this, the ecology of the fungus is crucial to the wider ecology and health of the woodland.

Professor Boddy has set up conflicts between different species of fungus and watched as one defeated the other. "We have arranged fights between scores of different fungi and have a sort of league table," Boddy says. "Some are much better fighters than others. But just because one fungus is top of the table it doesn't mean it's going to win every time. Just like sports teams, some fungi have their bogey opponents who they simply can't beat."

When we think of a fungus we usually envisage a mushroom or toadstool, or occasionally a bracket fungus clinging to the side of a tree. But these external manifestations are merely the fruiting bodies - the structures that produce the spores. The main body of the fungus consists of fine, filamentous "hyphae", the mass of which is called the mycelium. Sometimes many hyphae will intertwine to form visible string-like mycelial cords. The mycelium grows out across the forest floor in search of new resources.

When one part of the mycelium discovers a new source of food, it can send a chemical message to the rest of the network to despatch resources to that point, so that enzymes can be manufactured to digest a newly-found cache of dead wood or fallen leaves.

"During their growth and search for new resources, fungi will inevitably meet one another," says Professor Boddy. It is here that combat can occur. But the interaction between the mycelia of two fungi is more complicated than a simple competition for food. "We prefer the term `combat' to `competition'," she continues, "because one fungus can attack another in a very aggressive way, not just to secure an item of nutrition but to defend an entire territory. Once the fungus has marked out its territory it can then consume the resources within it at its own leisure."

A number of different scenarios may unfold when two fungal mycelia meet. If the two fungi are of the same species, they might mate. If they have already mated, then they often reach a kind of compromise, and a stand- off ensues, with neither advancing into the other's territory. If fungi of different species meet - and in British woods there are hundreds of different species - battle commences with one eventually gaining the upper hand and advancing into the opponent's territory. Sometimes there will be stalemate if both are equally adept fighters.

"If you take a dead branch from a tree or from the forest floor and cut a section through it, you will see elaborate patterns of black or even orange lines," says Professor Boddy. "These mark the boundaries between the territories of individual fungi growing in the wood - they are the battle-fronts where the mycelia have been slogging it out."

To understand precisely how fungi fight one another, the Cardiff researchers have been carrying out a series of experiments in the laboratory where trays of soil or dishes of agar are inoculated with different fungi and their interactions are observed. As the fungi grow, their mycelia advance towards each other until they meet.

"One scenario is like hand-to-hand combat," Professor Boddy says. "A hypha of one fungus coils around a hypha of its opponent and actually penetrates it, almost literally sucking the life out of it." This phenomenon is termed mycoparasitism.

Other battles resemble chemical warfare. "Here, when the hyphae or mycelial cords meet there is a release of chemicals," says Professor Boddy. "We don't know the identity of most of these, but there is a lot of interest among chemicals companies, as these substances could possibly be supplied as "natural" fungicide. It is clear that some very exotic chemicals are produced in response to a fight. Some of these are spectacularly pigmented."

There appear to be two main strategies for chemical warfare between fungi. "Sometimes you see very close interaction between opposing hyphae and, presumably, there is some subtle release of chemicals that results in one or other retreating, or else deadlock," Professor Boddy says. "On the other hand you can get a massive release of hugely powerful enzymes that obliterate the opponent very quickly. All the hyphae in that region are destroyed."

Intriguingly, volatile chemicals are sometimes released by the fighting hyphae which attract insects. "What we don't know is whether it is somehow in the fungus's interest to attract these insects, or whether it is merely coincidence," Professor Boddy says. "For example insects called springtails are attracted to certain fighting fungi, and burrow into the zone where the mycelia are interacting. We are looking into this to see if there is any advantage to either of the combatants or the insects."

The Cardiff team has shown that different parts of the mycelial network are capable of communicating with one another, and that during fighting there is active communication throughout the organism.

"We set a fungus to fight with an opponent and then inoculated it with a labelled phosphorus compound, which we can trace," Professor Boddy says. "When the mycelium met its opponent we saw that phosphorus from all over the mycelium was mobilised and rapidly transported to the battle-front. The organism was devoting large internal resources to the fight. However, when it appeared that it was losing the battle, it rapidly shunted its resources as far away as possible, presumably so that the enemy did not capture them."

Working with Dr Jonathan Leake of the University of Sheffield, the Cardiff team has also looked at the fighting strategies of a class of fungi that form associations, called mycorrhizae, with plants. These fungi live in intimate contact with the roots of plants, providing the plant with nutrients and water from the soil and receiving sugars in return.

"What we have seen here is that when a mycorrhizal fungus meets an enemy in the soil, the allocation of carbon to the interaction zone rapidly reduces," says Professor Boddy says. "We presume that this is so that the resource is not gambled with. It isn't clear whether it is the plant, which supplies the carbon, that is controlling this decision, or whether it is the fungus."

Even when a fungus is not involved in a fight, the mycelium is constantly responding to external stimuli. "We have seen, for example, that if one part of the mycelium encounters a new resource, it will colonise it and send a message to the other regions of the mycelium to send supplies to enable it to start digesting," Professor Boddy says.

Until about 25 years ago, mycologists generally considered that the vast network of fungal mycelia in the soil was simply an indistinguishable mass of fungal matter. "It's only relatively recently that we have begun to appreciate that fungi are individual organisms that interact with one another in the same way as other organisms," Professor Boddy says. "But it will be many years before we fully understand these interactions".