Fungi

Most of the time fungi hide away in the murky depths of the soil and are not visible. But their microscopic, mouldy strands are growing through bits of organic matter and soil, through leaf litter, dead wood and dung and into carcasses of dead animals. Here, the fungi quietly go about their job of decomposing detritus and recycling the nutrients back to the soil.

It is only when these microscopic strands merge together to form the fruiting bodies of mushrooms and toadstools that we become aware of this invisible network beneath our feet. The spores borne on these fruiting bodies disperse in the wind, or in or on, the bodies of animals feeding on them e.g. springtails, flies.

Fungal fruiting bodies are of many different and beautiful forms: mushrooms, toadstools, bracket fungi, puffballs, earthstars, coral fungi, large boletes and delicate crinoline fungi. Fungal fruiting bodies can also be rather gross. The starfish fungus smells revolting so as to attract blowflies which feed on the squishy mess in the centre of the fungus and transport its spores from place to place. Other soil fungi don't produce such conspicuous fruiting bodies, but form their spores or fruiting bodies within the soil e.g. the edible and delicious truffle which has to be sniffed out with the truffle-hunter's pigs or dogs.

Fungi grow by dividing and spreading out into a network of branches which grow through organic matter. Fungi can link different parts of the soil profile together. They can grow out from a piece of leaf litter, down into the soil, up into a dung pat and link all these together. It can absorb nutrients in one spot and transport them to another. This process is called translocation of nutrients and it is important in nutrient cycling. The networks of fungal filaments (mycelia) can explore their environment much more effectively than can soil bacteria which occur either as individual cells or as sedentary colonies.

The main food of fungi is organic matter in leaf litter, dung, soil and carrion. They (along with bacteria) are the main decomposer groups in soil because of their vast collective mass supported by the variety of digestive enzymes they can secrete onto organic matter to break it down.

Some fungi are edible and collecting wild fungi is a favourite pastime in Europe where, over the centuries, they have sorted out, by trial and error, those that are safe to eat - presumably the "errors" died! Australian fungi are a bit more problematical as we have not built up such a long-standing bed of knowledge. There are no simple tests for edibility. Some old wives tales are: they are harmless if you can peel them, if they fail to turn dark on soaking in cold water or if a silver spoon cooked with the fungi, doesn't turn black. I wouldn't depend on these guidelines for edibility. To put these rules of thumb on a more scientific basis, don't eat mushrooms with yellow or greenish coloured gills, or have caps with yellow centres or caps which are thimble-shaped when young, but which smell of disinfectant (iodine) when stored in a plastic bag. Even then, it is best to stick to the fungi you can buy in the supermarket - there are many varieties to choose from these days.

Some soil fungi form a positive relationship with plant roots. They live in roots, but also extend their filament networks out into the soil where they absorb nutrients and water which they bring back to the plant to use. What the fungus gets out of this relationship is food in the form of sugars. This mutually beneficial relationship (or symbiosis) between root and fungus is called a mychorrizal association

Not all soil fungi are goodies. The potato famine in Ireland in the mid-1800's was caused by a soil fungus. One in every nine people perished from starvation. Also, some soil-borne decomposer fungi can grow in grain and peanuts when these are stressed by drought. The fungus contaminates the grain and nuts with a toxin (aflatoxin) which causes liver damage in animals and humans.

Poisonous fungi: Only a small proportion of Australian fungi are poisonous but since some fungi have fatal effects it's best to stick to the supermarket varieties. Recently, a prominent Australian newspaper in its lifestyle section had to quickly publish a correction when it got the captions to two fungi mixed up. The paper had incorrectly labelled the lethal fly agaric as "edible"!

Quirky bit: Some microscopic fungi eat animals. Hard to imagine but the nematode-eating fungi grow little loops which soil nematode worms swim through. The loop tightens like a lassoo around the worm and the fungus oozes digestive enzymes onto it before absorbing the products of digestion into itself. Other nematode-eating fungi produce sticky knobs which stick to and trap nematodes.

The biggest organism in the world is invisible!

The largest organism in the world lives in the soil and is invisible to the naked eye for most of the time. The organism is a fungus that lives in the soil in the forests of northern Michigan and weighs over 100 tonnes and covers 15 ha. It is heavier than an elephant and rivals the blue whale. How did they discover it? Scientists collected the visible bits of the fungus when it popped up above-ground as a toadstool (Armillaria bulbosa). They mapped where they had collected them and then did a DNA analysis on the tissue. They found that over the 15 ha area that the fungal strands were genetically identical. The fungus is reputed to be around 1500 years old as well so it could well be the world’s oldest organism! How can scientists tell this? They estimated its growth rate and calculated the time it would take to cover 15 ha. This was the first “mega-fungus” to be reported and since then even bigger ones have been discovered – a similar soil fungus in Washington State covers 600 ha.

Australians scientists did a similar study and collected toadstools on the floor of a eucalypt forest and discovered soil fungi that cover a more modest area of a few hectares.

Of course, all the fungal strands by now may not all be joined up to form a huge single organism – connections in the microscopic fungal strands in the soil may have broken as it grew outwards from the original germination point in the forest 1500 years ago. But it’s a pity to spoil a good story!