Mushrooms that glow in the dark – bioluminescence in the forest

Yes, some mushrooms really do glow in the dark. The phenomenon is called bioluminescence: a chemical reaction takes place in the cells of the fungus, and its product is a faint, cold, greenish light. In Poland the easiest place to see it is damp, decaying wood overgrown with honey fungus mycelium – people used to call it “glowing rotten wood” or “foxfire”. More than a hundred species of glowing fungi have been described worldwide, and scientists are still working out what the light is for.

How do mushrooms produce light?

Bioluminescence is “cold light”. It comes not from heat, as in a light bulb, but from the energy of a chemical reaction. The principle is similar to that of fireflies, although the specific compounds are different:

• Luciferin, a molecule that oxidises and releases energy in the form of light.
• Luciferase, an enzyme that speeds up this reaction.
• Oxygen, needed to oxidise the luciferin.

In fungi, luciferin is made from simple compounds that are also found in plants. A few years ago scientists managed to decode the whole chemical pathway and even transfer fungal genes into plants, creating tobacco that glows throughout its life. This shows that the mechanism is relatively simple and well conserved.

Unlike fireflies, which flash, fungi usually glow continuously. The light is greenish and very faint: completely invisible by day, and noticeable at night only once your eyes have adjusted to the dark.

Which mushrooms glow?

Honey fungi and “glowing rotten wood”

In Polish forests the main source of the glow is honey fungi, including the honey fungus itself. Interestingly, what glows in them is mainly the mycelium, not the fruiting bodies that appear in autumn. The mycelium grows through dead trunks, roots and fallen branches, and when the wood is damp and well decomposed, whole sections of it can glow.

Hence the folk names: glowing rotten wood, or foxfire in the English-speaking tradition. For centuries people have come across mysteriously glowing pieces of wood in the forest without always knowing where the light came from. Ancient authors, Aristotle among them, already described glowing wood, although the cause was unknown at the time.

The jack-o’-lantern mushroom

The jack-o’-lantern mushroom (Omphalotus olearius) is a warmth-loving fungus that is rare in Poland. It has intensely orange fruiting bodies with gills that can glow faintly in the dark. It is poisonous, and its shape and colour can resemble a chanterelle – more proof that a pretty colour guarantees nothing. The jack-o’-lantern usually grows in clusters at the base of broadleaf trees or on their roots, while chanterelles grow singly or in groups on the ground.

Tropical fungi

The most spectacular glowing fungi grow in tropical forests. Some small species of bonnets (Mycena) and their relatives glow with their whole fruiting bodies, so that in a dark forest they look like scattered little lanterns. Most of the photos of glowing mushrooms circulating online come from places like these.

Why do mushrooms glow?

There is no final answer to this question yet. Three hypotheses are discussed most often, and they do not rule each other out.

Hypothesis 1: attracting insects

The light may attract night-flying insects that carry spores to new places. This hypothesis is supported by experiments in South America in which artificial “mushrooms” lit by green LEDs attracted more insects than unlit ones. In some species the intensity of the glow also follows a daily rhythm – it is stronger at night, when insects are active.

Hypothesis 2: a side effect of metabolism

Wood-decaying fungi carry out intense chemical processes that produce harmful reactive oxygen species. The light-producing reaction could help neutralise them, and the light would just be a side effect. This hypothesis explains well why in honey fungi it is mainly the mycelium hidden in the wood that glows – and it has no need to attract any insects.

Hypothesis 3: deterrence

Some researchers suggest that the light could deter animals that eat fungi, or attract predators that hunt such animals. This hypothesis has the least strong evidence.

It is possible that bioluminescence serves different functions in different species – or that it evolved once for one reason and later came to serve another.

How to see glowing mushrooms

Seeing foxfire with your own eyes is a memorable experience, but it takes patience.

• When: late summer and autumn, after rainy days, when the wood is well soaked.
• Where: in broadleaf and mixed forests, near old stumps and fallen, heavily decomposed wood. Clues are autumn clusters of honey fungi and black, bootlace-like strands under the bark: these are honey fungus rhizomorphs.
• How: on a dark night, ideally with no moon and far from any lamps. Switch off your torch and wait ten to fifteen minutes for your eyes to adjust to the darkness. The glow really is faint: in photos it usually shows up only with a long exposure.

Safety in the forest at night

• Don’t go alone, and tell someone where you are going.
• Take a charged phone and a torch (red light spoils your night vision less).
• Stick to paths you know; it is easy to lose your bearings at night.
• Respect wildlife and the local rules for being in the forest after dark.

You will find more practical tips in our guide to mushroom safety.

Don’t take rotten wood home

It is tempting to take a glowing piece of wood with you. Better not: outside the damp forest it quickly dries out and goes dark, and decaying wood is home to mycelium, insects and many other organisms. Take a photo, save the spot on the Gribo map and come back next year.

Light tells you nothing about whether a mushroom is safe

Bioluminescence is a natural curiosity, not an identification feature of “good” or “bad” mushrooms. Both poisonous species and species that foragers pick can glow. Whether a mushroom can be eaten is decided by a mycologist, not by its colour, smell or glow in the dark.

Sources

• Foxfire – Wikipedia
• Bioluminescence – Wikipedia