In order to defy climate change and the bark beetle, more deciduous trees are being planted in Swiss forests. If possible, their wood should be used several times before it ends up as firewood, thus releasing the previously bound CO₂ back into the atmosphere. At present, however, hardwood is still too often used directly for heating. Innovative ideas for a more sustainable cascade use are therefore needed. One possibility is to equip the natural material with new properties – in technical terms: functionalities – and transform it into magnetic, waterproof or electricity-generating wood, for instance.

A team led by fungal researcher Francis Schwarze from Empa's Cellulose & Wood Materials lab in St. Gallen is currently pursuing another idea for a new type of composite material based on hardwood: luminous wood. In addition to applications in technical fields, the luminous wood could be processed into designer furniture or jewelry.

Entangled living materials

This has been achieved thanks to a parasite: The honey fungus is a pathogen that causes white rot in trees and is therefore actually a wood pest. Some species produce the natural substance luciferin, which is stimulated to glow in a two-stage enzymatic process. Wood permeated by fungal threads therefore emits a green light.

"Naturally luminous wood was first described around 2,400 years ago by the Greek philosopher Aristotle," says Schwarze. Strictly speaking, the interwoven structure of fungus and wood can be described as a natural biohybrid, a combination of living materials. "Artificially produced composite materials of this kind would be interesting for many types of application," says the Empa researcher. But what nature seems to achieve effortlessly has so far been (too) challenging for biotechnology. Now, for the first time, the Empa team has succeeded in inducing and controlling the process in the laboratory.

From the woods to the lab

Biotechnologist Francis Schwarze has tracked down the glowing mushrooms in nature, analyzed them in the laboratory and deciphered their genetic code. The ringless honey fungus (Desarmillaria tabescens) turned out to be particularly powerful. After preliminary tests with different types of wood, Schwarze started with balsa wood (Ochroma pyramidale), a wood with a particularly low density. Using spectroscopy, the researchers observed how the fungus degrades lignin in the balsa wood samples, which is responsible for stiffness and compressive strength. However, X-ray diffraction analyses showed that the stability of the wood does not diminish as a result: The cellulose, which provides tensile strength in the wood, remained intact.

Wanted: a moist environment

The biohybrid of fungus and wood develops its maximum luminosity when incubated for three months. Desarmillaria likes it particularly moist: The balsa wood samples absorbed eight times their weight in moisture during this time. The enzyme reaction in the wood finally gets triggered when in contact with air. The glow unfolds its full splendor after about ten hours, emitting green light with a wavelength of 560 nanometers, as Empa researcher Giorgia Giovannini from the Biomimetic Membranes and Textiles lab determined during fluorescence spectroscopy analyses. The fascinating process currently lasts around ten days. "We are now optimizing the laboratory parameters in order to further increase the luminosity in the future," says the Empa researcher.

Natural bioluminescence
In nature, bioluminescence occurs in a wide variety of organisms. The light is produced thanks to chemical processes that release energy in the form of both light and heat. If one compares the light-generating reactions in nature on the basis of their so-called quantum yield, the firefly is the winner with a value of 40%, luminous jellyfish achieve 17%, and luminous mushrooms reach 2%.

Luminescent mushrooms
Over 70 species of fungi show bioluminescence. They produce a glow known as "foxfire" in rotting wood. The term is a French-English hybrid of "faux" and "fire" for "false fire". The purpose of bioluminescence in fungi is not entirely clear. It may be to attract insects to spread fungal spores.

It is difficult to find luminescent wood in nature because ubiquitous artificial light sources at night make it difficult to spot. Empa researcher Francis Schwarze advises looking for dead wood under moist autumn leaves on a moonless fall night in a mixed beech forest. With a bit of luck, the mushroom and its glowing wood supper can be discovered.

Luminous squid
The small squid Watasenia scintillans measures just eight centimeters, but it's big on camouflage: Light-emitting cells are scattered across its underside. These photophores confuse predatory fish that live on the seabed. When looking up towards the surface of the water, the luminous squid cannot be spotted by predators.

Fireflies
These insects, which are up to two centimetres in size, find each other for mating thanks to their light. Fireflies not only have a translucent abdomen, they also have a reflective layer on the inside. This "built-in mirror" also reflects the bioluminescence outwards. This makes Lampyris noctiluca the shining star among the bioluminescent creatures.

Giant honey fungus
The honey fungus is one of the most amazing creatures on Earth. It may sprout inconspicuously on the forest floor in the classic mushroom shape, adorned only with a decorative strip around the style, like a bracelet, which gives it the Latin name "Armillaria".

Much more impressive, however, is its web of black strands that it draws over wood and ground. The fungal threads form thick, meter-long bundles, surrounded by a black melanin-containing protective layer. These so-called rhizomorphs search for new habitats and food sources.

The largest living organism in the world, a 2400-year-old honey fungus network, covers an area of several square kilometers in the US state of Oregon. The largest mushroom in Europe can be found in Switzerland on the Ofen Pass. This 1000-year-old honey fungus covers an area the size of 50 football pitches.

Refining hardwood by bioluminescence: How to make wood glow