Medieval peasants enjoyed pleasant shivers over cider as they swapped stories. Fanciful explanations for the puzzling lights are part of folklore. Dancing elves used the lights to decorate their parties. Jack O'Lantern was a trickster whose pranks were a bit too much not only for St. Peter but even for the devil so he is forced to wander the marshes forever carrying a lantern. We like to think we are less superstitions in the twenty first century but contemporary yarns on the internet suggest otherwise. Across the Atlantic, the tale of Jack O'Lantern has morphed into local tales of a woman who wanders the forest with a lantern, seeking her lover who died in the Civil War. Abandoned railroads, hospitals and mills attract reports of ghosts and spooky green lights. And everyone knows, when an alien spacecraft crashes it gives off a pale green glow.
What does science say about these green glows? Very likely they are caused by luminescent fungi. Luminescent fungi thrive on dead wood and decaying leaves. This would explain why green glows appear around abandoned buildings, railroads and graves. The favored colors are in the blue to green range. Honey mushrooms emit an eerie green ligh whereas Panellus stipticus shines in a delicate blue suggestive of a ballerina's tutu. The intensity varies from barely visible to dark adjusted eyes and bright enough for reading. The light may be steady or it may flicker. Although mycologists know of about 40 luminescent species, hikers and gardeners are continually reporting as yet unknown varieties. The luminosity depends on the substrate, the geographic location, the availability of water and other factors.
The light is caused by a chemical reaction. The main players are oxygen, luciferin, ATP and luciferase. ATP functions as a tiny rechargeable molecular battery that exhausts its charge in nanoseconds . Luciferin actually refers to a class of biomolecules. In response to a pulse of energy from ATP, a luciferin has the ability to briefly combine with and then release oxygen like a partner at a square dance. An enzyme in the luciferase class acts like the caller in the square dance, directing the choreography but not participating. In the brief molecular rendezvous with oxygen, luciferin transforms an ATP energy pulse to a packet of light that it emits. The exact color depends on the particular luciferin. The luciferins found in fungi belong to the the class of greenish pigments called the flavins.
Science has not yet determined the adaptive function of luminance in fungi. One suggestion is that its light attracts small insects that carry the spores. Another is that, in the case of toxic fungi, it be may a signal to fungivores to stay away. A third idea is that the light attracts ecological allies that repel predators. Finally, it may be an accidental byproduct of metabolism.
These and other questions are still unanswered because research into the fungal kingdom is only at its beginning but it promises to reveal a world as entracing and magical in its own ways as the imagined dances of the elves.
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