Bioluminescence: foxfire, rebel angels and the sexual preferences of fireflies

By Jason Fowler, via Flickr

Yesterday a friend sent me this text message from a camping trip: “Hey, we found some chunks of wood that glowed in the dark. Never seen that before. Any idea what might cause that?”

I had a few ideas. Bioluminescence is pretty common in the ocean, so I’d come across it before. In my diving instructor days, I’d entertain divers at the end of every night dive by dimming our torches and stirring the water up into softly glowing clouds like miniature underwater galaxies.

By Mike (Red Tide at midnight), via Wikimedia Commons

This type of bioluminescence is caused by tiny plankton floating in the water column, mostly dinoflagellates (single-celled algae) and a few planktonic crustaceans.

Dinoflagellates. By Maria Antónia Sampayo, Instituto de Oceanografia,
Faculdade Ciências da Universidade de Lisboa, http://planktonnet.awi.de, via Wikimedia Commons

When disturbed, they switch on their lights as an escape strategy. (See Quantum Immortal’s excellent post on bioluminescence for a great overview of the various reasons organisms use bioluminescence, and the science behind it.)

Larger aquatic animals can bioluminesce, too ⎯ especially deepwater species. As seen in Pixar’s Finding Nemo, deepwater anglerfish have a bioluminescent lure that they use to draw prey close to their mouth. 

Cartoon anglerfish. www.pixarpost.com

Real anglerfish. Monterey Bay Aquarium

The anglerfish doesn’t produce the light itself ⎯ the esca, the blob at the end of the “fishing rod”, contains millions of bioluminescent bacteria that do the work instead.

The hatchetfish, meanwhile, has bioluminescent scales on its belly that act as a cloaking mechanism: it uses counter-illumination, matching the light intensity of the surface of the water hundreds of metres above it, so that when predators view it from below, it nearly disappears into the blue.

Argyropelecus hemigymnus
(permission of prof. Francesco Costa, via Wikimedia Commons) www.professorecosta.com

There are a few examples of bioluminescent animals on land, too ⎯ fireflies being the most well-known, glowworms being another (with "glowworm" being a generic term for a variety of insect species). Like marine species, these insects bioluminesce for various reasons, including to identify other members of their species, as a form of sexual selection (female fireflies prefer males with brighter and more frequent flashes), to warn predators that they are toxic, and to attract prey.

But back to our glowing wood chips:

By Jason Fowler, via Flickr

My gut said the bioluminescence was caused by a fungus, and it turns out that on land, bioluminescence happens exclusively (probably) in insects and fungi. A quick Internet search revealed my hunch to be correct, and I texted my friend back:

“It’s a species of bioluminescent fungus, most likely from the genus Armillaria. Some people call it foxfire or faerie fire.”

Armillaria is the most common genus of bioluminescent fungi in Ontario (and much of Canada). There are 10 species in the genus, but many resemble on another and are often collectively called honey mushrooms.

Honey mushrooms.
By Tomas Čekanavičius, GFDL (www.gnu.org/copyleft/fdl.html), via Wikimedia Commons

Armillaria generally infect dead or decaying trees and stumps by sending out shoelace-like strands of mycelia that infect the roots, then moving up through the trunk. Healthy trees can usually stave off infection, but Armillaria can be very destructive in forests. Unlike other bioluminescent mushrooms, in Armillaria it is only the mycelia, not the fruiting-bodies (mushrooms) that produce light. (Check out this University of Georgia page on foxfire for more information on bioluminescence in Armillaria.)

Fungal bioluminescence is less well understood than its insect counterpart. Since mushrooms don’t move around, signaling a mate is unnecessary ⎯ though it could attract insects, which help to spread spores. But bioluminescence is costly. In a sense, it is the reverse of photosynthesis: while photosynthesis takes energy (light), carbon dioxide and water and turns it into sugar and oxygen, bioluminescence takes sugar and oxygen and turns it into energy (light) and water. It is somewhat analogous to cellular respiration, which converts sugar and oxygen into energy (heat and chemical), water and carbon dioxide. Since bioluminescence uses a completely different pathway than cellular respiration, though, it does not produce carbon dioxide as a waste product, so the comparison with both cellular respiration and photosynthesis can only go so far.

Because they can’t photosynthesize (with at least one exception, again from the marine world), animals do cellular respiration to harness the energy stored in the sugars they eat. They need the energy to power reactions in their cells so they can stay alive. So why would a fungus waste precious sugar on producing light? Bioluminescene must have some pretty strong advantages for fungi, but the jury is still out on exactly what those advantages are.

Fun fact: the generic name for the various molecules responsible for bioluminescence is called “luciferin” (the catalyzing enzyme is “luciferase”). At first I thought that this name revealed something of the social attitudes of the time towards organisms diabolical enough to produce their own light, but the chemistry of bioluminescence seems to have been understood too recently for scientists to choose molecular names based on Christian views of good and evil. In pre-Christian days, the Latin noun lucifer referred to the morning star (Venus), or as an adjective meant “bringer of light”, which makes the name of this class of molecules a lot more à propos, and a lot more poetic.

(Looking back into Abrahamic mythology, Jewish tradition identifies the fallen lucifer as a Babylonian king, while certain Christian traditions evolved to identify him as the rebel angel who fell from heaven and became Satan, the personification of evil.

William Blake's illustration of Lucifer for the Divine Comedy, via Wikimedia Commons

Not surprisingly, pre-Abrahamic mythology also referred to the morning star as a god who tried to occupy the throne of the supreme god and, when unable to do so, descended to rule the underworld.) Meanwhile, all this light-bringing business brings to mind the Greek Titan Prometheus. Of course, I’m not the first person to see the parallels here.

But I digress. Greek mythology aside, there are several classes of luciferins, basically one for each different type of bioluminescent organism. The luciferin responsible for light production in mushrooms is illudin. Illudin is also responsible for those mushrooms being toxic. Interestingly, it appears to have some anti-tumour properties and is currently being experimented with as a potential cancer treatment. Not bad for a few chunks of wood rotting on the forest floor.