Life Science

Bioluminescent Fungi Glow to Trick Insects

When it comes to ‘catfishing‘, bioluminescent Agaricales fungi have the process down to a science. Each night, they emit a steady green light in an effort to attract lovesick beetles. Despite the ruse, the fungi and insects have the same motivation: the drive to reproduce.

The mesmerizing glow Agaricales fungi have been observed for hundreds of years. Reports go back as far as the writings of Aristotle in which he describes glowing rotten wood (which we now know was caused by bioluminescent mycelium).

BBC Earth fungi clip

Mycena chlorophos bioluminescent fungi against a dark background
Mycena chlorophos bioluminescent fungi
Image by Steve Axford/BBC

Discovering Why Bioluminescent Fungi Glow

Despite all that time knowing that these bioluminescent fungi exist, the underlying evolutionary justification for their appearance wasn’t understood until very recently. A study published in Current Biology in 2015 managed to shine some much-needed light on this subject (sorry, I couldn’t resist).

Out of the approximately 100,000 documented fungal species, only around 75 are known to exhibit this luminescence. The assumption up to this point was that this small number of fungi were producing their glow around the clock as some obscure byproduct of their metabolism. The authors of the Current Biology study, however, observed that bioluminescent fungi were generating light in a regular circadian rhythm. They glowed through the night and then stopped glowing during the day. That implied that the glow wasn’t just some energetically expensive metabolic byproduct, but rather a trait with a very deliberate purpose.

Mycena chlorophos bioluminescent fungi
Mycena chlorophos bioluminescent fungi
Image by Steve Axford/BBC

Anyone who has been around a light source at night knows that light tends to attract living things, particularly insects. The researchers hypothesized that the fungi might be glowing to exploit that tendency of insects for some purpose. They went a step further and postulated that that purpose might be for reproduction.

Insects attracted to the bioluminescent fungi would presumably crawl all over the mushrooms and become covered in fungal spores that they would then transport to different parts of the environment. Just as flowering plants have evolved alongside the foraging and/or reproductive behavior of animals to further their reproductive success, so too, perhaps, were the fungi.

To test their theory, they built artificial acrylic mushrooms which produced a green light (lmax 530 nm) similar to the natural bioluminescence produced by the fungi. They then placed illuminated and non-illuminated artificial mushrooms out in the forest test area and coated them in a sticky resin.

When they returned over the course of five nights, they discovered that the illuminated decoys not only ensnared some insects as expected, they ensnared more than 3 times the number of insects as their dark counterparts.

With this new evidence in hand, the researchers have since followed up with camera studies to investigate this relationship between the fungi and the arthropods. Their efforts are being supplemented by those of naturalist filmmakers like the team producing Planet Earth II who have made some extraordinary visual records of this behavior.

Researchers have since demonstrated that by glowing at night, the fungi are exploiting the reproductive strategies of insects to advance their own reproductive goals in a marvelous way. The insects are drawn to the light, mistaking the fungi as potential mates. In their search, they inadvertently fulfill the reproductive needs of another species: the glowing fungi!

If you’d like to learn more, National Geographic covered some of this work in the clip below:

National Geographic coverage of the work to understand bioluminescent fungi ecology.

Bioluminescent Fungal Luciferins

Since the original study in 2015, much more is known about the exact mechanism behind how bioluminescent fungi produce their distinctive green glow. A team of researchers from Russia and Brazil collected bioluminescent fungi and isolated the compounds responsible for the illumination.

Like other bioluminescent organisms, the fungi were using luciferins to create their light. Luciferins are chemical compounds that, when combined with oxygen, produce an excited (“unstable”) oxyluciferin compound. This compound effectively has “more energy than it wants to have” and emits a photon (particle of light) when it breaks down into a more stable form.

Getting luciferin and oxygen to combine requires a helper – an enzyme called a luciferase. In case you need a biochemistry refresher, you can think of enzymes as a set of hands that pull other molecules close to one another. They are usually very specific and only grab a particular set of molecules and arrange them in a particular way.

Bioluminescent fungi use this same general process, but they bend the rules a little. First, they grab a secondary metabolite called hispidin. An enzyme – hispidin-3-hydroxylase (H3H) – then uses oxygen and NAD(P)H (an “energy-carrying” molecule) to produce the fungal luciferin: 3-hydroxyhispidin. The fungal luciferin then follows the usual pathway of being enzymatically combined with oxygen to produce an excited oxyluciferin which quickly simmers down and produces light.

A diagram showing the molecular pathway that produces bioluminescence in fungi
Figure and caption from Kaskova et. al (2017).
General mechanism of the fungal bioluminescence and synthesis of the oxyluciferin (2). Hispidin is hydroxylated by a styrylpyrone hydroxylase [hispidin-3-hydroxylase (H3H)] in the presence of O2 and NAD(P)H, producing 3-hydroxyhispidin (1) (11), the fungal luciferin, which is enzymatically oxidized by O2, giving an HEI that decomposes in CO2 and the excited oxyluciferin. Fluorescence emission gives the ground-state oxyluciferin (2). The oxyluciferin was synthesized in two steps from 3,4-dimethoxybenzalacetone. LiHMDS, lithium bis(trimethylsilyl)amide; THF, tetrahydrofuran. (B) Comparison of HPLC-PDA-ESI-MS profiles of the enzymatic reaction (after 15 min) and the synthetic oxyluciferin. mAU, milliarbitrary unit. (C) Mass spectra of the luciferin 1 (Rt = 10.1 min, m/z = 261 [M − H]) and compound 2 (Rt = 12.2 min, m/z = 249 [M − H]). (D) Matching of the fungal bioluminescence (BL) spectrum and the fluorescence (FL) spectrum of 2 in acetone. The absorption spectrum of 2 is shown for reference (λmax = 380 nm).

Everything seems pretty straightforward, but the luciferase enzyme used by the fungi has some unusual…flexibility. Whereas many enzymes are very picky about what molecules they interact with, fungal luciferases seem to be “promiscuous”. They have the ability to bind to several types of substrates which can produce different colors!

So, while fungi in nature emit a green glow, the enzymes they use can actually accept more than one type of luciferin. They could theoretically glow orange, yellow, blue, etc. For bioengineers, that is a very exciting find and you can count on more attention being given to bioluminescent fungi in the future.

This post was updated from an earlier version published in 2017.

References and Further Reading

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