Glowing in the dark costs energy. So why do mushrooms bioluminescence?
To answer this question, researchers from Brazil and the U.S. focused on the pale green light emitted from fungi. They had a hunch that the light attracted insects. Fungi produce tiny spores to spread themselves, much like seeds from trees. Insects can help transport the fungal spores. The researchers were also curious if glowing brighter attracted more insects.
The fungi Neonothopanus gardneri, glows strongly at the bottom of coconut palms in Brazil, near the transition ecosystems by the Amazon forest. Down below the palms, the mushrooms are exposed to less windflow, so they need to find another way to spread their spores in order to be reproductively successful.
Originally it was thought that these mushrooms glowed all the time. Constant bioluminescence uses a lot of energy, so the researchers wanted to examine if this assumption was true.
The researchers grew the fungus in the lab under a normal light-dark cycle. Then, to test whether that was true, they left the fungi in complete darkness. The mushrooms glowed in a cycle. Much like our circadian rhythm, which contributes to our jet lag, the mushrooms also had a circadian rhythm that maintained itself on a 22 hour cycle, that corrects itself to a 24 hour cycle based on temperature.
Since bioluminescence is so energetically costly, the mushrooms only glow at night. They don’t glow bright enough during the day to be extra visible. An added bonus is that spores prefer to become active and grow at night when it is more humid.
Filming the mushrooms at night showed that they were swarmed by rove beetles, but the researchers observed the beetles may have been attracted by smells or something else, not necessarily the glowing.
To test if bioluminescence played a role in attracting the beetles, the researchers placed plastic mushrooms lit within by a green LED light at the same wavelength as the glowing of the mushrooms.
Beetles swarmed the fake glowing mushrooms, confirming that the glow-in-the-dark mushrooms are investing their energy wisely. Bioluminescence is one of the factors attracting insects to the mushrooms to help them spread their spores.