Most mushrooms grow in the dark, but some also glow in the dark, and researchers say they've discovered how they do it — and why.

For thousands of years people have seen a glow often described as "cold fire" in decaying wood on forest floors, the result of bioluminescence in mushrooms.

It's been observed in more than 70 species of the common fungi, researchers say.

The chemical processes behind the glowing mushrooms is imperfectly understood, they say, but there's another puzzle scientists have wondered about, namely, why a mushroom would want to glow in the dark. What's the advantage of being your own biological night light?

A team of Brazilian and U.S. researchers writing in the journal Current Biology, say they know why: it's a form of advertising.

The bioluminescent glow is like a neon advertising sign; in the case of mushrooms, it's meant to attract insects that can then spread the mushrooms' spores and allow them to reproduce and spread.

"It appears that fungi make light so they are noticed by insects who can help the fungus colonize new habitats," says Cassius Stevani of Brazil's Instituto de Química-Universidade de São Paulo.

And calling the effect a "night light" is appropriate, the researchers note. Mushrooms don't turn it on indiscriminately; rather, a circadian clock controlled by temperature regulates it, allowing the mushrooms to conserve energy, only switching their glow on when the surroundings become dark enough for insects to spot it.

Bioluminescence has evolved many times independently in a number of life forms including fungi, bacteria, fish and insects, the researchers say.

"Most of these make light in their own way, that is, with biochemistry that is unique to each organism," says molecular biologist and geneticist Jay Dunlap of Dartmouth College.

For their study, the scientists looked at Neonothopanus gardneri, which grows around the bases of young coconut palms trees in Brazilian forests.

Also known as "flor do coco" or coconut flower, it's one of the largest and brightest of all bioluminescent mushrooms, making it an ideal study subject, the researchers said.

They fashioned their own fake N. gardneri mushrooms out of acrylic resin, then equipped some with green LED lights to mimic the natural glow of the real versions.

After placing them in forest locations where the real mushrooms grow and watching over five nights, they determined that their LED-equipped "glow" mushrooms attracted almost three times as many insects as those without lights.

Insects crawling over the real bioluminescent mushroom can disperse the fungal spores, helping the fungi species spread and survive, the researchers explain.

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