It’s one of the ocean’s most beautiful and striking mysteries: Why do corals fluoresce?
In shallow waters, they glow a brilliant pink and purple. In deeper waters, corals turn red and green against a dim blue background. The view is most unforgettable at night with a flashlight and mask filter, when the fluorescent corals provide a “psychedelic adventure.”
Jörg Wiedenmann, a coral reef scientist at University of Southampton, had previously found that the pink and purple fluorescence in shallow waters act as a kind of sunscreen. The fluorescent pigments absorb damaging wavelengths of light and emit it as pink or purple light, protecting the single-celled organisms called zooxanthellae that live symbiotically inside coral. Zooxanthellae are photosynthetic and they provide the coral with food in exchange for shelter.
This “sunscreen” effect was interesting, but it didn’t explain why corals fluoresce in deeper water, where light was not intense enough to harm zooxanthellae. In fact, in those dim blue waters, the problem was more likely too little light. Wiedenmann and his colleagues now have a new study, in which they present a novel function for deep water fluorescence. And again, it has to do with the zooxanthellae: Coral may be converting blue light into orange-red light that penetrates deeper into the coral tissue, where photosynthetic zooxanthellae live. Fluorescence, by definition, is the absorption of light in one color and the emission in another.
There’s something elegant in these explanations if true. So intertwined is the coral-zooxanthellae relationship that fluorescence is the result of two species living and working together.