Glow With the Flow
Deep in the ocean, coral survive by making light in the darkness
MOST TYPES of coral depend on the sun to survive and use algae living inside them to make food from light. But little sunlight reaches the deep water where many coral live. They have an elegant way of making due: emitting their own light.
Recent research shows that some coral living far below the surface can absorb blue light, a wavelength that penetrates most deeply into the ocean, and re-emit it in a red-orange glow. This process is known as fluorescence (which is not the same as bioluminescence, another way organisms produce light). A new paper published in the journal Proceedings of the Royal Society B shows that this red-orange light reaches deeper within the coral than blue light, enabling algae to photosynthesize and produce chemical energy. The coral apparently employs this trick to get more energy out of the meager light penetrating the deep sea.
Jörg Wiedenmann, an oceanographer at England’s University of Southampton and the study’s lead author, discovered glowing coral while researching anemones. He was “blown away by a rainbow of colors” when he placed them under ultraviolet light, he says. Subsequent research showed that chemicals that absorb and re-emit light, known as photo-convertible red fluorescent proteins, are in the coral’s surface.
At a study site in the Mediterranean, 30 percent of coral at a depth of 150 feet had these red fluorescent proteins. The proportion of coral with this “glowing” ability increased with depth.
To determine whether coral containing the red fluorescent proteins had a survival advantage over those without them, the researchers simulated the condition of a deep-sea reef in the lab. Over the course of two years, fluorescing coral had a higher survival rate, suggesting that their glowing ability helps them produce more food from the light they receive.
Some shallow-water coral can fluoresce but do so to protect themselves from too much sunlight. Pigments “affect a number of functional traits in corals,” says Michael Kühl, an ecologist at the University of Copenhagen, who wasn’t involved in the paper. Fluorescing pigments help expand the habitats where coral can thrive, he says.