Glowing corals in the Red Sea.
Glowing corals in the Red Sea. Jörg Wiedenmann

Zooxanthellae have a pretty sweet deal. The photosynthetic algae live within corals, and supply their hosts with food in exchange for shelter and nutrients. In shallow waters, the corals even produce glowing fluorescent proteins that act as sunscreen for their microscopic tenants. Corals found as deep as 165 meters (541 feet) also glow, but their algae don’t really need sun protection because it’s so dark at that depth. Until now, these deep glowing corals were a bit of a mystery, but new research shows that, like their shallower cousins, these corals are also trying to help their algae out.

“Corals need special features to adjust to life in these low-light depths for the benefit of their vital photosynthetic partners,” said Jörg Wiedenmann, a researcher at University of Southampton and coauthor of the report, in a press release. In the dark depths, most of the light that penetrates from the surface is in the blue part of the spectrum. There, the corals’ proteins absorb this blue light and emit orange-red light instead. The organge-red light provides more fuel for photosynthesis for the resident algae. In a lab, these orange-red fluorescing corals were more likely to survive than neighbors that don’t glow.

And now researchers are exploring other uses for these fluorescent proteins. They can also be used to help cancer and HIV researchers, who use such glowing proteins to help identify living cells under a microscope. It may not be as great a deal as the one zooxanthellae get, but it’s not bad.