Living on a coral reef is a bit like living in a tanning bed. As the sun's rays shine through the water and reflect off the reef, they strike corals, their symbiotic photosynthetic algae, and other inhabitants from above and below. So what keeps these creatures from being fried? A new study suggests that coral acts as a sunscreen, absorbing UV light and limiting the harm it inflicts on the reef's denizens.
Past studies have found that the calcium carbonate exoskeletons of corals that make up reefs fluoresce under ultraviolet light, suggesting that they absorb UV rays. To see if this material protects the organisms that live on reefs, marine biologist Ruth Reef of the University of Queensland in Australia and colleagues studied sea anemones (Aiptasia pulchella). These relatives of corals have similar tissues and are also home to symbiotic photosynthetic algae.
The team placed the anemones on top of either coral skeletons or white plumbing tape in the lab. Unlike the tape, the skeletons absorbed nearly all the harmful UV rays, emitting it as yellow fluorescent light, the researchers reported 25 November online in PLoS ONE. What's more, anemones placed on the coral received four times less UV radiation and showed about seven times less damage to their DNA than those placed on the tape. The team saw this effect even when they ground the skeletons into a fine powder, suggesting that the protection is due to the chemistry of the coral exoskeleton, not to the scattering of UV rays from its rough, complex surface.
Many of the photosynthetic organisms that live in the oceans also produce calcium carbonate, and they may have begun doing so to protect themselves from ultraviolet radiation, says Reef. "Calcification began circa 600 million years ago, when UV levels were much higher than today." Moreover, during the Cambrian explosion roughly 530 million years ago, the abundance and diversity of skeletons increased, which might have reflected "a need for [coral building] that arose during that time when many organisms moved to shallow, oxygen-rich waters where UV levels were high," she adds. "This new property is thus an additional and unexpected adaptation of a host to a symbiotic way of life," says expert Denis Allemand, the scientific director of the Scientific Center of Monaco, an oceanographic institute.
Scorpions, spiders, and other creatures also fluoresce when exposed to UV light, Reef and colleagues note, suggesting that the sunscreen effect has evolved more than once.