In the case of corals, however, the “sunscreen” may come in the form of fluorescent proteins—and the protection is not from sunburn but from a fatal phenomenon known as bleaching.
Bleaching occurs when coral polyps, the animals that make coral reefs, expel photosynthetic algae under environmental stress, including too much sunlight. Under normal conditions, the algae, which give coral its brown color, are important symbiotic partners: embedded under the polyps’ skin, they produce carbohydrates that the polyps depend on for most of their energy.
Corals, in turn, need sunlight to fuel the algae’s photosynthesis. But too much sunlight can damage the algae’s photosynthetic machinery, leading them to produce damaging free radicals. This stress, combined with stress from warming seawater, can lead to bleaching.
Dr. Anya Salih of the University of Western Sydney, who served as the scientific adviser for artist Lynette Wallworth’s installation Coral: Rekindling Venus, says that corals’ own fluorescent proteins, which absorb high-energy light and emit it as lower-energy light of longer wavelengths, may offer some protection.
For example, cyan fluorescent proteins absorb ultraviolet light, which Salih says can “damage photosynthesis,” and release it as lower-energy blue light, effectively shielding the corals’ algae. When Salih sampled corals during the mass coral bleaching event of 1998, during which 25 percent of Great Barrier’s inshore reefs were reported to have severe bleaching, she found that those with fluorescent proteins retained higher levels of algae than those without them. Still, many questions remain.
“Within coral reef science, this is still a fairly controversial topic,” says Salih. “The mechanisms of photoprotection are quite complex, and we’re still trying to unravel them.”
Corals, along with other marine animals, cyanobacteria, and algae, also have a different compound—mycosporine-like amino acids, or MMAs—that absorbs ultraviolet light. At least one human sunscreen, Helioguard 365, uses two MMAs from red algae.
Fluorescent proteins, meanwhile, may also serve an important function in the shade. Within a coral polyp, the algae are sandwiched between layers of coral tissue. In the sun, Salih found, fluorescent proteins form a layer on top of the algae, acting like a reflective shield. But in the shade, the fluorescent proteins appear underneath or around the algae, amplifying light and directing it back toward the algae, perhaps to aid in photosynthesis.