The Mysterious Glow from Coral Reefs

The health of coral reefs around the world has been threatened from a variety of natural and human-produced sources. In order to more accurately decipher the factors that lead to healthy growth of corals beginning at the earliest life stages, scientists are unraveling the important role that light plays in their environment.

Corals are known to produce fluorescent proteins, which absorb high-energy light and re-emit it in a rainbow of colors, but their purpose remains a mystery. High amounts of light, and in particular higher energy wavelengths of light such as ultraviolet radiation and blue light, can damage corals and cause oxidative stress, which is thought to lead to coral bleaching and possibly death.

A new study produced from a collaboration between scientists at Scripps Institution of Oceanography at UC San Diego and the National Museum of Marine Biology and Aquarium (NMMBA) in Taiwan, has found a key difference in fluorescence color between young and adult individuals of a reef-building coral. The finding helps scientists trace the patterns of fluorescence in corals from early life through mature adulthood, as well as understand more about where young corals settle.

The researchers collected recently fertilized adult colonies of Seriatopora hystrix, a common Indo-Pacific shallow-water brooding coral, from Nanwan Bay in Kenting, Taiwan. The corals were maintained in aquaria until they released their brooded larvae about two weeks later. The researchers studied adult and larvae fluorescence properties in NMMBA laboratories by exposing them to a variety of wavelengths of light, including ultraviolet radiation, indigo, and blue light.

According to results described in the journal PLOS (Public Library of Science) ONE, Melissa Roth and Dimitri Deheyn of Scripps and Tung-Yung Fan of NMMBA have shown fluorescence differences between young and mature corals for the first time. While adults express the proteins that produce cyan (blue-green) fluorescent light, their coral larvae express the proteins that produce green fluorescent light, reflecting differences in their capacity to absorb different light energy between larvae and adults.

The results were surprising, Roth said, because they may be inconsistent with a “shielding” role for fluorescent proteins in this species, which is still one of the most accepted functions associated with fluorescent proteins in corals.

“The larvae experience a greater amount of deleterious indigo light (higher energy) in the water column compared to the adults on the seafloor, yet the larvae lack expression of fluorescent proteins that absorb such high-energy light,” said Roth, who conducted this research as part of her Ph.D. thesis while at Scripps and is now a postdoctoral scientist at Lawrence Berkeley National Laboratory and UC Berkeley. “While it is known that blue light and ultraviolet radiation contribute to damage to corals, the effects of indigo light are unknown.”

The fact that the larvae mainly absorb and fluoresce under blue light, while adults mainly under indigo light, is puzzling admits Deheyn, a project scientist in the Marine Biology Research Division at Scripps. The researchers concluded that fluorescence might have an optical-spectral function in absorbing certain colors of light that otherwise may be damaging to the corals, but the fluorescent proteins also have biochemical functions that remain to be characterized.

The study further determined that although coral colonies produce larvae with the physiological capacity to settle in a variety of habitats, the younglings tend to gravitate to environments that match the ambient light characteristics of their parents, a key to their settlement success and subsequent growth stages.

“These data highlight the importance of environmental conditions at the onset of life history and parent colony effects on coral larvae,” the authors noted in their report.

The new study is the latest product of an academic cooperation agreement signed by Scripps/UC San Diego and NMMBA in 2008. In 2012 Birch Aquarium at Scripps and NMMBA, sister aquariums, received an award from the American Association of Museums and the U.S. Department of State to connect youth in both countries under an international coral reef conservation program.

The PLOS ONE research was supported by the National Science Foundation East Asia and Pacific Summer Institutes for US Graduate Students program; the National Science Council in Taiwan; and the Air Force Office of Scientific Research, Natural Materials, Systems and Extremophiles program.

-- Mario C. Aguilera