Coral Bleaching Study Explains Different Responses To Climate Change
April 24, 2013

Coral Bleaching Study Reveals Why Some Survive And Others Die

Brett Smith for - Your Universe Online

When corals become stressed, they expel their symbiotic algae companions in a process known as “bleaching.” Corals can survive the bleaching, but it leaves them highly vulnerable and often results in die-off.

As a changing climate threatens to bleach the corals of the world´s oceans on a massive scale, a team of researchers from Northwestern University has found that some corals facilitate bleaching through the light-scattering properties of their skeletons, according to their report in the open access journal PLOS ONE.

"We have solved a little piece of the puzzle of why coral reefs are bleaching and dying," said lead author Luisa A. Marcelino. "Our research is the first to show light-scattering properties of the corals are a risk factor."

Based on a study of almost one hundred different species of corals, the scientists used optical technology designed for early cancer detection to determine that reef-building corals disperse light in different ways compared to the symbiotic algae that live within their coral exoskeletons. The corals that are less efficient at light scattering are more likely to retain their symbiotic algae under stressful conditions — like climate change — compared to those types whose skeletons scatter light most efficiently. However, under normal conditions the opposite was found to be true — with more efficient corals tending to thrive.

"Coral reefs are like the rain forests of the oceans — the consequences will be catastrophic if coral reefs are lost in great numbers," said co-author Vadim Backman, a physicist and professor of biomedical engineering at Northwestern. "Corals are also optical machines. By identifying how much light the skeletons of individual coral species reflect, we have learned which species are more resilient under stress."

The health of a coral´s symbiotic algae is crucial to its survival, with the algae providing nutrients to the coral and receiving both shelter and light in return.

To calculate the amount of light diffraction and amplification inside the skeletons of 96 different coral species, the team used a low-coherence enhanced backscattering (LEBS) technique devised by Backman. The groundbreaking technique allowed the researchers to determine that light transport at the micro level translates directly into how fast light amplification increases with the loss of algae.

After their analysis was complete, the team constructed a family tree of corals that illustrated how bleaching and light scattering relate to the evolutionary history of corals. As corals whose skeletons scatter light most efficiently thrive under normal conditions, evolution has tended to favor these particular species. More evolved corals tend to grow faster, leading to an exoskeleton that is more conducive to light scattering.

"We found that bleaching and light scattering are associated across the history of reef corals," said co-author Mark W. Westneat, a coral reef fish biologist and curator of zoology at the Field Museum in Chicago. "This important mechanism occurs repeatedly in all major coral groups, regardless of relationship or evolutionary age."

The specimens used in the study were taken from the Field Museum, which includes scores retained from both the Chicago Columbian Exposition and World's Fair of 1893 and the Smithsonian Institution. Scientists believe that living reef corals originated about 220 million years ago, and that contemporary reefs are descendants of these older lineages.