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World: Corals Can Reestablish Symbiosis With Algae From Their Environments After Bleaching, Study Says |
| June 10, 2004 |
Source: Vancouver Aquarium Marine Science Centre
 | This photo compares the gorgonian corals used in the study after the bleaching episode (inset) and following their recovery.
Photo: University at Buffalo |
Researchers from the University at Buffalo have found that corals can develop new symbiotic relationships with algae from their surrounding environments after an episode of bleaching.
Scientists have known that corals can recover from bleaching episodes, or the whitening of corals due to environmental stresses, but they did not know why. The research provides new evidence that corals may make use of several recovery mechanisms.
Corals, which are in fact tiny anemone-like animals, are able to survive due to their relationship with tiny single-celled algae called zooxanthellae (zo-zan-thel-ly), which live inside the corals and provide them with energy through the process of photosynthesis. The relationship is a symbiotic one, meaning both coral and algae benefit from living together.
But environmental stresses such as dramatic changes in light or sea temperature can cause the corals to eject their symbiotic algae as a stress response, leaving the coral's white skeleton visible through its clear tissue. This process, known as bleaching, can destroy a coral reef when it occurs on a large scale.
Prior research has not been able to determine whether recovery from bleaching was caused by the action of the few remaining symbiotic algae within the coral tissue, or whether corals could acquire entirely new ones from the surrounding aquatic environment.
"Our data show that corals have the potential to take up new symbionts, providing a mechanism for resilience in the face of environmental change," said Mary-Alice Coffroth, Ph.D., associate professor of biological sciences in UB's College of Arts and Sciences, and senior author on the paper.
Coffroth and her co-author, Cynthia L. Lewis, induced bleaching in gorgonian soft corals (a type of sea rod common to Caribbean reefs) by keeping them in darkness in the laboratory.
After 12 weeks of darkness, the densities algae cells in the coral had plunged to less than one percent of their population density when the corals were healthy. The lack of light had prevented the algae from photosynthesizing, which deprived the corals of the energy required for survival. This triggered the ejection of the algae that led to the bleaching episode.
During the six weeks following the induced bleaching, the corals were exposed to new symbiotic algae that were added to the aquarium water. The researchers found that at the end of this period, the densities of algae cells within the coral showed a significant increase, demonstrating that the coral animals were able to establish symbiotic relationships with these new, or exogenous, algae.
"We found cell densities within the coral had increased between nine and 31 times the level measured immediately following the bleaching episode," said Lewis.
"The symbiosis had begun to reestablish itself," added Coffroth.
While their study was successful, the researchers warned that the fact that an individual coral population can survive an episode of bleaching does not necessarily mean that the larger ecosystem is healthy.
"Nevertheless, these data demonstrate that these animals may have the resilience to recover from bleaching episodes," said Coffroth.
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