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Corals can reestablish symbiosis with algae after bleaching, UB research shows
By ELLEN GOLDBAUM
Contributing Editor
Corals can develop new symbiotic relationships with algae from their environments after they've undergone bleaching, the process by which corals whiten as a result of environmental stress, UB biologists report in the current issue of Science.
Single-celled algae called zooxanthellae convert sunlight into energy for the corals. These tiny plants impart rich hues of oranges, reds, purples and yellows to corals and help them build reef structures.
The research provides evidence that corals may have multiple mechanisms that facilitate recovery from bleaching induced by environmental stresses.
Scientists have known that corals can recover from bleaching episodes, but they did not know why.
It has not been clear whether recovery resulted from the few remaining symbiotic algae, or algal symbionts, remaining within the coral tissue since early development, or if coral could acquire entirely new ones from their aquatic environments.
UB researchers, working with the Caribbean octocoral, Briareum sp., have found that corals can recover from bleaching (inset).
Corals survive and thrive because of the symbiotic relationship they develop with the single-celled algae called zooxanthellae, which live inside them and help supply them with food.
But certain environmental stresses, such as high or low light or sea temperatures, can lead to a reduction in algal densities or loss of pigmentation, leaving the coral's white skeleton visible through the clear tissue.
"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, associate professor in the Department of Biological Sciences in the College of Arts and Sciences, and senior author on the paper.
Coffroth and her co-author, Cynthia L. Lewis, who recently received her master's degree in biological sciences from UB, 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, cell densities of symbionts in the coral had plunged to less than 1 percent of their population density when the corals were healthy.
During the six weeks following "bleaching," the corals were exposed to algal symbionts that were added to the aquarium water.
The researchers found that at the end of this period, symbiont cell densities 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 re-establish itself," added Coffroth.
The UB researchers caution that the survival of individual colonies and populations of coral should not be construed as a demonstration that an entire ecosystem is healthy.
"Nevertheless, these data demonstrate that these animals may have the resilience to recover from bleaching episodes," said Coffroth.
The coral used in the research was harvested from the Florida Keys National Marine Sanctuary and the laboratory work was conducted at the Keys Marine Laboratory in Long Key, Fla.
The work was supported by grants from the National Science Foundation.