Coral Can Have Too Much Of A Good Thing
October 15, 2012

Too Many Algal Symbionts Could Harm Coral Reefs

redOrbit Staff & Wire Reports - Your Universe Online

While the single-celled algae that live inside corals typically play a vital role in keeping the reefs healthy, a new study suggests that an overabundance of the symbiotic organisms could have a negative effect on them.

According to scientists at the University of Miami (UM) Rosenstiel School of Marine & Atmospheric Science, these algal symbionts provide corals with the energy needed to build larger reef frameworks.

When temperatures become too hot, they are expelled during bleaching episodes that can result in the death of many corals, though it had been believed that reefs with a greater number of the symbionts would be better protected from these episodes as they had more algae to release.

The new study, which has been published in the journal Nature Climate Change, suggested that the opposite was true.

"We discovered that the more symbiotic algae a coral has, the more severely it bleaches, showing that too much of a good thing can actually be bad," Ross Cunning, a Ph.D. student at the university and the lead author of the study, said in a prepared statement Sunday. "We also learned that the number of algae in corals varies over time, which helps us better understand coral bleaching risk."

Cunning and colleagues used cauliflower coral (Pocillopora damicornis) collected from the coast of Panama during their study, and monitored it for a period of six months while increasing the heat until the coral became bleached. They also analyzed the DNA samples of the reefs to determine the ratio of algal cells to coral cells.

"Corals regulate their symbionts to match the environment in which they are found, and this study shows there is a real cost to having too many," explained Andrew Baker, associate professor at UM's Rosenstiel School and co-author of the study. "There are real-world implications of this. Corals will be more vulnerable to bleaching if they are found in environments which increase the number of symbionts, such as coastal reefs polluted by wastewater and runoff."

"If we can improve water quality, we might be able to buy some time to help these reefs avoid the worst effects of climate change," he added. "Other environmental changes, including ocean acidification as a result of increasing carbon dioxide emissions, might also influence bleaching vulnerability in ways we haven't thought of before."