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Tiny Polyps Need Two Kinds of Carbon to Survive Coral Bleaching

Posted on: Wednesday, 30 April 2008, 03:00 CDT

How well ocean reefs recover from the growing damage caused by warming sea temperatures depends both on how much the tiny coral polyps can eat and how healthy they can keep the microscopic algae that live inside their bodies, said researchers who conducted a recent study. The research focuses on the key role that carbon plays on the recovery of damaged coral reefs.

Andrea Grottoli, an assistant professor of earth sciences at the Ohio State University, tested two types of coral, Montipora capitata and Pontes compressa.

"Corals get carbon in two ways, either through photosynthesis by the algae kept inside their bodies, or by feeding on the Zooplankton that comprise their diets," she said.

But when seawater temperatures climb, the coral can either jettison the algae altogether, or the algal cells can lose the pigments essential for photosynthesis. Without the algae, corals appear white, which is often referred to as "bleached."

Grottoli placed samples of both healthy and bleached corals of both types in tanks, mimicking actual ocean conditions. In one set of experiments, she pumped in seawater containing higher-than- normal levels of a carbon isotope, C-13. In another, she fed the corals Zooplankton that were also heavily laced with the carbon isotope.

"We could track the carbon and determine if it was coming from either the photosynthetic process or from the animals' feeding, and then see how it was ultimately used by the animals," she said.

The experiments showed that healthy corals had much more of the seawater-labeled carbon.

"But we could also see that in the healthy coral, the carbon was transferred into the algae, where it is used for photosynthesis and ultimately ends up in the animals' skeleton," she continued. "So the corals are using photosynthetic carbon for calcification and to meet their daily metabolic demands."

The carbon consumed while feeding, however, is not ending up in the skeleton, she said. Instead, it is ending up both in the tissue of the coral polyp or inside the algae. With bleached samples, the coral is apparently feeding carbon to the algae.

"This suggests that there is a great deal more coupling between the coral and the algae than we had thought," said Grottoli. "Once the coral gets the carbon from feeding into its system, it locks it in, using it for energy storage and tissue growth and, when bleached, to feed the algae.

"All corals need both photosynthesis and feeding for recovery, and the rate of those two processes is the key to whether the coral can actually meet all its metabolic demands and ultimately recover."

Source: Sea Technology

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