Back in the lab, they simulated the temperatures that had been recorded at Admiral Reef during the cold weather event, testing the different corals' physiological responses at 12 C and 16 C (61 F), and then, after the corals' exposure to the cold, returned the temperature to 20 C (68 F). They found that although responses varied depending on the coral species, in general the stress of extended cold temperatures had an effect similar to that of high temperatures.
Kemp explained that corals depend on Symbiodinium, a type of symbiotic algae that lives inside them, for nutrition. Through photosynthesis, the algae produce sugars, which are passed on to the corals. "The cold temperatures inhibited photosynthesis in the algae, leading to a potential net loss of carbon transferred from the algae to the coral," said Kemp. He said that each coral species had its own unique type of Symbiodinium, some of which were better able to tolerate and recover from cold temperatures than others.
All of the corals experienced a significant decrease in photosynthesis at 12 C. Siderastrea siderea and M. faveolata were able to handle the 16 C temperatures, but P. astreoides was not, and did not show signs of recovery once the temperature was returned to 20 C. Siderastrea siderea was the only coral able to recover.
"Corals and their symbiotic algae have a range of stress tolerance," said Kemp. "Some can handle moderate stress, some are highly sensitive, and some are in between. But extreme cold is
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University of Georgia