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Seasonal acclimation of thermal performance in two species of reef-building corals

Thermal performance curves describe the relationship between temperature and the rate of biological processes. These relationships can vary among species and environments, allowing organisms to acclimatize to their local thermal regime. This study quantified the seasonal variation in the thermal performance of several coral and symbiont-dominated physiological traits for the thermally tolerant coral species Porites cylindrica and the thermally sensitive coral species Acropora valenciennesi. Photosynthesis rates, respiration rates, maximum photosystem II (PSII) quantum yield and electron transport rates were measured in winter and summer on coral fragments exposed to an acute temperature increase and decrease up to 5 degrees C above and below the average seawater temperature in each season. Results showed that colonies of A. valenciennesi acclimated primarily by shifting their optimal temperature to a higher temperature in summer, whereas colonies of P. cylindrica had broader thermal breadth during summer. For symbionts within both species, performance was higher at all temperatures in summer, while the thermal optima and performance breadth remained unchanged. Despite these changes in thermal performance, the thermal optima of most traits did not match the ambient environmental temperature, but fell between the summer and winter temperatures. Overall, these results showed that both coral species were physiologically plastic in response to temperature change, but that there are constraints on the rate or capacity for acclimation that prevent a perfect match between the average temperature of the environment and the thermal optimum of the species.