Physiological Response to Thermal Stress of the Caribbean Corals Orbicella annularis and Porites astreoides

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Background and Objective:The physiological response of corals to ocean warming is an essential component of their overall resiliency to climate change.It is important to understand how increasing sea surface temperature will affect the mutualistic relationship between corals and their endosymbionts dinoflagellates (Symbiodinium spp.).This study focused on the effects of temperature stress on the photophysiology of two Caribbean corals differing in life history strategies: Orbicella annularis (O.annularis) and Porites astreoides (P.astreoides).Materials and Methods: A total of 12 fragments of each species were collected from the Puerto Morelos, Mexico shoreline.Six fragments of each species were placed under ambient light and a water temperature of 28EC and the other six under ambient light and an elevated temperature of 32EC for 10 days.Maximum pressure over photosytem II (Qm) was estimated daily while chlorophyll content and Symbiodinium densities were measured at the end of the experiment.Results: Qm values were considerably higher in O. annularis at 32EC when compared to O. annularis at 28EC.In contrast, Qm values for P. astreoides did not differ significantly between treatments.Interestingly, Qm values of P. astreoides at 32EC was similar to that of O. annularis at 28EC.Orbicella annularis showed higher Symbiodinium densities and significantly higher concentrations of chlorophyll a at 28EC than at 32EC.In P. astreoides, no differences on chlorophyll content and Symbiodinium densities were found between temperature treatments.Porites astreoides showed higher chlorophyll a content and Symbiodinium densities than O. annularis under high temperature stress.Conclusion: Low Qm values but high chlorophyll concentrations and Symbiodinium densities within P. astreoides after induced temperature stress provides a physiological basis of the opportunistic strategy of this species.However, the significant increase in Qm, lower chlorophyll a concentration and reduced Symbiodinium densities observed in O. annularis at 32EC highlights its vulnerability to ocean warming.If O. annularis, which is considered one of the main builders of Caribbean coral reefs, disappears and is replaced by relatively small and weedy P. astreoides, the structural heterogeneity of Caribbean coral reefs will be reduced.