Photosymbioses between marine cnidarians ( e.g. sea anemones and corals) and Symbiodiniaceae dinoflagellates are influenced by abiotic factors like light and temperature. The anemone Exaiptasia diaphana is widely used as a model for understanding the Symbiodiniaceae-Cnidaria photosymbiotic system. Despite extensive research on many facets of this photosymbiosis, key photophysiological processes, such as photosystem II (PSII) photoinactivation and repair mechanisms, remain poorly characterized. In this study, we used individuals of E. diaphana harbouring endosymbiotic dinoflagellates from the genus Breviolum to characterize the susceptibility to photoinactivation and repair capacity (measured by photoinactivation and repair rates, k PI and k REC , respectively) of PSII in the symbiont under thermal stress (16 °C and 32 °C) using in vivo imaging of chlorophyll fluorometry and lincomycin as a protein synthesis inhibitor. The results showed that the repair capacity of PSII of the symbionts ( k REC ) was higher than the susceptibility to photoinactivation ( k PI ) despite of the level of thermal stress applied. Elevated temperatures increased the vulnerability of PSII to damage and reduced its repair capacity. In contrast, cold stress decreased the susceptibility to PSII damage but considerably reduced the efficiency of repair mechanisms. Extreme temperatures affected both PSII repair capacity, decreasing k REC , and photoinactivation, increasing k PI under high temperatures (greater susceptibility to photodamage) and decreasing k PI under low temperatures (lower susceptibility to photodamage). By characterizing the sensitivity of the E. diaphana – Breviolum photosymbioses to thermal stress, this study provides insights on the photophysiological vulnerability and resilience cnidarian-dinoflagellate in general, contributing to the understanding of the potential impacts of environmental change. • In Exaiptasia symbionts, PSII repair exceeds photodamage, even under thermal stress. • Extreme temperatures affect both PSII photoinactivation and PSII repair. • Heat stress increases PSII photodamage and reduce PSII repair capacity. • Cold stress lowers PSII photodamage but also weakens PSII repair mechanisms.
Silva et al. (Fri,) studied this question.