[A-CG41] Coastal Ecosystems -2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG41-07] Photoinhibition as a strategy for corals to mitigate heat stress and resulting ROS production

Reimi Terayama¹, *Agostini Sylvain¹, Takashi Nakamura², Tomihiko Higuchi³, Ikuko Yuyama⁵, Sung-Yin Yang¹, Toshihiro Miyajima³, Hiroyuki Fujimura⁴ (1.Shimoda Marine Research Center, University of Tsukuba, 2.Tokyo Institute of Technology, 3.AORI, University of Tokyo, 4.University of the Ryukyus, 5.Yamaguchi University)

Keywords:coral bleaching, photoinhibition, reactive oxygen species

Elevated water temperature due to climate change is causing massive bleaching events around the world. Two main factors that lead to coral bleaching are strong light and heat stress. Recent studies suggested that the bleaching response starting with a decrease in the carbon fixation due to heat damages of the Rubisco enzyme. Under strong light, this cause the photosystem electron transfer exceed the calvin cycle capacity. This excess in energy would cause ROS production. Photoprotective mechanisms such as photoinhibtion could mitigate the production of ROS, by restricting the electron transfer upstream carbon fixation. The loss of zooxanthellae could therefore be secondary protection mechanisms to reduce ROS production when other mechanisms are overwhelmed. Although both mechanisms reduce the photosynthesis rates of corals, photoinhibition could allow a more rapid recovery compare to the loss of zooxanthellae. We recently a theoretical model highlighting the dynamics of bleaching and the importance of photoinhibition as a protective mechanisms. However the lack of studies on the change in both photosystem efficiency and photosynthetic rates under natural diurnal cycle and temperature stress, limits the validity of the model. Here we present experimental data to validate and tune the theoritical model. Specimens of the corals A. solitaryensis and P. heronensis were incubated in the laboratory under diurnal cycle and three temperature treatment, 26ºC, 30ºC, and sustained 30ºC for one-week. In a first experiment we investigated the dynamics photosystem efficiency and, gross snd net photosynthesis using PAM fluorometry and microsensors techniques. Secondly we investigated the difference in oxygen and carbon based photosynthesis rates during bleaching under the same conditions of light and temperature. All corals bleached under the increased temperature treatment. Our results showed that high light intensity cause enhanced photoinhibition and in consequence, a reduction of the photosynthesis rates and electron transports rates in PSII. High temperature treatment enhanced photoinhibition and further decreased net and gross photosynthesis rates. The comparision of net photosynthesis rates in oxygen and carbon units showed a smaller difference under heat stress which was associated with enhanced photoinhibition. Our result suggests that photoinhibition can mitigate heat stress by reducing the difference between electron transfer rates from the photosystems and the capacity of the calvin cycles. Our observation mostly matches the previously develloped model, and will allow its tuning.