[A-CG40] Coastal Ecosystems 2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG40-P02] Diurnal cycle of photosynthesis and Photoinhibition of hermatypic corals under heat stress

*Akihiro Hirata¹, Reimi Terayama^2,1, Tomihiko Higuchi⁴, Takashi Nakamura³, Hiroyuki Fujimura⁵, Sylvain Agostini¹ (1.Shimoda Marine Research Center, University of Tsukuba, 2.International Christian University , 3.Tokyo Institute of Technology, 4.AORI, The University of Tokyo, 5.University of the Ryukyus)

Keywords:photoinhibition, coral bleaching, gross photosynthesis rate

INTRODUCTION
global temperatures and anthropogenic disturbances have led to an increase in massive bleaching events over the past century. Coral bleaching refers to the loss of zooxanthellae or their pigment due to various stressors including heat stress. As zooxanthellae produces most of the nutriments required by the coral host, the lack of zooxanthellae prevents autotrophy. Heat stress induced coral bleaching is thought to be caused by an excessive production of reactive oxygen species (ROS). Under heat stress ROS are produced when excess electrons generated during the electron transfer process of the photosynthesis react with the surrounding oxygen. One prevention mechanism coral to limit ROS production is photoinhibition, which prevents excess energy from flowing into the photosystem and downstream rubisco processes. Here, we measured gross and net photosynthesis rates, and Y(II) to understand the role of photoinhibition during heat stress.

MATERIALS AND METHODS
In this study, Acropora solitaryensis and Porites heronensis were used based on their local abundance, growth strategies, and different resilience to environmental stress. O₂ microsensors were used to measure gross and net photosynthesis, and pulse-amplitude modulation (PAM) to measure the photosystem II efficiency (YII) as an indicator of photoinhibition. The first experiment was conducted to measure gross photosynthesis rates within coral tissue from the coral skeleton to coral surface of A. solitaryensis and P. heronensis to find the optimal depth for measuring gross photosynthesis rates. During the second experiment measured gross photosynthesis rates were measured for both coral species under a combination of ranging light intensities and temperatures of 26°C, 30°C, and 30°C (1 week incubation).

RESULTS AND DISCUSSION
Gross photosynthesis rates within the tissue showed a maximum at 15 µm depth and decreased for the deeper parts of the tissue. However, no significant differences were observed for gross photosynthesis calculated from the individual measurements within the tissue and, the measurement done on the surface of the tissue multiplied by the tissue thickness (50 µm coral tissue thickness). This indicates that gross photosynthesis rates can be estimated from measurements at the coral surface. Therefore, subsequent measurements were taken only at the coral surface. Coral of both species maintained at 30°C for one week completely bleached, with zooxanthellae density in P. heronensis showing a higher decrease compared to A. solitaryensis. The gross photosynthesis rate and net photosynthesis rate increased with increasing light intensity in low light intensity, but it reaches a plateau near the maximum light intensity. Y(II) decreased at high light intensities, indicating photoinhibition which could explain the plateau in photosynthetic rates. Under heat stress gross photosynthesis rate decreased and Y(II) decreased compared to under control conditions. The decrease in Y(II) under light and heat stress suggests that photoinhibition may contribute to a reduction in ROS production. However, the simultaneous decrease in zooxanthellae densities indicates that corals use both protective mechanisms.