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

[ACG41-02] Effects of OA on the warm temperate coral species Acropora solitaryensis under different light regimes

*Iijima Layla¹, Joshua Heitzman¹, Guinther Hiromu Mitushasi¹, Shigeki Wada¹, Agostini Sylvain¹ (1.Shimoda Marine Research Center, University of Tsukuba)

Keywords:Ocean acidification, Light availability, Warm temperate, High latitude, Poleward expansion, Acropora solitaryensis

Rising seawater temperatures due to climate change is leading to the poleward expansion of hermatypic coral species. Increased atmospheric CO₂ concentration drives ocean acidification (OA), the decrease in oceanic pH resulting from CO₂ being absorbed into oceans. As the effects of OA are more pronounced towards the poles, poleward-expanding corals may be more rapidly affected. Furthermore, higher latitudes tend to have less light availability in comparison to tropical and subtropical coral reefs, which may decrease coral habitable areas at high latitudes. Ocean acidification increases the energetic cost of calcification, and as corals relies on the energetic input of photosynthesis from their symbiont, lower light level could further enhance the negative effects of OA. Therefore, we investigated the interaction of future pH conditions with different light conditions, factors which could determine the potential of high latitudes as a coral refugia. Colonies of the warm-temperate coral species Acropora solitaryensis were sampled at Shikine island (Tokyo Prefecture, Japan, 34° 19' 34" N 139° 12' 36" E). Each colony was fragmented into four microcolonies and acclimatized prior to the experiment for two weeks in the experimental tanks with their respective light levels and flowing seawater. Four experimental treatments were chosen by fully crossing two pH (Present day, 8.1 pH; IPCC SSP3-7.0 scenario, 7.85 pH) and two light conditions (high light, 5 m depth, 8 mol photons m^-2 d^-1; low light, 15 m depth, 3 mol photos m^-2 d^-1). Coral host and symbiont physiology and metabolism were assessed to understand how the energy acquisition pathways of A. solitaryensis change under these stressors. Our results showed that low light conditions led to decreased metabolism (photosynthesis and respiration) and growth (skeletal growth and calcification). The only significant difference observed in OA conditions regardless of light condition, was observed for the symbiont maximum photosystem II efficiency (F_vF_m), which increased under OA (p = 0.012). Corals reared under OA and low light availability showed an increase, albeit non-significant, in photosynthetic activity, host protein, ETSA and net calcification compared to corals reared under present day pH and low light, and were comparable to present day pH and high light. Although the combined effects of OA and decreased light availability could negatively affect coral energetic homeostasis, OA had either a neutral or positive effect on colonies reared in low-light conditions. Autotrophs like zooxanthellae (coral symbiont) and macroalgae that have poor carbon concentration mechanisms may benefit from OA in the form of CO₂ enriched seawater. In conclusion, the combined effects of OA and low light may not have a compounding effect on the coral A. solitaryensis, suggesting this species may not have a limited habitable depth range at higher latitudes.