Exposure to a gradient of warming and acidification highlights physiological, molecular, and skeletal tolerance thresholds in Pocillopora acuta recruits

Ocean warming and acidification are among the biggest threats to the persistence of coral reefs. Organismal stress tolerance thresholds are life stage specific, can vary across levels of biological organization, and also depend on natural environmental variability. Here, we exposed the early life stages of Pocillopora acuta in Kāne‘ohe Bay, Hawai‘i, USA, a common reef-building coral throughout the Pacific, to projected ocean warming and acidification scenarios. We measured ecological, physiological, biomineralization, and molecular responses across the critical transition from larvae to newly settled recruits following 6 days of exposure to diel fluctuations in temperature and pH in Control (26.8-27.9°C, 7.82-7.96 pH _Total ), Mid (28.4-29.5°C, 7.65-7.79 pH _Total ) and High conditions (30.2-31.5°C, 7.44-7.59 pH _Total ). We found that P. acuta early life stages are capable of survival, settlement, and calcification under all scenarios. The High conditions, however, caused a significant reduction in survival and settlement capacity, with changes in the skeletal fiber deposition patterns. In contrast to a limited impact on the expression of biomineralization genes, the dominant transcriptomic response to the High conditions relative to the two other treatments included depressed metabolism, reduced ATP production and increased activity of DNA damage-repair processes. Collectively, our findings indicate that corals living in environments with large diurnal fluctuations in seawater temperature and pH, such as Kāne‘ohe Bay, can tolerate exposure to moderate projected increased temperature and reduced pH. However, under more severe environmental conditions significant negative effects on coral cellular metabolism and overall organismal survival jeopardize species fitness and recruitment.