In this study we analyzed the physiological responses of coralline algae to ocean acidification (OA) and global warming, by exposing algal thalli of three species with contrasting photobiology and growth-form to reduced pH and elevated temperature. The analysis aimed to discern between direct and combined effects, while elucidating the role of light and photosynthesis inhibition in this response. We demonstrate the high sensitivity of coralline algae to photodamage under elevated temperature and its severe consequences on thallus photosynthesis and calcification rates. Moderate levels of light-stress, however, were maintained under reduced pH, resulting in no impact on algal photosynthesis, although moderate adverse effects on calcification rates were still observed. Accordingly, our results support the conclusion that global warming is a stronger threat to algal performance than OA, in particular in highly illuminated habitats such as coral reefs. We provide in this study a quantitative physiological model for the estimation of the impact of thermal-stress on coralline carbonate production, useful to foresee the impact of global warming on coralline contribution to reef carbon budgets, reef cementation, coral recruitment and the maintenance of reef biodiversity. This model, however, cannot yet account for the moderate physiological impact of low pH on coralline calcification.

In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2015) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation is 2016-05-26.

Supplement to: Vásquez-Elizondo, Román Manuel; Enríquez, Susana (2016): Coralline algal physiology is more adversely affected by elevated temperature than reduced pH. Scientific Reports, 6, 19030