Inorganic carbon physiology underpins macroalgal responses to elevated CO(2)

Beneficial effects of CO(2) on photosynthetic organisms will be a key driver of ecosystem change under ocean acidification. Predicting the responses of macroalgal species to ocean acidification is complex, but we demonstrate that the response of assemblages to elevated CO(2) are correlated with inorganic carbon physiology. We assessed abundance patterns and a proxy for CO(2):HCO(3)(−) use (δ(13)C values) of macroalgae along a gradient of CO(2) at a volcanic seep, and examined how shifts in species abundance at other Mediterranean seeps are related to macroalgal inorganic carbon physiology. Five macroalgal species capable of using both HCO(3)(−) and CO(2) had greater CO(2) use as concentrations increased. These species (and one unable to use HCO(3)(−)) increased in abundance with elevated CO(2) whereas obligate calcifying species, and non-calcareous macroalgae whose CO(2) use did not increase consistently with concentration, declined in abundance. Physiological groupings provide a mechanistic understanding that will aid us in determining which species will benefit from ocean acidification and why.