Inorganic carbon physiology underpins macroalgal responses to elevated CO₂

Beneficial effects of CO₂ 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 CO₂ are correlated with inorganic carbon physiology. We assessed abundance patterns and a proxy for CO₂:HCO₃^- use (δ¹³C values) of macroalgae along a gradient of CO₂ 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 HCO₃^- and CO₂ had greater CO₂ use as concentrations increased. These species (and one unable to use HCO₃^-) increased in abundance with elevated CO₂ whereas obligate calcifying species, and non-calcareous macroalgae whose CO₂ 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.