Inorganic carbon physiology underpins macroalgal responses to elevated CO2

Beneficial effects of CO2on 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 CO2are correlated with inorganic carbon physiology. We assessed abundance patterns and a proxy for CO2:HCO3−use (δ13C values) of macroalgae along a gradient of CO2at 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 CO2had greater CO2use as concentrations increased. These species (and one unable to use HCO3−) increased in abundance with elevated CO2whereas obligate calcifying species, and non-calcareous macroalgae whose CO2use 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.