Inorganic carbon physiology underpins macroalgal responses to elevated CO ₂

Abstract

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 ¹³C 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.