Acidification, not carbonation, is the major regulator of carbon fluxes in the coccolithophore Emiliania huxleyi

A combined increase in seawater [CO(2)] and [H(+)] was recently shown to induce a shift from photosynthetic HCO(3) (−) to CO(2) uptake in Emiliania huxleyi. This shift occurred within minutes, whereas acclimation to ocean acidification (OA) did not affect the carbon source. To identify the driver of this shift, we exposed low‐ and high‐light acclimated E. huxleyi to a matrix of two levels of dissolved inorganic carbon (1400, 2800 μmol kg(−1)) and pH (8.15, 7.85) and directly measured cellular O(2), CO(2) and HCO(3) (−) fluxes under these conditions. Exposure to increased [CO(2)] had little effect on the photosynthetic fluxes, whereas increased [H(+)] led to a significant decline in HCO(3) (−) uptake. Low‐light acclimated cells overcompensated for the inhibition of HCO(3) (−) uptake by increasing CO(2) uptake. High‐light acclimated cells, relying on higher proportions of HCO(3) (−) uptake, could not increase CO(2) uptake and photosynthetic O(2) evolution consequently became carbon‐limited. These regulations indicate that OA responses in photosynthesis are caused by [H(+)] rather than by [CO(2)]. The impaired HCO(3) (−) uptake also provides a mechanistic explanation for lowered calcification under OA. Moreover, it explains the OA‐dependent decrease in photosynthesis observed in high‐light grown phytoplankton.