Figure 4.

Schematic illustration of the ocean acidification (OA)‐dependent regulations in external inorganic carbon (C_i) fluxes of diploid, low‐light acclimated (2N LL; a, b) and high‐light acclimated (2N HL; c, d) Emiliania huxleyi under acclimation light. Sizes of arrows with solid lines reflect the measured photosynthetic and respiratory fluxes of CO ₂ and HCO₃ ⁻. Sizes of arrows with dashed lines reflect estimated fluxes. (a) Low‐light acclimated cells mainly use external HCO₃ ⁻ as photosynthetic substrate under acclimation pH and achieve similar rates of calcification (Calc) and photosynthesis (PS). (b) When exposing low‐light acclimated cells to OA, cells increase CO ₂ uptake for photosynthesis, whereas HCO₃ ⁻ uptake is downscaled due to the increased H⁺ levels. If HCO₃ ⁻ fluxes into photosynthesis and calcification were downscaled to the same degree, photosynthesis would disproportionally increase over calcification. (c) High‐light acclimated cells perform higher rates of photosynthesis under acclimation light than low‐light acclimated cells. The increased C_i demand is covered by additional HCO₃ ⁻ uptake. (d) When exposing high‐light acclimated cells to OA, they are not able to increase CO ₂ uptake rates, but nevertheless experience the H⁺‐driven decrease in HCO₃ ⁻ uptake. As a consequence of the decreased overall C_i supply, photosynthesis and presumably calcification experience C_i shortage and thus decrease.