Photosynthetic Characteristics of the Coccolithophorid Emiliania Huxleyi (Prymnesiophyceae) Exposed to Elevated Concentrations of Dissolved Inorganic Carbon1

Light‐saturated photosynthesis (Pmax) of Emiliania huxleyi (Lohmann) Hay et Mohler is known to be carbonlimited at natural concentrations of dissolved inorganic carbon (DIC). In the present study, light‐limited and light‐saturated photosynthetic rates of E. huxleyi were studied at three concentrations of DIC (2.4, 7.4, and 12.4 mM) for high‐calcite (Cin/Ctot=0.48) and low‐calcite (Cin/Ctot=0.08) cells of the same strain. The photosynthetic efficiency (α) and the maximum quantum yield (θmax)A increased by more than a factor of 2 from the lowest to the highest DIC level. Pmax a, and θmax were always higher for the high‐calcite than for the low‐calcite cells at identical DIC levels. This may indicate that the calcifcation process acts as an extra supplier of CO2 for photosynthesis making the CO2 shortage at natural DIC levels a little smaller for high‐calcite than for low‐calcite E. huxleyi. A dependency of θmax on DIC has not previously been shown for marine phytoplankton. θmax is a key parameter in recent biooptical models of phytoplankton productivity, and the results from the present study are therefore important for modeling the productivity of E. huxleyi.