Photosynthesis and Calcification by Emiliania huxleyi (Prymnesiophyceae) as a Function of Inorganic Carbon Species

To test the possibility of inorganic carbon limitation of the marine unicellular alga Emiliania huxleyi (Lohmann) Hay and Mohler, its carbon acquisition was measured as a function of the different chemical species of inorganic carbon present in the medium. Because these different species are interdependent and covary in any experiment in which the speciation is changed, a set of experiments was performed to produce a multidimensional carbon uptake scheme for photosynthesis and calcification. This scheme shows that CO2 that is used for photosynthesis comes from two sources. The CO2 in seawater supports a modest rate of photosynthesis. The HCO3- is the major substrate for photosynthesis by intracellular production of CO2 (HCO3- + H+ → CO2 + H2O → CH2O + O2). This use of HCO3- is possible because of the simultaneous calcification using a second HCO3- , which provides the required proton (HCO3- + Ca2+ → CaCO3 + H+). The HCO3- is the only substrate for calcification. By distinguishing the two sources of CO2 used in photosynthesis, it was shown that E. huxleyi has a K½ for external CO2 of ‘‘only’’ 1.9 ± 0.5 µM (and a Vmax of 2.4 ± 0.1 pmol·cell-1·d-1). Thus, in seawater that is in equilibrium with the atmosphere ([CO2] = 14 µM, [HCO3-] = 1920 µM, at fCO2 = 360 µatm, pH = 8, T = 15° C), photosynthesis is 90% saturated with external CO2. Under the same conditions, the rate of photosynthesis is doubled by the calcification route of CO2 supply (from 2.1 to 4.5 pmol·cell-1·d-1). However, photosynthesis is not fully saturated, as calcification has a K½ for HCO3- of 3256 ± 1402 µM and a Vmax of 6.4 ± 1.8 pmol·cell-1·d-1. The H+ that is produced during calcification is used with an efficiency of 0.97 ± 0.08, leading to the conclusion that it is used intracellularly. A maximum efficiency of 0.88 can be expected, as NO3- uptake generates a H+ sink (OH- source) for the cell. The success of E. huxleyi as a coccolithophorid may be related to the efficient coupling between H+ generation in calcification and CO2 fixation in photosynthesis.