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 unicel lular alga Emiliania huxleyi (Lohmann) Hay and Mohler, its carbon acquisiti on was measured as a function of the different chemical species of inorgani c carbon present in the medium. Because these different species are interde pendent and covary in any experiment in which the speciation is changed, a set of experiments was performed to produce a multidimensional carbon uptak e scheme for photosynthesis and calcification. This scheme shows that CO2 t hat 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 + O-2). This use of HCO3- is possible because of the simultan eous calcification using a second HCO3-, which provides the required proton (HCO3- + Ca2+ --> CaCO3 + H+). The HCO3- is the only substrate for calcifi cation. By distinguishing the two sources of CO2 used in photosynthesis, it was shown that E. huxleyi has a K-1/2 for external CO2 of "only" 1.9 +/- 0 .5 mu M (and a V-max of 2.4 +/- 0.1 pmol.cell(-1).d(-1)). Thus, in seawater that is in equilibrium with the atmosphere ([CO2] = 14 mu M, [HCO3-] = 192 0 mu M, at fCO(2) = 360 mu atm, pH = 8, T = 15 degrees C), photosynthesis i s 90% saturated with external CO2. Under the same conditions, the rate of p hotosynthesis 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 saturate d, as calcification has a K-1/2 for HCO3- of 3256 +/- 1402 mu M and a V-max of 6.4 +/- 1.8 pmol.cell(-1).d(-1). The H+ that is produced during calcifi cation is used with an efficiency of 0.97 +/- 0.08, leading to the conclusi on that it is used intracellularly. A maximum efficiency of 0.88 can be exp ected, as NO3- uptake generates a H+ sink (OH- source) for the cell. The su ccess of E. huxleyi as a coccolithophorid may be related to the efficient c oupling between H+ generation in calcification and CO2 fixation in photosyn thesis.