Table 1 Percentage of the total photosynthetic energy budget dedicated to components of calcification.

The budget is presented for two main coccolithophore species (E. huxleyi and C. pelagicus). PIC, particulate inorganic carbon.

ProcessE. huxleyiC. pelagicus
Ca2+ transport3% (CV pH of 8) to 20% (CV pH of 7.5)*≫20%
HCO3 transport5%Undocumented but expected to be significant to sustain high PIC production rate
H+ (removal) transport<5%§5%*
Polysaccharide generation7%0.2%

*Measured by Anning et al. (30).

†Estimated from E. huxleyi, assuming a 10-fold higher PIC production rate.

‡Because there is no direct measurement of HCO3 accumulation in the cytoplasm, we used measurement of total cellular dissolved inorganic carbon (DIC) by Sekino and Shiraiwa (31), which is equivalent to a 10-fold accumulation. Following the electrochemical potential gradient equation for HCO3, ΔμHCO3 = RTlnCo/Ci + zFV (in kilojoules per mole), where Δμ is the electrochemical potential gradient, R is the gas constant, F is the Faraday constant, z is the valency, T is the temperature, Co and Ci are the external and internal concentrations of HCO3, and V is the membrane potential (measured at −50 mV); a 10-fold HCO3 concentration gradient across the membrane corresponds to ΔμHCO3 ~ 10 kJ/mol. Considering that 1 mol of adenosine triphosphate (ATP) provides ~ 50 kJ per mole of energy for transport, moving 1 mol of HCO3 against its electrochemical potential gradient then requires 0.2 ATP. Assuming a requirement of 3.2 ATP per mole for CO2 fixation and that 1 mol of transported HCO3 produces 1 mol of CO32− and a 1:1 calcification/photosynthesis ratio, the cost of HCO3 transport in terms of ATP required to fix 1 mol of CO2 by photosynthesis is thus equal to 0.2/3.2 ~ 5%.

§Estimated from C. pelagicus, assuming a lower PIC production rate, resulting in lower generation of H+.