Process | E. huxleyi | C. pelagicus |

Ca^{2+} transport | 3% (CV pH of 8) to 20% (CV pH of 7.5)* | ≫20%^{†} |

HCO_{3}^{−} transport | 5%^{‡} | Undocumented but expected to be significant to sustain high PIC production rate |

H^{+} (removal) transport | <5%^{§} | 5%* |

Polysaccharide generation | 7% | 0.2% |

Total | 20–37% | ≫25% |

*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 HCO_{3}^{−} 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 HCO_{3}^{−}, ΔμHCO_{3}^{−} = *RT*ln*Co*/*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 HCO_{3}^{−}, and *V* is the membrane potential (measured at −50 mV); a 10-fold HCO_{3}^{−} concentration gradient across the membrane corresponds to ΔμHCO_{3}^{−} ~ 10 kJ/mol. Considering that 1 mol of adenosine triphosphate (ATP) provides ~ 50 kJ per mole of energy for transport, moving 1 mol of HCO_{3}^{−} against its electrochemical potential gradient then requires 0.2 ATP. Assuming a requirement of 3.2 ATP per mole for CO_{2} fixation and that 1 mol of transported HCO_{3}^{−} produces 1 mol of CO_{3}^{2−} and a 1:1 calcification/photosynthesis ratio, the cost of HCO_{3}^{−} transport in terms of ATP required to fix 1 mol of CO_{2} 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^{+}.