All species of coccolithophore appear to respond to perturbations of carbonate chemistry in a different way. Here, we show that the degree of malformation, growth rate and stable isotopic composition of organic matter and carbonate produced by two contrasting species of coccolithophore (<i>Gephyrocapsa oceanica</i> and <i>Coccolithus pelagicus ssp. braarudii</i>) are indicative of differences between their photosynthetic and calcification response to changing DIC levels (ranging from ~1100 to ~7800 μmol kg<sup>−1</sup>) at constant pH (8.13 ± 0.02). <i>Gephyrocapsa oceanica</i> thrived under all conditions of DIC, showing evidence of increased growth rates at higher DIC, but <i>C. braarudii</i> was detrimentally affected at high DIC showing signs of malformation, and decreased growth rates. The carbon isotopic fractionation into organic matter and the coccoliths suggests that <i>C. braarudii</i> utilises a common internal pool of carbon for calcification and photosynthesis but <i>G. oceanica</i> relies on independent supplies for each process. All coccolithophores appear to utilize bicarbonate as their ultimate source of carbon for calcification resulting in the release of a proton. But, we suggest that this proton can be harnessed to enhance the supply of CO<sub>2</sub>(aq) for photosynthesis either from a large internal HCO<sub>3</sub><sup>-</sup> pool which acts as a pH buffer (<i>C. braarudii</i>), or pumped externally to aid the diffusive supply of CO<sub>2</sub> across the membrane from the abundant HCO<sub>3</sub><sup>-</sup> (<i>G. oceanica</i>), likely mediated by an internal and external carbonic anhydrase respectively. Our simplified hypothetical spectrum of physiologies may provide a context to understand different species response to changing pH and DIC, the species-specific ε<sub>p</sub> and calcite "vital effects", as well as accounting for geological trends in coccolithophore cell size.