Multispecies expression of coccolithophore vital effects with changing CO ₂ concentrations and pH in the laboratory with insights for reconstructing CO ₂ levels in geological history

Abstract. The coccolith sedimentary and micropalaeontological archive has fostered great interest in palaeoclimate applications. Indeed, the geochemistry of coccolith calcite has the potential to reconstruct both palaeo-CO2 concentrations and palaeo-temperature of seawater. Studying coccolith geochemistry aims at better understanding the changes in the vital effect of coccoliths with changes in environmental parameters, especially the carbonate chemistry of seawater. To this aim, we need to deconvolve the biological imprint from the environmental signals recorded in the composition of coccoliths. We have undertaken culture experiments of four coccolithophore strains with various sizes and growth rates, grown under eight CO2/pH conditions typifying the CO2 evolution of the Cenozoic era. We propose an assessment of the expression of the vital effects for Emiliania huxleyi, Gephyrocapsa oceanica, Helicosphaera carteri and Coccolithus braarudii with simultaneous changes in dissolved inorganic carbon (DIC) and pH in the medium resulting in variations in dissolved CO2 (CO2 aq) availability to the cells. We have identified a distinct isotopic response of C. braarudii to pCO2 levels on both sides of the 600 ppmv (pH 7.89) condition. We propose that this discrepancy is the result of a modification of the proton efflux across the plasma membrane through voltage-dependent proton channels. We further show that as the CO2 level rises and pH decreases (from 200 to 500 ppmv and from 8.29 to 7.96 pH units, respectively), a significant increase in δ13Ccoccolith of C. braarudii is expressed, along with a coeval decrease in δ13Corg. The constant physiological parameters of C. braarudii (growth rate, particulate inorganic carbon (PIC) and particulate organic carbon (POC)) across the 200 to 500 ppmv interval support the idea that the change in δ13Ccoccolith is the consequence of a lower fractionation between dissolved CO2 and organic matter. Meanwhile, the small cells of E. huxleyi and G. oceanica are less carbon-limited and do not exhibit any change in their carbon vital effects with changes in carbonate chemistry of the environment across the whole CO2 spectrum. Using this biogeochemical framework, we have established a calibration between CO2 aq concentration and the differential vital effect (Δδ13C) between isotopically invariant small G. oceanica and large coccoliths C. braarudii, whose vital effect is CO2-dependent at low CO2. The CO2–Δδ13C transfer equation allows palaeo-pCO2 reconstructions based on isotope changes explained by physiological processes, especially at low and medium CO2 levels.