Isotopic fractionation of carbon in the coccolithophorid Emiliania huxleyi

Relating atmospheric CO₂ to δ13C of calcifying phytoplankton is often used as a proxy to reconstruct paleo-CO₂. Therefore, a firm undertanding of how living cells fractionate carbon under different environmental conditions (known as vital effects) is necessary when interpreting δ13C values. In this study, we measured the isotopic fractionation of carbon in organic matter (εp) in the globally distributed, bloom-forming coccolithophorid Emiliania huxleyi grown in continuous culture under nutrient-replete conditions with growth rate limited by light or temperature. At a constant temperature of 18°C, growth followed a hyperbolic function of irradiance. At low irradiance levels, changes in εp were highly correlated with growth rate. However, as growth became light-saturated, εp declined with increasing light intensities. When temperature was increased from 7 to 18°C at a constant photon flux density, equilibrium partial pressure CO₂ concentrations ([pCO₂]) decreased from 17 to 13 μM, and εp values declined from 25 to 19‰. As temperature was increased further to 26°C, [pCO₂] declined to 10 μM and εp increased to 25‰. This non-linear pattern in isotopic fractionation is consistent with the induction of a carbon-concentrating mechanism at low [pCO₂] that replenishes the internal inorganic carbon pool with isotopically lighter carbon. In this study, we present an empirical model that predicts this non-linear behavior, and we validate this model with experimental data. These results suggest extreme variability in the isotopic fractionation of carbon in the bulk organic pool in E. huxleyi that precludes the reconstruction of pCO₂ from isotopic measurements without a priori knowledge of temperature.