Relating atmospheric CO 2 to δ 13 C of calcifying phytoplankton is often used as a proxy to reconstruct paleo-CO 2 . Therefore, a firm undertanding of how living cells fractionate carbon under different environmental conditions (known as vital effects) is necessary when interpreting δ 13 C values. In this study, we measured the isotopic fractionation of carbon in organic matter (ε p ) in the globally distributed, bloom-forming coccolithophorid Emiliania huxleyi grown in continuous culture under

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... 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 ε p were highly correlated with growth rate. However, as growth became light-saturated, ε p declined with increasing light intensities. When temperature was increased from 7 to 18°C at a constant photon flux density, equilibrium partial pressure CO 2 concentrations ([pCO 2 ]) decreased from 17 to 13 µM, and ε p values declined from 25 to 19 ‰. As temperature was increased further to 26°C, [pCO 2 ] declined to 10 µM and ε p increased to 25 ‰. This non-linear pattern in isotopic fractionation is consistent with the induction of a carbon-concentrating mechanism at low [pCO 2 ] 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 pCO 2 from isotopic measurements without a priori knowledge of temperature. KEY WORDS: Emiliania huxleyi · Calcifying phytoplankton · Isotopic fractionation of carbon · Vital effects · Paleo-CO 2 reconstruction · Temperature · Carbon-concentrating mechanism Resale or republication not permitted without written consent of the publisher