Carbon isotope fractionation by marine phytoplankton in culture: The effects of CO₂ concentration, pH, temperature, and species

Closed cultures of marine phytoplankton were established under variable conditions of CO₂ concentration, temperature, growth rate (by light limitation), and pH (but with nearly identical [CO₂aq]) in order to assess the relative influence of these variables on the extent of carbon isotope fractionation relative to dissolved inorganic carbon sources. Culture biomass was not allowed to increase beyond levels that would significantly affect the dissolved carbon system in the closed cultures. In experiments with Skeletonema costatum and Emiliania huxleyi, increasing CO₂ concentrations led to increased carbon isotope discrimination (resulting in organic matter progressively depleted in δ¹³C, i.e., a greater, more negative ϵ_p). ϵ_p values for E. huxleyi were 8–10‰ less than for S. costatum under identical conditions. For the S. costatum cultures, there was nearly a 20 ‰ range in [CO₂aq]‐dependent ϵ_p. The effect was nonlinear with a leveling off at high [CO₂aq]. Over a pH range of 7.5–8.3 but at a constant [CO₂aq] there was a variation in carbon isotope fractionation by S. costatum of about 9 ‰ with a minimum at pH 7.8–7.9. There was a temperature effect of ∼8‰ on fractionation even after equilibrium temperature dependency of δ¹³C of CO₂aq was taken into account. No growth rate effect was found for S. costatum over a modest range of growth rates. Culture experiments used to determine the carbon isotope fractionation by phytoplankton species must be conducted under well‐defined conditions of temperature, pH, and CO₂ concentrations. Hindcasts of ancient atmospheric pCO₂ from measurements of δ¹³C of organic carbon in marine sediments will require careful calibration because of the variety of possible factors that influence δ¹³Corg.