Carbon isotope fractionation by marine phytoplankton in culture: The effects of CO₂ concentration, <i>p</i>H, temperature, and species

Abstract

Closed cultures of marine phytoplankton were established under variable conditions of CO 2 concentration, temperature, growth rate (by light limitation), and p H (but with nearly identical [CO 2 aq]) 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 CO 2 concentrations led to increased carbon isotope discrimination (resulting in organic matter progressively depleted in δ 13 C, 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 [CO 2 aq]‐dependent ϵ p . The effect was nonlinear with a leveling off at high [CO 2 aq]. Over a p H range of 7.5–8.3 but at a constant [CO 2 aq] there was a variation in carbon isotope fractionation by S. costatum of about 9 ‰ with a minimum at p H 7.8–7.9. There was a temperature effect of ∼8‰ on fractionation even after equilibrium temperature dependency of δ 13 C of CO 2 aq 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, p H, and CO 2 concentrations. Hindcasts of ancient atmospheric p CO 2 from measurements of δ 13 C of organic carbon in marine sediments will require careful calibration because of the variety of possible factors that influence δ 13 Corg.