Relationship between aqueous CO2 concentrations and stable carbon isotope discrimination in the diatoms Chaetoceros calcitrans and Ditylum brightwellii

Recent work has focused on a possible relationship between marine organic δ13C and the concentration of carbon dioxide dissolved in seawater [CO2(aq)]. This relationship is based on the general assumption that diffusive CO2 uptake is the main pathway of photosynthetic carbon acquisition by phytoplankton. This study found an inverse linear relationship between [CO2(aq)] and organic δ13C in the marine diatoms Chaetoceios calcitrans and Ditylum brightwellii. However, the relationship was not a function of diffusive CO2 use by these diatoms, which have previously been shown to be HCO3– users. Our findings underline the importance of understanding the mechanisms of phytoplankton carbon acquisition in interpreting carbon isotope data.