Sources of inorganic carbon for marine microalgal photosynthesis: A reassessment of δ13C data from batch culture studies of Thalassiosira pseudonana and Emiliania huxleyi

Abstract

A reevaluation of previously published analyses of stable carbon isotope fractionation by batch cultures of Thalassiosira pseudonana and Emiliania huxleyi indicates that the Rayleigh distillation model was used to model CO2 uptake incorrectly. Correct use of the model shows that the relationship between the δ13C of the particulate organic carbon (δ(p)) and the concentration of the dissolved inorganic carbon (DIC) at the time of harvest can be equally well described by a model assuming bicarbonate or CO2 uptake. The lack of a correlation between growth rate and δ(p) in the T. pseudonana results suggests that growth rate and the intracellular CO2 concentration are directly proportional. Theoretical considerations indicate that the T. pseudonana cultures started at a pH of 9.2 would have become CO2 limited before harvest and that this species must have the ability to utilize bicarbonate when CO2 becomes limiting. The similarity of the T. pseudonana δ(p) results from cultures started at pH 8.2 and 9.2 suggests that the form of DIC entering the cells was the same in both sets of experiments. The results are consistent with (1) uptake of bicarbonate or (2) uptake of CO2, with external carbonic anhydrasemediated conversion of bicarbonate to CO2 supplementing the uncatalyzed supply of CO2 when the latter becomes limiting. Analysis of the δ13C of both particulate organic carbon and coccolith carbon in the case of E. huxleyi suggests that the cells were taking up primarily bicarbonate at low growth rates, but that at high growth rates the DIC used for photosynthesis was derived almost entirely from uptake of CO2. The DIC utilized for coccolith formation seems to have been substantially diluted by isotopically light DIC derived from respiration at high growth rates.