SOURCES OF INORGANIC CARBON FOR MARINE MICROALGAL PHOTOSYNTHESIS - A REASSESSMENT OF DELTA-C-13 DATA FROM BATCH CULTURE STUDIES OF THALASSIOSIRA-PSEUDONANA AND EMILIANIA-HUXLEYI

A reevaluation of previously published analyses of stable carbon isoto pe fractionation by batch cultures of Thalassiosira pseudonana and Emi liania 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 delta(13)C of the particulate organic car bon (delta(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 delta(p) in the T. pseudonana results suggests that g rowth rate and the intracellular CO2 concentration are directly propor tional. Theoretical considerations indicate that the T. pseudonana cul tures started at a pH of 9.2 would have become CO2 limited before harv est and that this species must have the ability to utilize bicarbonate when CO2 becomes limiting. The similarity of the T. pseudonana delta( p) results from cultures started at pH 8.2 and 9.2 suggests that the f orm of DIC entering the cells was the same in both sets of experiments . The results are consistent with (1) uptake of bicarbonate or (2) upt ake of CO2, with external carbonic anhydrase-mediated conversion of bi carbonate to CO2 supplementing the uncatalyzed supply of CO2 when the latter becomes limiting. Analysis of the delta(13)C of both particulat e organic carbon and coccolith carbon in the case of E. huxleyi sugges ts that the cells were taking up primarily bicarbonate at low growth r ates, but that at high growth rates the DIC used for photosynthesis wa s derived almost entirely from uptake of CO2. The DIC utilized for coc colith formation seems to have been substantially diluted by isotopica lly light DIC derived from respiration at high growth rates.