Résumé
The stable carbon isotope composition of particulate organic matter expressed as δ13C was measured in cultures of 13 species of marine microalgae in different phylogenetic groups. The effects of salinity variations and changes in photoperiod were also assayed for three of them (i.e. Skeletonema costatum, Amphidinium opercularum and Isochrysis galbana); the effect of nature of nitrogen supply (nitrate. ammonium) was studied for one (S.costatum). These environmental parameters were chosen because of their variability in the ocean and their possible effects on δ13C values of phytoplankton organic carbon. Batch culture conditions and sampling time after inoculum were strongly controlled in order to provide cells in good physiological state which were comparable from one culture to the other. In the same way, sampling was limited to the first 2 days of exponential growth, in order to avoid a possible dissolved inorganic carbon (DIC) limitation. Carboxylase activities [of the enzyme ribulose 1,5-bisphosphate carboxylase oxygenase (Rubisco), and the three β carboxylases: phosphoenolpyruvate carboxylase (PEPC), phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate carboxylase (PC)] and total chlorophyll a concentrations were assayed simultaneously. The δ13C values observed were between −30.2‰ and −12.7‰ i.e. comparable to those observed in the world's oceans. The isotopic composition of phytoplankton organic carbon was shown to be under the influence of the parameters tested but δ13C variations are specific to the species considered. The nature of β carboxylase found in each species, or systematic position, could not be linked to the isotopic composition of organic carbon. No linear or single correlation between δ13C variations and environmental modifications were observed and there is no evidence for a simple and universal relation between δ13C of phytoplankters and their environment. In monospecific cultures as in the field, δ13C fractionation by Rubisco (and eventually by PEPCK) may be counterbalanced by other mechanisms.