INTERSPECIFIC VARIABILITY AND ENVIRONMENTAL INFLUENCE ON PARTICULATE ORGANIC-CARBON DELTA(13) IN CULTURED MARINE-PHYTOPLANKTON

The stable carbon isotope composition of particulate organic matter expressed as delta(13)C 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 operculatum 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 delta(13)C 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 beta carboxylases: phosphoenolpyruvate carboxylase (PEPC), phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate carboxylase (PC)] and total chlorophyll a concentrations were assayed simultaneously. The delta(13)C values observed were between -30.2 parts per thousand and -12.7 parts per thousand, 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 delta(13)C variations are specific to the species considered. The nature of beta 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 delta(13)C variations and environmental modifications were observed and there is no evidence for a simple and universal relation between delta(13)C of phytoplankters and their environment. In monospecific cultures as in the field, delta(13)C fractionation by Rubisco (and eventually by PEPCK) may be counterbalanced by other mechanisms.