Effects of growth rate, CO2 concentration, and cell size on the stable carbon isotope fractionation in marine phytoplankton

Stable carbon isotope fractionation (epsilon(p)) was measured in four marin e diatom and one dinoflagellate species of different cell sizes. Monospecif ic cultures were incubated under high-light and nutrient-replete conditions at 16 h : 8 h and 24 h : 0 h light/dark cycles in dilute batch cultures at six CO2 concentrations, [CO2,aq], ranging from ca. 1 to 38 mu mol kg(-1). In all species, epsilon(p) increased with increasing [CO2,aq]. Among the di atoms, the degree of CO2-related variability in epsilon(p) was inversely co rrelated with cell size. Isotopic fractionation in the dinoflagellate diffe red in several aspects from that of the diatoms, which may reflect both mor phological and physiological differences between taxa. Daylength-related ch anges in instantaneous growth rate, defined as the rate of C assimilation d uring the photoperiod, affected epsilon(p) to a similar or greater extent t han differences in experimental [CO2,aq] in three of the species tested. In contrast, the irradiance cycle had no effect on epsilon(p) in 2 other spec ies. With the exception of Phaeodactylum tricornutum, growth rate of all sp ecies declined below a critical [CO2,aq]. At these concentrations, we obser ved a reversal in the CO2-related epsilon(p) trend, which we attribute to a decline in carbon assimilation efficiency. Although uncatalyzed passive di ffusion of CO2 into the cell was sufficient to account for gross carbon upt ake in most treatments, our results indicate that other processes contribut e to inorganic carbon acquisition in all species even at [CO2,aq] > 10 mu m ol kg(-1). These processes may include active C transport and/or catalyzed conversion of HCO3- to CO2 by carbonic anhydrase. A comparison of ur result s with data from the literature indicates significant deviations from previ ously reported correlations between epsilon(p) and mu/[CO2,aq], even when d ifferences in cellular carbon content and cell geometry are accounted for. Copyright (C) 1999 Elsevier Science Ltd.