Abstract:
Stable carbon isotope fractionation (#p) was measured in four marine diatom and one dinoflagellate species of different cell sizes. Monospecific 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 μmol kg-1. In all species, #p increased with increasing [CO2,aq]. Among the diatoms, the degree of CO2-related variability in #p was inversely correlated with cell size. Isotopic fractionation in the dinoflagellate differed in several aspects from that of the diatoms, which may reflect both morphological and physiological differences between taxa. Daylength-related changes in instantaneous growth rate, defined as the rate of C assimilation during the photoperiod, affected #p to a similar or greater extent than differences in experimental [CO2,aq] in three of the species tested. In contrast, the irradiance cycle had no effect on #p in 2 other species. With the exception of Phaeodactylum tricornutum, growth rate of all species declined below a critical [CO2,aq]. At these concentrations, we observed a reversal in the CO2-related #p trend, which we attribute to a decline in carbon assimilation efficiency. Although uncatalyzed passive diffusion of CO2 into the cell was sufficient to account for gross carbon uptake in most treatments, our results indicate that other processes contribute to inorganic carbon acquisition in all species even at [CO2,aq] > 10 μmol kg-1. These processes may include active C transport and/or catalyzed conversion of HCO3- to CO2 by carbonic anhydrase. A comparison of our results with data from the literature indicates significant deviations from previously reported correlations between #p and μ/[CO2,aq], even when differences in cellular carbon content and cell geometry are accounted for.