Inorganic carbon acquisition and isotope fractionation of marine phytoplankton with emphasis on the coccolithophore Emiliania huxleyi

Abstract

This thesis investigates inorganic carbon acquisition and isotope fractionation of marine phytoplankton with emphasis on the calcifying coccolithophore Emiliania huxleyi. In dilute batch culture experiments with E. huxleyi a strong CO2 dependence on the ratio of particulate inorganic carbon (PIC) to particulate organic carbon (POC) was observed. The decrease in the PIC/POC ratio with increasing CO2 concentration is caused by stimulation in photosynthesis and constant or decreasing rates in calcification. Isotope fractionation (ep) showed a low sensitivity to CO2 concentration. Carbon specific growth rates and PFD positively correlated with ep. Moreover, a L:D cycle of 16:8h resulted in lower ep values compared to continuous light. These responses are best explained by invoking active carbon acquisition in E. huxleyi. The mechanisms of carbon acquisition were investigated in E. huxleyi, the diatom Skeletonema costatum and the flagellate Phaeocystis globosa by membrane-inlet mass spectrometric techniques. In vivo activities of carbonic anhydrase (CA), photosynthetic O2 evolution, CO2 and HCO3- uptake rates were measured in cells acclimated to different pCO2 levels. While half-saturation concentrations for O2 evolution and the light-stimulation in CA activity indicate a carbon concentrating mechanism (CCM) in all three species, large differences were obtained with regard to the efficiency and regulation of their CCMs. Large changes in CCM regulation were also caused by different photoperiods. Rates of photosynthesis doubled under L:D cycles compared to continuous light, an effect that was often accompanied by a higher contribution of HCO3- uptake. These results indicate that carbon acquisition plays a larger role in phytoplankton productivity and ecology than previously recognized. In view of the observed taxon-specific differences in carbon acquisition CO2-related changes in seawater chemistry are expected to modify phytoplankton species succession and distribution.