Isotopic fractionation during carbon acquisition in dinoflagellates: a new proxy for pCO₂?

Atmospheric CO₂ concentrations (pCO₂) have markedly changed over geological timescales, thereby considerably influencing Earth’s climate and ecosystems. Reconstructing pCO₂ is therefore one of the major challenges for the scientific community today. This thesis aims at investigating if past pCO₂ can be reconstructed based on the carbon isotopic composition of dinoflagellates. In order to do so, experiments were carried out with four dinoflagellate species and correlations between the carbon isotopic composition in the dinoflagellates and the CO₂ concentrations in the growth medium were determined. Next to CO₂, nitrogen concentration and light availabilities were modified and thereby their influences on the carbon isotopic fractionation were tested. Furthermore, inorganic carbon fluxes were measured by means of membrane inlet mass spectrometry. Such in vivo assays allowed for estimating the preferred inorganic carbon source (i.e. bicarbonate or CO₂) and the degree of CO₂ efflux/leakage during photosynthesis, both key determinants for carbon isotope fractionation. My results show a general positive correlation of dinoflagellate carbon isotopic fractionation with CO₂ availability. In addition to CO₂, carbon isotopic fractionation was found to be affected by growth rates and particulate organic carbon quotas. Accounting for these growth parameters improved the correlations significantly in most species. However, these correlations were found for dinoflagellates grown under replete conditions. Reduced light levels showed comparable CO₂ dependencies, but increased the overall carbon isotopic fractionation in one species, while under nitrogen limitation no CO₂ dependency was observed. Results of in vivo assays showed that carbon isotopic composition in dinoflagellates was influenced by CO₂ because increased CO₂ availability decreased the relative uptake rates of bicarbonate, i.e. the inorganic carbon species containing more ¹³C compared to CO₂, and increased leakage, i.e. the amount of CO₂2 diffusing out of the cell in relation to total inorganic carbon uptake, thereby preventing the accumulation of ¹³C. Results of this thesis are promising with respect to proxy development. However, measuring CO₂ dependencies of the carbon isotopic composition in dinoflagellates under higher CO₂ concentrations than tested here, and estimating the potential influence of other environmental factors than nitrogen and light, besides CO₂, are important next steps. In addition, it remains to be tested whether there is an isotopic offset between the carbon isotopic composition of the dinoflagellate cells and the dinoflagellate cysts. Such experiments could not be performed within the frame of this thesis but should certainly be considered in future work to allow an eventual application of the proxy.