Stable carbon isotope fractionation of organic cyst-forming dinoflagellates: Evaluating the potential for a CO2 proxy

Abstract Over the past decades, significant progress has been made regarding the quantification and mechanistic understanding of stable carbon isotope fractionation ( 13 C fractionation) in photosynthetic unicellular organisms in response to changes in the partial pressure of atmospheric CO 2 ( p CO 2 ). However, hardly any data is available for organic cyst-forming dinoflagellates while this is an ecologically important group with a unique fossil record. We performed dilute batch experiments with four harmful dinoflagellate species known for their ability to form organic cysts: Alexandrium tamarense , Scrippsiella trochoidea , Gonyaulax spinifera and Protoceratium reticulatum . Cells were grown at a range of dissolved CO 2 concentrations characterizing past, modern and projected future values (∼5–50 μmol L −1 ), representing atmospheric p CO 2 of 180, 380, 800 and 1200 μatm. In all tested species, 13 C fractionation depends on CO 2 with a slope of up to 0.17‰ (μmol L) −1 . Even more consistent correlations were found between 13 C fractionation and the combined effects of particulate organic carbon quota (POC quota; pg C cell −1 ) and CO 2 . Carbon isotope fractionation as well as its response to CO 2 is species-specific. These results may be interpreted as a first step towards a proxy for past p CO 2 based on carbon isotope ratios of fossil organic dinoflagellate cysts. However, additional culture experiments focusing on environmental variables other than p CO 2 , physiological underpinning of the recorded response, testing for possible offsets in 13 C values between cells and cysts, as well as field calibration studies are required to establish a reliable proxy.