Photosynthetic Carbon Isotope Fractionation of the Marine Dinoflagellate Alexandrium tamarense: A Chemostat Investigation of Taxonomic and Physiological Controls on the Stable Carbon Isotope Record

Abstract:

Interpretations of stable carbon isotope excursions in the sedimentary record are strengthened by laboratory culture studies investigating the photosynthetic carbon isotope fractionation (ɛ_p) of marine phytoplankton taxa with long geological records. These studies are essential for understanding organic matter δ¹³C signals in terms of environmental changes (e.g., atmospheric pCO₂ and nutrient availability) or taxonomic changes (e.g., algal species succession and community composition).

Dinoflagellates are among the most widespread and ecologically dominant primary producers in modern oceans and throughout the Mesozoic and Cenozoic. Compared to more recently evolved phytoplankton taxa, however, dinoflagellate carbon isotope fractionation has received relatively little mechanistic study. Several dilute batch culture experiments with dinoflagellates have investigated ɛ_p as a function of CO₂ availability, but the influences of changing growth rates, nutrient limitation, pH, and irradiance require further systematic exploration. We investigated stable carbon isotope fractionation in the marine dinoflagellate Alexandrium tamarense under nitrate-limited conditions in a chemostat culture system in which full DIC system parameters, including the concentration and δ¹³C value of CO₂, were determined. Growth rates were varied between experiments, and cells were grown under continuous light. Previously reported ɛ_p values for seven dinoflagellate species including A. tamarense ranged from approximately -1 to 14‰ in nutrient-replete batch culture studies ([CO₂] = 0-50 µmol kg^‑1). In contrast, in chemostat conditions we measured ɛ_p values on the order of 20‰ ([CO₂] = 20-30 µmol kg^-1). These experiments provide an initial step toward understanding the physiological controls on ɛ_p in dinoflagellates and illuminating the role of algal taxonomy in shaping the Phanerozoic stable carbon isotope record.