CARBON-ISOTOPE FRACTIONATION BY A MARINE DIATOM - THE INFLUENCE OF IRRADIANCE, DAYLENGTH, PH, AND NITROGEN-SOURCE

We examined the carbon-isotope composition of Thalassiosira pseudonana grown under different combinations of irradiance and daylength with nitrate or ammonium as the nitrogen source. The relationship between the concentration of dissolved inorganic carbon and delta(13)C of T. pseudonana strongly suggested that HCO, was the major source of carbon for growth regardless of pH or nitrogen species. The discrimination against C-13 (epsilon(p)) was significantly influenced by daylength, irradiance, and the interaction of daylength with irradiance. Discrimination was lower when daylengths were 16:8 and 20:4 L/D. Discrimination was minimal at irradiances of 25-50 mu mol photons m(-2) s(-1). Natural variation in the concentration of CO2(aq) seems unlikely to influence the delta(13)C of T. pseudonana, whereas daylength and irradiance probably influence the delta(13)C of natural samples of particulate organic matter. We suggest that geochemical models that use delta(13)C measurements from sedimented material to hindcast the partial pressure of CO2 may need reevaluation.