Carbon isotope fractionation by Emiliania huxleyi

The ratio of 12 C : 13 C was measured for Emiliania huxleyi cells grown under a range of irradiances in batch culture. Based on the assumption that HCO 3 was the carbon source, the maximum discrimination against 13 C in the decalcified E. huxleyi cells was 24.6‰ at 4.8 mol photons m −2 d −1 and the minimum discrimination was 17.9‰ at 0.5 mol photons m −2 d −1 . In calcified cells, the range of 13 C isotope discrimination was markedly lower (12–21‰). In both calcified and decalcified cells, the carbon isotopic composition of and carbon isotopic discrimination by E. huxleyi was a significant linear function of irradiance from 0.5 to 4.8 mol photons m −2 d −1 . The relationship between isotopic discrimination and irradiance in E. huxleyi provides new evidence that the 13 C of the oceanic POC samples should not be treated simply as a function of sea surface temperature or [CO 2 ] aq . We propose that the δ 13 C of E. huxleyi was controlled by irradiance through variation in the intracellular HCO 3 − ‐derived CO 2 supply which influenced the discrimination of ribulose 1,5‐bisphosphate carboxylase/oxygenase (rubisco) against 13 C.