Carbon isotope fractionation by Emiliania huxleyi

The ratio of l*C : 13C was measured for Emiliania huxleyi cells grown under a range of irradiances in batch culture. Based on the assumption that HCO, was the carbon source, the maximum discrimination against 13C in the decalcified E. huxleyi cells was 24.6?& at 4.8 mol photons m-* d-l and the minimum discrimination was 17.9?& at 0.5 mol photons m-* d-l. In calcified cells, the range of 13C isotope discrimination was markedly lower (12-217~). 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-* dI. The relationship between isotopic discrimination and irradiance in E. huxleyi provides new evidence that the 613C of the oceanic POC samples should not be treated simply as a function of sea surface temperature or [CO,],,. We propose that the P3C of E. huxleyi was controlled by irradiance through variation in the intracellular HC03-derived CO2 supply which influenced the discrimination of ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) against 13C. Stable carbon isotopes are increasingly used in research on a diverse range of topics, from ecosystem food-web studies (Fry and Wainright 199 1) to estimation of ancient carbon dioxide concentrations (Rau et al. 199 1). We still, however, have a poor understanding of the full range of factors which influence the 13C composition of phytoplankton and sedimentary and particulate organic C (POC). Phytoplankton studies have reported a wide range of 613C values for different species (Falkowski 199 l), and there have been indications that irradiance (Takahashi et al. 199 1; Thompson and Calvert 1994), temperature (Degens et al. 1968), growth rate (Fry and Wainright 199 l), cell leakiness (Badger et al. 1985), and carbon fixation pathway (Descolas-Gros and Fontugne 1985) may also influence isotopic carbon composition. The underlying mechanism for these responses is proposed to be (Berry 1989) the supply rate of COz to the C3 carboxylating enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco, EC 4.1.1.39). Given the increasing and widespread use of 6’ 3Cpoc, we believe that further research on environmental factors that may influence the carbon isotopic discrimination in marinc phytoplankton is crucial. Coccolithophorids in general and Emiliania huxleyi in particular are important components of the oceanic carbon cycle. E. huxleyi has a worldwide distribution, can form dense blooms at high latitudes, and seems likely to be the major source of the n-alkcnones found in ancient L Current address and address for reprint requests: CSIRODivision of Fisheries, P.O. Box 20, North Beach, 6020 WA,