The effects of varying CO2 concentration on lipid composition and carbon isotope fractionation in Emiliania huxleyi

We have measured the stable carbon isotopic composition of bulk organic matter (POC), alkenones, sterols, fatty acids, and phytol in the coccolithophorid Emiliania huxleyi grown in dilute batch cultures over a wide range of CO2 concentrations (1.1-53.5 mu mol L-1). The carbon isotope fractionation of POC (epsilon (POC)) varied by ca. 7 parts per thousand and was positively correlated with aqueous CO2 concentration [CO2aq]. While this result confirms general trends observed for the same alga grown in nitrogen-limited chemostat cultures, considerable differences were obtained in absolute values of epsilon (POC) and in the slope of the relationship of epsilon (POC) with growth rate and [CO2aq]. Also, a significantly greater offset was obtained between the delta C-13 of alkenones and bulk organic matter in this study compared with previous work (5.4, cf. 3.8 parts per thousand). This suggests that the magnitude of the isotope offset may depend on growth conditions. Relative to POC, individual fatty acids were depleted in C-13 by 2.3 parts per thousand to 4.1 parts per thousand, phytol was depleted in C-13 by 1.9 parts per thousand, and the major sterol 24-methylcholesta-5,22E-dien-3 beta -ol was depleted in C-13 by 8.5 parts per thousand. This large spread of delta C-13 values for different lipid classes in the same alga indicates the need for caution in organic geochemical studies when assigning different sources to lipids that might have delta C-13 values differing by just a few parts per thousand. Increases in [CO2aq] led to dramatic increases in the alkenone contents per cell and as a proportion of organic carbon, but there was no systematic effect on values of U-37(k') used for reconstructions of paleo sea surface temperature. Copyright (C) 2000 Elsevier Science Ltd.