Physiological responses of lipids in Emiliania huxleyi and Gephyrocapsa oceanica (Haptophyceae) to growth status and their implications for alkenone paleothermometry

The physiological responses of alkenone unsaturation indices to changes in growth status of E. huxleyi and G. oceanica strains isolated from a water sample of the NW Paci®c were examined using an isothermal batch culture system. In both E. huxleyi and G. oceanica the unsaturation index U37 K0 changed during the growth period, but the e€ects of this change were di€erent. This suggests that genotypic variation rather than the adaptation of the strains to the geographical environment of the sampling location is a major factor in determining the physiological responses to U37 K. Changes of U37 K0 were associated with those of the unsaturation indices of C38 and C39 alkenones, the abundance ratios of lower to higher homologues of alkenones, the abundance ratios of saturated to polyunsaturated n-fatty acids, the abundance ratio of ethyl alkenoate to alkenones, and sterol contents. These associations might be attributable to the physiological response of lipids for maintaining their ̄uidity. The degree of unsaturation both in alkenones and nfatty acids increased at day 8, possibly due to nutrient depletion. The ethyl alkenoate/total alkenone and ethyl alkenoate/C37 alkenone ratios increased abruptly at day 8 in both strains. These ratios should be useful in clarifying the relationship between the marine environment and its corresponding growth phase of batch culture. E. huxleyi and G. oceanica can be e€ectively distinguished using the U37 K-U38Et K diagram. # 2000 Elsevier Science Ltd. All rights reserved.