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 Pacific were examined using an isothermal batch culture system. In both E. huxleyi and G. oceanica the unsaturation index U37K′ changed during the growth period, but the effects of this change were different. 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 U37K′. Changes of U37K′ 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 fluidity. The degree of unsaturation both in alkenones and n-fatty 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 effectively distinguished using the U37K′-U38EtK diagram.