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 s ample of the NW Pacific were examined using an isothermal batch culture sys tem. In both E. huxleyi and G. oceanica the unsaturation index U-37(K') cha nged during the growth period, but the effects of this change were differen t. This suggests that genotypic variation rather than the adaptation of the strains to the geographical environment of the sampling location is a majo r factor iri determining the physiological responses to U-37(K'). Changes o f U-37(K') were associated with those of the unsaturation indices of C-38 a nd C-39 alkenones, the abundance ratios of lower to higher homologues of al kenones, 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 a lkenones and n-fatty acids increased at day 8, possibly due to nutrient dep letion. The ethyl alkenoate/total alkenone and ethyl alkenoate/C-37 alkenon e ratios increased abruptly at day 8 in both strains. These ratios should b e 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 U-37(K')-U-38Et(K) diagram. (C) 2000 Elsevier Science Ltd. All rights reserved.