Change of the unsaturation degree of alkenone and alkenoate during acclimation to salinity change in Emiliania huxleyi and Gephyrocapsa oceanica with reference to palaeosalinity indicator

Abstract

Laboratory cultured strains of Emiliania huxleyi EH2 (identical to NIES-837) and Gephyrocapsa oceanica GO1 (NIES-838) were grown at 20℃ under various salinity conditions ranging from 15‰ to 34‰, and were analyzed for long-chain (C₃₇-C₃₉) alkenones and (C₃₇-C₃₈) alkyl alkenoates. In both E. huxleyi EH2 and G. oceanica GO1, there were no tetra-unsaturated (C_37:4) alkenones, which were frequently identified at low salinity waters in literatures. The alkenone unsaturation index (U^k'₃₇) in E. huxleyi clearly changed under lower salinities such as 27‰ and 32‰, while the values in G. oceanica were almost constant in various salinities. From these results, the U^k'₃₇ variations affected by salinity should be paid attention in paleotemperature reconstruction from natural samples that E. huxleyi predominates, especially in lower saline environments. Interestingly, our culture experiments also showed that the alkenone chain-length ratio (K₃₇/K₃₈) for E. huxleyi and G. oceanica increased with decreasing salinities, although the ranges of these variations were small. These results suggest that the K₃₇/K₃₈ values were affected by the cellular and physiological factors in a single haptophyte cell, although this value varies mainly depending on the changes of haptophyte species and/or strains in natural environments. Hence, we suggest that the K₃₇/K₃₈ ratio can be more reliable as a paleosalinity indicator.