Changes in alkenone and alkenoate distributions during acclimatization to salinity change in Isochrysis galbana: Implication for alkenone-based paleosalinity and paleothermometry

Abstract

A cultured strain of Isochrysis galbana UTEX LB 2307 was grown at 20°C and 15°C under salinity of 35‰, 32‰, 27‰, 20‰, and 15‰, and analyzed for long chain (C37–C39) alkenones and (C37–C38) alkyl alkenoates. It was characterized by abundant C37 alkenones and C38 ethyl alkenoates (fatty acid ethyl ester: FAEEs) and a lack of C38 methyl alkenones. There were no tetra-unsaturated (C37:4) alkenones, which are frequently found in natural samples from low salinity waters. The alkenone unsaturation index (U37) did not vary in response to change in salinity at 15°C. The alkenone unsaturation index (U37) clearly changed in response to change in salinity at 20°C, but not at 15°C, where algal growth was and was not limited by temperature at 15°C and 20°C, respectively. The U37-temperature calibration for I. galbana UTEX LB 2307 is quite different from those of E. huxleyi and another strain of I. galbana (CCMP1323), while is resemble to that reported for C. lamellosa isolated from a Chinese lake. Also, variation in the alkenone chain length ratio values (K37/K38) did not correlate with salinity. These results implied that a high abundance of tetra-unsaturated alkenone and high K37/K38 values might be attributed to a taxonomic factor rather than a physiological response to salinity change. Interestingly, our culture experiments showed that the ethyl alkenoates/alkenones ratio (EE/K37) correlates with salinity. Hence, it is suggested that the EE/K37 ratios are affected by the cellular and physiological factors against salinity condition in single haptophyte cells.