C38 Ethyl Alkenoates as New Group II Isochrysidales Proxy for Detecting Mixed Alkenone Productions in Ocean Sediments

Abstract:

Alkenones are polyunsaturated long chain methyl and ethyl ketones produced by Isochrysidales, an order of haptophyte algae. Based on phylogenetic data, members of Isochrysidales have been classified into three groups, with each group showing significant differences in temperature calibrations and preferred growth environments. Alkenones have been widely used for paleo-sea surface temperature (SST) reconstructions in the past 40 years. However, such reconstructions remain challenging in coastal areas and high latitude ocean areas due to the potential mixed alkenone productions by Group II (coastal species) and Group III (marine species) Isochrysidales. Therefore, identification of mixed alkenone productions in sediments is important for assessing corresponding reconstruction results. However, there are currently no definitive biomarkers to indicate the alkenone contribution from Group II Isochrysidales for ocean sediments where the dominant alkenone producers belong to Group III Isochrysidales. Here, we systematically examine samples from cultures of Group II (Isochrysis nuda, Isochrysis litoralis, Ruttnera lamellosa, Isochrysis galbana, Tisochrysis lutea and one strain isolated from sea ice in Baffin Bay) and Group III (Emiliania huxleyi and Gephyrocapsa oceanica) Isochrysidales and environmental samples of Group I Isochrysidales. For the first time, we report the production of C₃₇ ethyl ester (C₃₇OEt) and C₃₈ ethyl ester (C₃₈OEt) (also called alkenoates) with extended chain lengths by Isochrysidales. While C₃₇OEt alkenoates could be found in all culture samples of Group II and Group III Isochrysidales, C₃₈OEt alkenoates are only found in Group II Isochrysidales. Our results indicate that C₃₈OEt alkenoates are specific biomarkers for Group II Isochrysidales, and could be easily detected with gas chromatography-mass spectrometry (GC-MS) under single ion monitoring (SIM) mode after hydrogenating corresponding samples. Importantly, the Group II strain isolated from ice has the highest production of C₃₈OEt alkenoates (percentage of C₃₈OEt/C₃₇Me is ~10 times higher than that of other Group II samples), allowing easier identification of its contribution to alkenone profiles of high latitude ocean sediments and subsequent study of environmental changes (e.g., temperature, sea ice).