Extended chain length alkenoates differentiate three Isochrysidales groups

Abstract

Abstract Alkenoates and alkenones are polyunsaturated long-chain esters and ketones produced by members of the Isochrysidales, an order of haptophyte microalgae. Based on phylogenetic data, members of Isochrysidales have been classified into three groups, with each group showing major differences in preferred growth environments (e.g., salinity). Systematic identification of chemotaxonomic differences among the three groups of Isochrysidales is thus important for paleoenvironmental reconstructions and disentangling mixed alkenone temperature signals. Here, we systematically examined alkenone/alkenoate profiles in culture samples of Group 2 and Group 3 Isochrysidales and environmental sample of Group1 Isochrysidales. Using GC columns with stationary phases of contrasting polarities, we discovered for the first time C37 and C38 ethyl alkenoates (C37OEt and C38OEt) in Group 2 and/or 3 Isochrysidales, extending the longest alkenoate chain length of C36OEt in previous publications. We show that Group 3 Isochrysidales only produce C37OEt, whereas Group 2 Isochrysidales produce both C37OEt and C38OEt alkenoates. Because Group 1 Isochrysidales do not produce alkenoates, C38OEt are specific biomarkers for Group 2 species. The identification of C37OEt and C38OEt in sediments from saline lakes and Chesapeake Bay where sequences of Group 2 DNA have been recovered further validates the use of C38OEt as a Group 2-specific biomarker.