Identification of double-bond positions in isomeric alkenones from a lacustrine haptophyte

Measurements of alkenone unsaturation ratios are widely used for paleotemperature reconstructions in ocean and lake environments. Previously, we reported the discovery of a series of tri-unsaturated alkenone positional isomers (Δ(14, 21, 28) ) from oligosaline and freshwater lakes in Greenland and Alaska. In this work we provide a detailed analysis of the structures and isotopic compositions (δ(13) C and δ(2) H) of the alkenones produced by the "Greenland haptophyte".

Alkenones were extracted from sediments of Lake BrayaSø, Greenland. Alkenone double-bond positions were determined by GC/EI-MS analysis of alkenone dimethyl disulfide and cyclobutylimine derivatives. Alkenones were purified by semi-preparative HPLC using a silver(I) thiolate stationary phase. Carbon and hydrogen isotope analysis was performed by gas chromatography/isotope ratio mass spectrometry (GC/IRMS).

A series of novel tri-unsaturated alkenone positional isomers were identified among four alkenone homologues (i.e. C37 Me , C38 Me , C38 Et , and C39 Et ) with double-bond positions at Δ(14, 21, 28) . The hydrogen isotope compositions (δ(2) H, VSMOW) of the tri-unsaturated positional isomers from C37 Me and C38 Et were slightly depleted (~ -11 ‰) relative to the common tri-unsaturated alkenone. The carbon isotope composition (δ(13) C, VPDB) of the tri-unsaturated positional isomers from the C37 Me , C38 Me , C38 Et , and C39 Et alkenones were significantly enriched (~ +4 ‰) relative to the common alkenones (di-, tri-, and tetra-unsaturated).

The novel tri-unsaturated alkenone positional isomers produced by the Greenland haptophyte possess Δ(14, 21, 28) double-bond positions, instead of the common Δ(7, 14, 21) double-bond positions. The hydrogen isotope values suggest the novel tri-unsaturated positional isomers could be biosynthetic precursors to the tetra-unsaturated alkenones (Δ(7, 14, 21, 28) ). However, the significantly higher carbon isotope values of the tri-unsaturated positional isomers relative to the common di-, tri- and tetra-unsaturated alkenones suggest these positional isomers may have different/additional biosynthetic precursors.