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

• Published in: Rapid Communications in Mass Spectrometry
• Main Authors: Dillon, James T., Longo, William M., Zhang, Yifan, Torozo, Rafael, Huang, Yongsong
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2015
• Subjects: Greenland; Alaska
• Online Access: http://dx.doi.org/10.1002/rcm.7414; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Frcm.7414; http://onlinelibrary.wiley.com/wol1/doi/10.1002/rcm.7414/fullpdf

Rationale 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”. Methods 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). Results A series of novel tri‐unsaturated alkenone positional isomers were identified among four alkenone homologues (i.e. C 37 Me , C 38 Me , C 38 Et , and C 39 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 C 37 Me and C 38 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 C 37 Me , C 38 Me , C 38 Et , and C 39 Et alkenones were significantly enriched (~ +4 ‰) relative to the common alkenones (di‐, tri‐, and tetra‐unsaturated). Conclusions 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 ...