title: Discovery of alkenones with variable methylene-interrupted double bonds: implications for the biosynthetic pathway.
abstract: Alkenones (C37-C40) are highly specific biomarkers produced by certain haptophyte algae in ocean and lacustrine environments and have been widely used for paleoclimate studies. Unusual shorter-chain alkenones (SCA; e.g., C35and C36) have been found in environmental and culture samples, but the origin and structure of these compounds are much less understood. The marine alkenone producer, Emiliania huxleyi CCMP2758 strain, was reported with abundant C35:2Me (∆12, 19) alkenones when cultured at 15°C (Prahl et al. 2006). Here we show, when this strain is cultured at 4°C-10°C, that CCMP2758 produces abundant C35:3Me, C36:3Me, and small amounts of C36:3Et alkenones with unusual double-bond positions of ∆7, 12, 19. We determine the double-bond positions of the C35:3Me and C36:3Me alkenones by GC-MS analysis of the dimethyl disulfide and cyclobutylamine derivatives, and we provide the first temperature calibrations based on the unsaturation ratios of the C35and C36alkenones. Previous studies have found C35:2Me (∆14, 19) and C36:2Et (∆14, 19) alkenones with three-methylene interruption in the Black Sea sediments, but this is the first reported instance of alkenones with a mixed three- and five-methylene interruption configuration in the double-bond positions. The discovery of these alkenones allows us to propose a novel biosynthetic scheme, termed the SCA biosynthesis pathway, that simultaneously rationalizes the formation of both the C35:3Me (∆7, 12, 19) alkenone in our culture and the ∆14, 19Black Sea type alkenones without invoking new desaturases for the unusual double-bond positions.
authors: Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island, 02912, USA.
authors: Yinsui
authors: Y
authors: Zheng
authors: Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island, 02912, USA.
authors: James T
authors: JT
authors: Dillon
authors: Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island, 02912, USA.
authors: Yifan
authors: Y
authors: Zhang
authors: Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island, 02912, USA.
authors: Yongsong
authors: Y
authors: Huang
mesh_terms: D015415:Biomarkers / Q000737:chemistry / Q000378:metabolism; D053898:Biosynthetic Pathways; D058087:Haptophyta / Q000737:chemistry / Q000378:metabolism*; D009930:Organic Chemicals / Q000737:chemistry / Q000378:metabolism
chemical_list: D015415:Biomarkers; D009930:Organic Chemicals
keywords: 
  Emiliania huxleyi
  ; biosynthetic pathways; culture; double-bond positions; shorter-chain alkenones