Discovery of alkenones with variable methylene‐interrupted double bonds: implications for the biosynthetic pathway

Abstract

Alkenones (C -C ) 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., C and C ) 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 C Me (∆ ) 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 C Me, C Me, and small amounts of C Et alkenones with unusual double-bond positions of ∆ . We determine the double-bond positions of the C Me and C Me 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 C and C alkenones. Previous studies have found C Me (∆ ) and C Et (∆ ) 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 C Me (∆ ) alkenone in our culture and the ∆ Black Sea type alkenones without invoking new desaturases for the unusual double-bond positions.

Optimizing the yield of transient mono-dimethyl disulfide adducts for elucidating double bond positions of long chain alkenones

“…We recently reported double bond positions of tri-unsaturated alkenone isomers (Dillon et al, 2016;Zheng et al, 2016) by imine derivatization of the alkenone carbonyl group and dimethyl disulfide (DMDS) double bond adduction, using a new gas chromatographic separation method that provides higher selectivity of derivatized alkenones than conventional methods (Longo et al, 2013). In contrast to the OsO 4 oxidation method (de Leeuw et al, 1980;Rontani et al, 2001, DMDS can separately and consecutively bind to individual double bonds in an alkenone molecule (termed DMDS-1), creating products with distinct mass fragments at any given double bond position without significantly decreasing the product volatility (Buser et al, 1983;Dillon et al, 2016).…”

“…In contrast to the OsO 4 oxidation method (de Leeuw et al, 1980;Rontani et al, 2001, DMDS can separately and consecutively bind to individual double bonds in an alkenone molecule (termed DMDS-1), creating products with distinct mass fragments at any given double bond position without significantly decreasing the product volatility (Buser et al, 1983;Dillon et al, 2016). To account for fragments that are structurally symmetric (i.e., ions which may be assigned to two different double bond positions on opposite sides of the alkyl chain based on the mass spectra for DMDS adducts), compounds can be further derivatized using cyclobutylimine to arrive at a structurally unique solution (Zheng et al, 2016).…”

“…Despite our initial success (Longo et al, 2013;Dillon et al, 2016;Zheng et al, 2016), there are major problems with published procedures for alkenone DMDS adduction. In our experience, applying published DMDS adduction procedures (Francis, 1981;Leonhardt and DeVilbiss, 1985;Yamamoto et al, 1991;Xu et al, 2001;van Soelen et al, 2014;Zhao et al, 2014) to alkenones, has frequently resulted in the lack of alkenone adducts in GC-MS analyses and even the complete absence of the original alkenones (i.e., total sample loss).…”

Alkenone Distributions and Hydrogen Isotope Ratios Show Changes in Haptophyte Species and Source Water in the Holocene Baltic Sea

“…The Littorina Sea phase is distinguished from other phases by the presence of the alkenones of shorter chain‐length as well as the absence of tri‐unsaturated isomers and C 39 and C 40 alkenones. The presence of the shorter chain alkenones is an indicator for the presence of Group II haptophytes (Zheng, Dillon, et al, ; Zheng, Huang, et al, ). The final shift in alkenone distribution corresponds to the change from the Littorina Sea into the Modern Baltic phase and represents a mixed Groups II and III signal indicated by the presence of the C 35:2 , C 36:2 , and C 39 ethyl alkenones (Figure ).…”

“…These shorter chain alkenones have also appeared in an E. huxleyi culture after being kept in cultivation for a few years (Prahl et al, ). In contrast to the C 37 ‐C 40 alkenones, the C 35 and C 36 are thought to be synthesized from the C 37:2 and C 38:2 , respectively, via β‐oxidation and produced by mutants of Group II species in the natural environment (Zheng, Dillon, et al, ; Zheng, Huang, et al, ). Finally, Group III produce the standard suite of C 37 methyl and C 38 ethyl and methyl alkenones, with low abundance of the C 39 ethyl alkenones in some cases (Longo et al, ).…”