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

Alkenones (C 37 –C 40 ) 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 35 and C 36 ) 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 CCMP 2758 strain, was reported with abundant C 35:2 Me (∆ 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 CCMP 2758 produces abundant C 35:3 Me, C 36:3 Me, and small amounts of C 36:3 Et alkenones with unusual double‐bond positions of ∆ 7, 12, 19 . We determine the double‐bond positions of the C 35:3 Me and C 36:3 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 35 and C 36 alkenones. Previous studies have found C 35:2 Me (∆ 14, 19 ) and C 36:2 Et (∆ 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 C 35:3 Me (∆ 7, 12, 19 ) alkenone in our culture and the ∆ 14, 19 Black Sea type alkenones without invoking new desaturases for the unusual double‐bond positions.