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[MIS19-23] Long-chain alkenediones found from marine sediments and their unsaturation ratios: key compounds to study the evolution of alkenone biosynthetic pathway?
Keywords:Alkenone, Alkenedione, Paleoceanography
We analyzed the alkenones and alkenediones in the hemipelagic sediments recovered from the NE Atlantic during the IODP exp. 339 to evaluate the variations in their compositions at 100,000-year scale from the late Miocene to Pleistocene. We measured using gas chromatography–flame ionization detector (GC–FID) and GC–mass spectrometry (GC–MS) equipping with a mid-polarity capillary column (VF-200 ms). The GC analysis using the mid-polarity column makes the peak shapes sharp and achieves the well chromatographic separations in the analyses of long-chain alkenones, and thus, several recent studies applied this technique (Longo et al., 2013; Furota et al., 2016).
The long-chain alkenones and the C38 alkenediones were detected from all the study samples. Interestingly, the carbon numbers of the alkenediones are only C38 in contrast to the alkenones. Mass fragmentogram shows that the C38 alkenedione has a methyl-ketone and ethyl-ketone groups at the each terminals of the carbon chain. In addition to C38:2 alkenedione as reported previously, we firstly identified tri-unsaturated (C38:3) alkenedione in the late Pliocene–Pleistocene sediments.
The relative abundances of the C38 alkenediones to alkenones were significantly higher in older sediments, and gradually decreased from the Miocene to Pleistocene. These results support the interpretation that the alkenediones could be produced by the ancient alkenone producers, and imply that those producers decreased from the Miocene to Pleistocene. The unsaturation ratios of C38 alkenediones, defines as C38:2/(C38:2 + C38:3), highly correlate with the unsaturation ratios of the C37 alkenones (UK’37) (R2 = 0.85). This fact strongly suggests that the alkenediones had a similar biological function and biosynthetic pathway related to the alkenones. Furthermore, the carbon number of alkenediones (C38) is averaged one for those of alkenones (C37–C39), and the positions of ketone groups are equal to those of alkenones. From these results, we conclude that the C38 alkenedione had the similar functions with the alkenones, and might be produced by primitive alkenone biosynthetic pathway.
Furota et al. (2016) Org. Geochem. 101, 166–175.
Longo et al. (2013) Org. Geochem. 65, 94-102.