Alkenones

Alkenones are long-chain, unsaturated ketones produced predominantly by marine haptophyte algae, especially coccolithophores such as Emiliania huxleyi. Their carbon skeleton is typically based on a C37 chain, with related C38 and C39 homologs present. Each molecule carries one or more trans (E) double bonds along the hydrocarbon chain and a ketone carbonyl group. The most studied members are the C37 alkenones with two or three double bonds (C37:2 and C37:3) and their ketone-bearing analogs.

Occurrence: Alkenones accumulate in the lipids of haptophytes and are well preserved in marine sediments, where they serve as biomarkers for coccolithophore productivity and paleoenvironments. They are detectable in modern oceans and in sediment records dating back to the Cretaceous.

Analytical and paleoceanographic significance: In sediment extracts, alkenones are quantified by gas chromatography–mass spectrometry. The primary application is a sea surface temperature proxy based on the relative abundances of alkenones with different unsaturations. For C37 homologs, indices such as U37k′ use the ratio of two- to three-double-bond forms (including ketone-bearing variants), calibrated to contemporary temperatures. Interpretations depend on regional coccolithophore assemblages and diagenetic stability; inaccuracies can arise from differential preservation or provenance effects.

Biogenesis and properties: Alkenones are relatively resilient lipids whose biosynthesis involves elongation and functionalization of long-chain lipids in haptophytes. Their persistence in sediments underpins their value for reconstructing past ocean conditions.