Cold-induced metabolic conversion of haptophyte di- to tri-unsaturated C alkenones used as palaeothermometer molecules.

The cosmopolitan marine haptophyte alga Emiliania huxleyi accumulates very long-chain (C-C) alkyl ketones with two to four trans-type carbon-carbon double bonds (alkenones). These compounds are used as biomarkers of haptophytes and as palaeothermometers for estimating sea-surface temperatures in biogeochemistry. However, the biosynthetic pathway of alkenones in algal cells remains enigmatic, although it is well known that the C tri-unsaturated alkenone (K) becomes dominant at low temperatures, either by desaturation of K or by a separate pathway involving the elongation of tri-unsaturated alkenone precursors. Here, we present experimental evidence regarding K synthesis. Using the well-known cosmopolitan alkenone producer E. huxleyi, we labelled K with C by incubating cells with C-bicarbonate in the light at 25 °C under conditions of little if any K production. After stabilisation of the C-K level by depleting C-bicarbonate from the medium, the temperature was suddenly reduced to 15 °C. The C-K level rapidly decreased, and the C-K level increased, whereas the total C-K level-namely [K + K]-remained constant. These C-pulse-chase-like experimental results indicate that C-K is converted directly to C-K by a desaturation reaction that is promoted by a cold signal. This clear-cut experimental evidence is indicative of the existence of a cold-signal-triggered desaturation reaction in alkenone biosynthesis.