Cold-induced metabolic conversion of haptophyte di- to tri-unsaturated C(37) alkenones used as palaeothermometer molecules

The cosmopolitan marine haptophyte alga Emiliania huxleyi accumulates very long-chain (C(37)-C(40)) 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(37) tri-unsaturated alkenone (K(37:3)) becomes dominant at low temperatures, either by desaturation of K(37:2) or by a separate pathway involving the elongation of tri-unsaturated alkenone precursors. Here, we present experimental evidence regarding K(37:3) synthesis. Using the well-known cosmopolitan alkenone producer E. huxleyi, we labelled K(37:2) with (13)C by incubating cells with (13)C-bicarbonate in the light at 25 °C under conditions of little if any K(37:3) production. After stabilisation of the (13)C-K(37:2) level by depleting (13)C-bicarbonate from the medium, the temperature was suddenly reduced to 15 °C. The (13)C-K(37:2) level rapidly decreased, and the (13)C-K(37:3) level increased, whereas the total (13)C-K(37) level—namely [K(37:2) + K(37:3)]—remained constant. These (13)C-pulse-chase-like experimental results indicate that (13)C-K(37:2) is converted directly to (13)C-K(37:3) 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.