Cold-induced metabolic conversion of haptophyte di- to tri-unsaturated C₃₇ alkenones used as palaeothermometer...

The cosmopolitan marine haptophyte alga Emiliania huxleyi accumulates very long-chain (C₃₇-C₄₀) 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 C₃₇ 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 ¹³C by incubating cells with ¹³C-bicarbonate in the light at 25 °C under conditions of little if any K_37:3 production. After stabilisation of the ¹³C-K_37:2 level by depleting ¹³C-bicarbonate from the medium, the temperature was suddenly reduced to 15 °C. The ¹³C-K_37:2 level rapidly decreased, and the ¹³C-K_37:3 level increased, whereas the total ¹³C-K₃₇ level-namely [K_37:2 + K_37:3]-remained constant. These ¹³C-pulse-chase-like experimental results indicate that ¹³C-K_37:2 is converted directly to ¹³C-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.