The possible metabolic role of C37 alkenones in Emiliania huxleyi

Ratios of C37 alkenones (U37K′ratios) produced by the haptophyte microalgae Emiliania huxleyi and Gephyrocapsa oceanica are often used as proxies for past sea surface temperatures. Previous studies suggested that these lipids might be either membrane molecules or metabolic storage molecules. In order to assess their cellular role, we cultured two strains of E. huxleyi at 12:12 and 0:24 light/dark cycles (North Pacific clone 55a and Sargasso Sea clone BT6). For both strains, alkenone concentrations (ng/ml) generally increased through both logarithmic and stationary phase in cultures with 12:12 light/dark cycles. This pattern of increasing concentration closely resembles the pattern of increasing triacylglycerol concentration observed in cultures of other marine phytoplankton. Triacylglycerols are used as metabolic storage molecules by many marine algae, but are under-represented or absent in E. huxleyi. When the E. huxleyi cultures were placed in continuous darkness, their alkenone concentrations fell. These findings, that alkenones increase in abundance from logarithmic to stationary phase and decrease in concentration when E. huxleyi is energy-deprived, suggest that Emiliania huxleyi uses alkenones for metabolic storage. Alkenone unsaturation (U37K′) ratios changed only slightly during net alkenone degradation in our dark experiments (by −0.035 and +0.046 U37K′ units in the dark incubations of BT6 and 55a, respectively), suggesting that metabolic utilization of C37 alkenones by E. huxleyi is unlikely to be a major source of uncertainty for paleotemperature estimation. If C37 alkenones are primarily used for metabolic storage, the temperature dependence of their unsaturation may result from differences in melting point, density, or enzymatic optima of biochemical pathways of the differently saturated alkenones.