NaBH4 reduction of alkenones to the corresponding alkenols: a useful tool for their characterisation in natural samples

During a search for phytoplanktonic lipid photoproducts in sediment extracts that had been reduced with NaBH4, we observed that the alkenones present had been reduced quantitatively to the corresponding unsaturated alcohols (alkenols). The silylated alkenols display better chromatographic characteristics than the corresponding alkenones and very useful EI mass spectra, which show strong fragment ions at m/z 117 or m/z 131 due to cleavage α to the functional group, allowing methyl and ethyl alkenols (and hence the parent alkenones) to be readily differentiated. Calculations of UK′37 values using the alkenols formed by reduction agreed within experimental error (±0.01 units) with values calculated from the original alkenones, suggesting that this might be a useful method for samples containing low contents of alkenones or where co-elution is a problem. Although small amounts of alkenols have been found in some sediments their presence does not appear to affect UK′37 calculations. We have found large amounts of alkenols (up to 20% of the amount of the corresponding alkenone) in extracts of some sediments from Camargue (France), which we suggest are probably formed by a non-selective bacterial reduction of alkenones under anoxic conditions. The possibility of a natural contribution of alkenols from haptophytes was tested and we were able for the first time to detect traces (less than 1% of the amount of the corresponding alkenone) of alkenols in non-reduced extracts of Gephyrocapsa oceanica and Isochrysis galbana. We presume that their biosynthesis is closely related to that of the alkenones. Studies of other haptophytes (particularly those from benthic or coastal environments) are needed to ascertain whether other species might contain higher contents of alkenols. The reduction-silylation technique has also proven to be very useful for the characterisation of previously unreported alkenones in microalgae, sediments and seawater. A re-examination of extracts of several haptophytes after this treatment allowed us to detect small amounts of a 35:1 methyl alkenone and 2 isomers of a 35:2 alkenone in G. oceanica, and monounsaturated C35–C38 alkenones in Emiliania huxleyi.