Biosynthetic effects on the stable carbon isotopic compositions of agal lipids: Implications for deciphering the carbon isotopic biomarker record

Thirteen species of algae covering an extensive range of classes were cultured and stable carbon isotopic compositions of their lipids were analysed in order to assess carbon isotopic fractionation effects during their biosynthesis. The fatty acids were found to have similar stable carbon isotopic compositions to each other in all the algae with the exceptions of the C28 fatty acid in Scenedesmus communis and the polyunsaturated fatty acids in Ampidinium sp. and Gymnodinium simplex which are 3.5 more depleted in 13C and up to 8 more enriched in 13C, compared to the C16 fatty acid, respectively. Phytol is consistently enriched in 13C by 2-5 compared with the C16 fatty acid in all algae. The sterols are, however, enriched in 13C by 0-8 compared to the C16 fatty acid, possibly due to a different pool of isopentenyl diphosphate in the cytosol. These large ranges in carbon isotopic compositions of compounds biosynthesized by the same eukaryotes can significantly complicate the interpretation of 13C values of sedimentary biomarkers