Microalgal lipid markers for paleoclimatic research

Abstract

Primary producers, mainly represented in the marine environment by microalgae, synthesize a wide variety of lipid constituents, some of which are recognized as ‘biomarkers’ that can be used to identify sources of organic matter. Furthermore, as phytoplankton rapidly reflect climate variations, the identification of organic matter in sediments may help to understand present or ancient specific environmental conditions. To highlight differences in some lipid constituents (hydrocarbons, sterols and fatty acids) of different taxa of microalgae, we determined these compounds in the diatom Cylindrotheca closterium and dinoflagellate Scrippsiella trochoidea isolated from the Adriatic Sea and grown in batch cultures. Total lipids were extracted in chloroform-methanol and, after separation, analysed by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). The only hydrocarbons present in these species were cis-3,6,9,12,15,18-heneicosahexaene (HEH) and squalene. Analysis of sterols in C. closterium revealed that only 4-desmethylsterols were present. Conversely, in S. trochoidea, 2 different groups of sterols (4-desmethylsterols and 4a-methylsterols) were identified. Besides the fatty acid 16:0, which predominated in both species, high percentages of 16:1n-7 and 20:5n-3 were observed in C. closterium, whereas 18:4n-3 and 22:6n-3 were detected in S. trochoidea. Differences in lipid patterns were observed between C. closterium and S. trochoidea. These differences might help understand the Adriatic Sea Basin’s paleoclimatic history.