LIPID CHEMISTRY OF ICELANDIC HOT-SPRING MICROBIAL MATS

Photosynthetic microbial mats were sampled from hot springs (43-66°C) at seven sites in three geographic areas of Iceland. The extractable lipids of thirteen representative samples have been analysed by GC-MS. Predominant components, include n-heptadecane, various branched alkanes, n-hexadecanol, phytol, C16, C16:1, C18, C18:1, C18:2 and C19:1 fatty acids. Phytyl hexadecanoate and octadecanoate occur in several samples. Three samples contain low levels of sterols, probably of allochthonous origin. No extractable hopanoids were detected. EI GC-MS/MS proved to be invaluable in determining the structures of the branched alkane components. Thus, the complete range of methylheptadecanes (C18), 2-methyloctadecane, mixtures of 9-, 8-, 7- and 6-methyloctadecanes, 10-, 9-, 8- and 7-methylnonadecanes and 9- and 10-methyleicosanes were detected. Additionally, dimethyl-branched alkanes occurred, with 5,13-dimethyl-heptadecane dominant; a structure confirmed by synthesis. 8- and 7-methylheptadecanes have often been reported as major constituents of cyanobacteria and contemporary sediments receiving cyanobacterial contributions, whereas in the case of the Icelandic mats, 5- and 6-methylheptadecanes or 5,13-dimethylheptadecane are normally the most abundant branched alkanes. The finding of such complex suits of C18 to C21 mid-chain methyl-branched alkanes in contemporary microbial mats supports the view that the suites of structurally-isomeric methyl-branched alkanes reported to occur in ancient sediments and oils, especially Late Precambrian to Early Cambrian samples, have a direct origin from cyanobacteria. Significant differences were noted between the lipid compositions of the Icelandic mats and published data for other microbial mats, presumably reflecting differences in their microbial communities. Hence, although modern hot spring microbial mats are valuable models for ancient stromatolites in terms of their lipid compositions, this study indicates that such stromatolites may have displayed considerable variability in the composition of their microbial communities and thus their lipid compositions. © 1990.