A new type of ether lipid comprising phenolic moieties in Botryococcus braunii. Chemical structure and abundance, and geochemical implications

In addition to recently described alkoxy ether lipids, the A race of Botryococcus braunii was shown to produce high molecular weight phenoxy ether lipids. The basic chemical structure of these alkoxy and phenoxy compounds is sharply different from those of all the ether lipids previously observed in living organisms. Eight homologous series of phenoxy ether lipids were identified in a B. braunii strain (OV7) isolated from a Bolivian lake. All the above series comprise 5-n-alkenyl-1,3-benzenediol units as phenolic moieties. Comparison with two other strains of A race revealed a strong strain dependence for both ether lipid nature and global production. Some strains can thus produce very large amounts of ether lipids corresponding either to alkoxy or phenoxy compounds. Indeed, the OV7 strain appears to be the most prolific producer of lipids containing a phenolic moiety so far identified in living systems. The algaenan (i.e. the insoluble and non-hydrolysable macromolecular material building up B. braunii outer walls) isolated from the OV7 strain was analysed by a combination of spectroscopic and pyrolytic methods. Phenoxy ether lipid condensation and cross-linking were thus shown to contribute to OV7 algaenan formation. B. braunii ether lipids should play a major geochemical role in two ways. Firstly, as algaenan precursors, since the latter macromolecular material is selectively preserved during fossilization and thus plays a major role in the genesis of B. braunii-derived kerogens. Secondly, due to diagenetic transformations, resulting in ether lipid incorporation into kerogens, as demonstrated in the Balkash coorongite and the Ribesalbes Oil Shale. In addition, it appears that the above materials are both derived from alkoxy ether lipid-rich B. braunii strains. The observations on the OV7 strain show that a substantial contribution of phenolic units in a kerogen does not necessarily indicate a terrestrial contribution. Such a feature should also be a signature of some B. braunii populations; nevertheless, the chemical structure of such units would then be sharply different. Finally, these observations further support the affinity of Gloecapsomorpha prisca, the main contributor to Ordovician organic matter, with B. braunii and account for the two conspicuous features of Kukersites, i.e. a large contribution of phenolic units dominated by 5-n-alkyl-1,3-benzenediol moieties.