Geochemical significance of the occurrence of dinosterol and other 4-methyl sterols in a marine diatom

Abstract

The 4-methyl sterol 4 α,23,24 R-trimethyl-5 α-cholest-22E-en-3 β-ol (dinosterol) is commonly used as an unambiguous biomarker for organic matter derived from dinoflagellates in sediments and seawater. Consequently, the presence in petroleum of the saturated hydrocarbon dinosterane, which is thought to be derived from dinosterol, is used as evidence for the presence of dinoflagellate organic matter in petroleum source rocks. Here we report the first identification of dinosterol (or its C-24 epimer since the two are not separable on conventional non-polar capillary columns) in a laboratory culture of a marine diatom Navicula sp. (CS-46c). Although it was only a minor constituent (2.0–3.6% of total sterols), this result indicates that diatoms may be a source of dinosterol in marine sediments particularly those lacking obvious contributions of organic matter from dinoflagellates. Navicula sp. also contained the 4-methyl sterols 4α,24-dimethyl-5α-cholest-22E-en-3β-ol (9.7–12.6%), 4α,24-dimethyl-5α-cholestan-3β-ol (2.0–3.4%) and 4α,23,24-trimethyl-5α-cholestan-3β-ol (dinostanol or its C-23/C-24 epimer; 0.3–0.6%). The 4-desmethyl sterol fraction included the common diatom sterol 24-methylcholesta-5,22E-dien-3β-ol (20.2–30.5%), but the major sterol was 24-ethylcholest-5-en-3β-ol (31.0–38.6%). This provides another example of a marine algal source for C 29 sterols. Minor amounts of 23,24-dimethylcholesta-5,22E-dien-3β-ol were also found. Ultrastructural studies were made, but no evidence could be obtained for a dinoflagellate symbiont, and the fatty acid and pigment distributions were typical of marine diatoms. An unusual compound tentatively identified as a C 34 tetra-unsaturated straight-chain alcohol was also detected.