Abstract
Several marine algae of the class Pelagophyceae produce the unusual marine sterol 24‐propylidenecholesterol, mainly as the (24__E__)‐isomer. The (24__Z__)‐isomer had previously been considered as a specific biomarker for Aureococcus anophagefferens, the ‘brown tide’ alga of the Northeast coast of the USA. To test this hypothesis and to generate chemotaxonomic information, the sterol compositions of 42 strains of pelagophyte algae including 17 strains of Aureococcus anophagefferens were determined by GC analysis. A more comprehensive sterol analysis by HPLC and ^1^H‐NMR was obtained for 17 selected pelagophyte strains. All strains analyzed contained 24‐propylidenecholesterol. In all strains belonging to the order Sarcinochrysidales, this sterol was found only as the (E)‐isomer, while all strains in the order Pelagomonadales contained the (Z)‐isomer, either alone or together with the (E)‐isomer. The occurrence of Δ^22^ and 24__α__‐sterols was limited to the Sarcinochrysidales. The first occurrence of Δ^22^‐24‐propylcholesterol in an alga, CCMP 1410, was reported. Traces of the rare sterol 26,26‐dimethyl‐24‐methylenecholesterol were detected in Aureococcus anophagefferens, and the (25__R__)‐configuration was proposed, based on biosynthetic considerations. Traces of a novel sterol, 24‐propylidenecholesta‐5,25‐dien‐3__β__‐ol, were detected in several species.