Mechanism of non-isoprenoid hydrocarbon biosynthesis in Botryococcus braunii

The green unicellular alga Botryococcus braunii shows unusually high concentrations of non-isoprenoid very long chain hydrocarbons. The structure of such hydrocarbons, the relative efficiency of various long chain fatty acids as precursors, the relationship between fatty acid and hydrocarbon concentrations (over the different physiological stages of the alga achieved during batch cultures) and the preferential localization of fatty acids lead to the conclusion that all the major non-isoprenoid hydrocarbons of B. braunii derive from the same direct precursor, oleic acid. Feeding experiments, using doubly labelled oleic acid, show that the whole carbon chain of the latter is incorporated into final hydrocarbons; accordingly such compounds do not originate from a head-to-head condensation mechanism with oleic acid acting as donor. Various features (regarding chiefly the systematic occurrence of a terminal double bond in B. braunii hydrocarbon, their close specific activities after feeding and the large inhibition in their production achieved using dithioerythritol) show that the biosynthesis of B. braunii hydrocarbons probably takes place via an elongation-decarboxylation mechanism related to that operating in some higher plants.