BIOSYNTHESIS OF NORMAL-ALKATRIENES IN BOTRYOCOCCUS-BRAUNII

In addition to very long chain n-alkadienes with omega-9 and terminal unsaturations, the A race of the hydrocarbon-rich microalga Botryococcus braunii produces substantial amounts of C29 and C31 n-alkatrienes. The latter are dominated by a C29 isomer comprising one terminal and two cis-omega-7,cis-omega-9 unsaturations. The present study indicates (i) although linoleic acid is the only 18:2 acid present in significant amounts in B. braunii, no trienic hydrocarbons directly derived from this acid (hence comprising cis-omega-6,cis-omega-9 internal unsaturations) are normally formed by this microalga. (ii) The elongation-decarboxylation system of B. braunii is selective, it does not accept linoleic acid as a direct precursor and even growth in the presence of large amounts of exogenous linoleic acid does not induce the production of the corresponding trienes. (iii) As previously demonstrated in the case of dienes, oleic acid is also the direct precursor of B. braunii trienes. (iv) Trienes are unlikely to be formed via desaturation of dienes. Introduction of the additional double bond probably occurs at an earlier stage of the elongation-decarboxylation pathway, either via direct desaturation of oleic acid (e.g. into a cis-omega-7,cis-omega-9 acid) or via desaturation of the very long chain fatty acyl derivatives formed by elongation of oleic acid. (v) The formation of radioactive dienic and trienic hydrocarbons following feeding with linoleic acid results neither from the prior degradation of this acid into acetate units nor from its prior reduction into oleic acid. (vi) Under normal growth conditions, that is in the absence of exogenous linoleic acid, the formation of hydrocarbons from this acid is at most a minor process.