Mid-chain hydroxy long-chain fatty acids in microalgae from the genus Nannochloropsis

Extractable and bound lipids of four species of microalgae from the genus Nannochloropsis have been examined by capillary GC-mass spectrometry. In addition to previously described unusual C-28-C-34 alkyl diols and n-alcohols, small quantities of compounds identified as C-28-C-34 monohydroxy fatty acids were detected both in free and bound form. For each carbon chain-length member of the series, a single positional isomer predominated, which was identified as the omega 18-isomer from characteristic mass spectral fragment ions. The position of the hydroxyl group at omega 18 rather than at a constant position relative to the carboxylic acid group indicates that the series results from chain-elongation (or perhaps chain-shortening) of a particular hydroxy fatty acid, rather than hydroxylation of a range of fatty acids. Furthermore, two dihydroxy fatty acids, identified as 15,16-dihydroxydotriacontanoic and 16,17-dihydroxytritriacontanoic acids, were also found in the products from acid hydrolysis of the cell residue. Possible biosynthetic correlations between these hydroxy and dihydroxy fatty acids and the long-chains diols also produced by these algae are discussed. Our data suggest that hydroxy acids may be precursors of the diols via reduction of the acid group. However, another pathway must be invoked in order to explain the formation of some diol isomers. (C) 1997 Elsevier Science Ltd.