Hydrogen isotopic fractionations during desaturation and elongation associated with polyunsaturated fatty acid biosynthesis in marine macroalgae

Compound-specific hydrogen isotopic compositions of saturated, monounsaturated and polyunsaturated fatty acids in natural marine macroalgae have been determined in order to clarify hydrogen isotopic fractionations during their desaturation and elongation associated with polyunsaturated fatty acid biosynthesis. Compound-specific hydrogen isotopic compositions (δD) of saturated, monounsaturated and polyunsaturated fatty acids have been determined for natural marine macroalgae including two brown algae (Heterokontophyta) and two red algae (Rhodophyta). δD values of individual fatty acids from four macroalgae exhibit a wide variation ranging from - 189‰ to +48‰. Generally, stearic (18:0), arachidic (20:0) and behenic acids (22:0) are much more enriched in D by up to ∼180‰ relative to myristic (14:0), palmitic (16:0), octatetraenoic [18:4(n - 3)] and eicosapentaenoic acids [20:5(n - 3)]. Other fatty acids such as oleic [18:1(n - 9)], lenoleic [18:2(n - 6)] and linolenic acids [18:3(n - 3)] fall isotopically between these fatty acids. This wide δD variation of fatty acids is probably explained by the hydrogen isotopic fractionation during desaturation being much larger than that during elongation in the network of polyunsaturated fatty acid biosynthesis. A large hydrogen isotopic fractionation during desaturation may cause D-enrichment in the remaining hydrogen of the residual fatty acids, which could be controlled by the relative flux into their desaturates.