Stable carbon isotope ratios of fatty acids in seagrass and redhead ducks

Fatty acids were extracted from roots and rhizomes of the seagrass, Halodule wrightii, and from subcutaneous fat tissues of eight redhead ducks (Aythya americana) collected either in Texas or South Dakota. Stable carbon isotope ratios (δ13C) of individual fatty acids were measured by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). In cases where individual fatty acids were not completely resolved by the chromatographic column, argentation chromatography was used as a pre-separation technique to separate saturated fatty acids from unsaturated fatty acids. The majority of fatty acids extracted from roots and rhizomes of H. wrightii and from subcutaneous fat of redhead ducks had δ13C values that clustered about the δ13C value measured for the total fatty acid fraction of the parent tissue. Normal and anteiso isomers of the saturated 17:0 fatty acid had the most negative δ13C values in the organisms studied. Texas redhead ducks which likely had consumed a diet comprised primarily of roots and rhizomes of H. wrightii for at least 1 month had δ13C values of fatty acids that were more positive than those of the identical fatty acids in the seagrass. This discrepancy in δ13C indicates that fatty acids in ducks are synthesized from molecules with more positive δ13C values, such as carbohydrates and/or proteins, rather than by direct incorporation of fatty acids from the diet, or else there are isotopic fractionations associated with the biosynthesis and catabolism of these compounds.