Gulf of Mexico hydrocarbon seep communities: XI. Carbon isotopic fractionation during fatty acid biosynthesis of seep organisms and its implication for chemosynthetic processes

The individual fatty acids of mytilids and vestimentiferan ( Escarpis sp.) from hydrocarbon seeps exhibit light δ 13Cvalues: from −56.9 to −49.0‰ for the mytilids and from −38.6 to −31.6‰ for the vestimentiferan. Unsaturated fatty acids have lighter δ 13C than saturated ones. The variations in δ 13C are up to 5.1–6.7‰ (mytilids) and 7.0‰ (vestimentiferan) within a single specimen. It is suggested that a kinetic isotopic effect in the biosynthesis of fatty acids and intermolecular isotope fractionation during fatty acid desaturation and elongation are responsible for the observed distribution pattern. Fatty acids are depleted in 13C relative to the gills of the mytilids, whereas fatty acids of the vestimentiferan are enriched relative to trophosomes. The difference in δ 13C of fatty acids between mytilids and vestimentiferan reflects the differences in substrates (methane vs. CO 2), and the different chemosynthetic processes of the invertebrates.