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 δ¹³Cvalues: from -56.9 to -49.0‰ for the mytilids and from -38.6 to -31.6‰ for the vestimentiferan. Unsaturated fatty acids have lighter δ¹³C than saturated ones. The variations in δ¹³C 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 δ¹³C of fatty acids between mytilids and vestimentiferan reflects the differences in substrates (methane vs. CO₂), and the different chemosynthetic processes of the invertebrates.