GULF-OF-MEXICO HYDROCARBON SEEP COMMUNITIES .11. 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 deltaC-13-values: from -56.9 to -49.0 parts per thousand for the mytilids and from -38.6 to -31.6 parts per thousand for the vestimentiferan. Unsaturated fatty acids have lighter deltaC-13 than saturated ones. The variations in deltaC-13 are up to 5.1-6.7 parts per thousand (mytilids) and 7.0 parts per thousand (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 C-13 relative to the gills of the mytilids, whereas fatty acids of the vestimentiferan are enriched relative to trophosomes. The difference in deltaC-13 of fatty acids between mytilids and vestimentiferan reflects the differences in substrates (methane vs. CO2), and the different chemosynthetic processes of the invertebrates.