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 s p.) 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 p arts per thousand for the vestimentiferan. Unsaturated fatty acids hav e lighter deltaC-13 than saturated ones. The variations in deltaC-13 a re up to 5.1-6.7 parts per thousand (mytilids) and 7.0 parts per thous and (vestimentiferan) within a single specimen. It is suggested that a kinetic isotopic effect in the biosynthesis of fatty acids and interm olecular isotope fractionation during fatty acid desaturation and elon gation are responsible for the observed distribution pattern. Fatty ac ids are depleted in C-13 relative to the gills of the mytilids, wherea s fatty acids of the vestimentiferan are enriched relative to trophoso mes. The difference in deltaC-13 of fatty acids between mytilids and v estimentiferan reflects the differences in substrates (methane vs. CO2 ), and the different chemosynthetic processes of the invertebrates.