Novel fatty acids originated from the vent bivalves, such as vent clam Calyptogena phaseoliformis and vent mussels Bathymodiolus platifrons and Bathymodiolus japonicus collected from hydrothermal vents in the deep-sea, were determined by using gas chromatography-mass spectrometry analysis of the 4,4-dimethyloxazoline derivatives. The major fatty acids present in the C. phaseoliformis lipids belong to the n-4 family non-methylene interrupted polyunsaturated fatty acids (NMI-PUFA): 20:3n-4,7,15, 20:4n-1,4,7,15, and 21:3n-4,7,16, while two Bathymodiolus mussels contained high levels of methylene interrupted PUFA: 20:3n-4,7,10, 20:4n-1,4,7,10, and 21:3n-4,7,10. The vent bivalves probably have an intrinsic mechanism in vivo so as to maintain the fluidity of the high-melting fatty acids in the membrane lipids by exogenous n-4 family PUFA adaptation as substitutes. Such special kinds of fatty acids are assimilated by the symbiotic chemosynthetic bacteria, which use geothermal energy and minerals from the cold-seep vents. Their unique fatty acid compositions corresponding to the novel n-4 family PUFA markedly differs from those of the reported lipid compositions of other marine animals, which depend on the marine grazing food chain originating from phytoplankton. The present findings confirm a perfect and closed novel food chain in the cold-seep bivalves and in their symbionts, which are independent from the photosynthetic food chain.
