Origins of long-chain polyunsaturated fatty acids in the hydrothermal vent worms Ridgea piscesae and Protis hydrothermica

Long-chain polyunsaturated fatty acids (PUFAs) are essential nutrients for all marine animals that have previously been studied. Most marine animals obtain long-chain PUFAs from their diets (i.e. as products of photosynthetic processes) and few are known to be able to produce these compounds de novo. Deep-sea vent organisms live in an environment that is relatively isolated from photosynthetic sources of PUFAs, yet some species are known to contain substantial amounts of these compounds. To further understand the origins of PUFAs in deep-sea vent animals, we studied 2 annelid species; a vestimentiferan tubeworm (class Pogonophora) with endosymbiotic bacteria, and a tubicolous serpulid polychaete that feeds heterotrophically. The vestimentiferan tubeworms Ridgeia piscesae from the Juan de Fuca Ridge are rich in the monounsaturated fatty acids 16:1n-7 and 18:1n-7, the polyunsaturated fatty acids (PUFAs) 20:5n-3 and 20:4n-6, and the non-methyleneinterrupted PUFAs 20:2∆5, 13. They also contain small but significant amounts of the PUFAs 18:2n-6, 18:3n-3 and 22:6n-3. δ13C values of ca –14‰ for 16:1n-7 and 18:1n-7 suggest that these fatty acids are synthesised by chemosynthetic processes at the vent site. A δ13C value of –22‰ for 20:5n-3 in R. piscesae makes it impossible to conclusively attribute the origin of this fatty acid to either the photic zone or hydrothermal vents. The lipids of the serpulid polychaete Protis hydrothermica from the East Pacific Rise are also rich in 16:1n-7 and contain the unusual PUFAs 18:3n-7. They also contain significant amounts of 20:1n-13 and the PUFAs 20:3n-9, 20:5n-3 and 22:6n-3. Other than 22:6n-3, which had a δ13C of –22‰, the fatty acids in P. hydrothermica had δ13C values ranging from –33‰ to –41‰. This is consistent with the fatty acids of P. hydrothermica, other than 22:6n-3 but including 20:5n-3, originating from a chemoautotrophic carbon source in the hydrothermal vent. It cannot be excluded that the 22:6n-3 in P. hydrothermica originates in the photic zone. Potential origins of the long-chain PUFAs in hydrothermal vent animals, whether produced by photoor chemotrophic processes or by proor eukaryotic organisms, are considered.