Evidence for polyunsaturated fatty acid biosynthesis in the ragworm (Nereis virens) and the lugworm (Arenicola marina

Abstract

There is increasing evidence suggesting that heterotrophic marine organisms could also be potential sources of polyunsaturated fatty acids (PUFAs), in addition to marine bacteria and microalgae, including two polychaete species such as the lugworm (Arenicola marina) and the ragworm (Nereis virens). The capacity for PUFAs production by these species is, therefore, essential to understand the roles of polychaetes in relation to PUFA fluxes in an estuarine food web. Two methods were adopted to study fatty acid biosynthesis by the worms: i) quantifying of PUFA fluxes in closed system (A. marina and N. virens) and ii) stable isotope labelling and mass spectrometry to investigate possible pathway (N. virens only). In the first method, the controlled amount of fatty acid inputs and outputs in the culture worms was determined together with GC/GC-MS analysis to calculate the amount of fatty acid gained per system box after subtracting food given fatty acids. Palmitic acid (C16:0) and monoenoic acids e.g. palmitoleic acid (C16:1n7), oleic acid (C18:1n9) and vaccenic acid (C18:1n7) were prominent fatty acids in the system. EPA was found to be accumulated over the period of experiment in both species. In the stable isotope experiment, 13C-labelled palmitic acid (C16:0) was incorporated into the feed pellets and used to trace the changes in the 13C/12C ratio in characteristic peaks of mass spectra. Significant changes in the 13C/12C ratio of key peaks in the spectra compared with the controlled start samples: palmitic acid (C16:0), stearic acid (C18:0), arachidonic acid (ARA: C20:4n6) and EPA (C20:5n3) within the 7 days period of experiment revealed a pathway for EPA production in N. virens. These findings would therefore corroborate of the concept that some marine animals are able to generate long-chain PUFAs such as ARA, EPA and DHA. N. virens and A. marina could be one of the very important PUFA sources within estuarine marine ecosystem.