Effects of salinity on the regulation of long-chain polyunsaturated fatty acid biosynthesis in the polychaete Platynereis dumerilii

Polychaetes are promising sources of n-3 long-chain (≥C₂₀) polyunsaturated fatty acid (LC-PUFA) due to their ability to synthesise these essential nutrients through elongases, front-end desaturases (Fed), and methyl-end desaturases. Salinity can influence lipid metabolism in aquatic invertebrates by triggering osmotic stress that alters membrane lipid composition. This study investigates the effect of salinity on LC-PUFA biosynthesis in the polychaete, Platynereis dumerilii, a model organism in Evolutionary and Developmental Biology. In vivo trials were conducted with juveniles under standard conditions (18 °C, 16 L:8D photoperiod, frozen spinach as feed), evaluating three salinity levels: 30 ‰, 35 ‰, and 40 ‰. After one month, survival and growth were assessed, and samples were collected to evaluate the modulation of n-3 LC-PUFA biosynthesis by analysing total lipids, fatty acid (FA) profiles, and gene expression of elongases, desaturases, and transcription factors (TF). Despite a uniform diet, P. dumerilii at 40 ‰ salinity exhibited elevated levels of n-3 LC-PUFA (20:4n-3, 20:5n-3, 22:5n-3). Variations in FA profiles suggest salinity plays a regulatory role in lipid metabolism. Gene expression analysis showed significant upregulation of a Fed, with Δ6/Δ8 activity, under high salinity (40 ‰), consistent with reduced levels of its substrates, eicosadienoic acid (20:2n-6) and eicosatrienoic acid (20:3n-3). Additionally, the TF EcR (Ecdysone receptor) showed increased expression, suggesting involvement in LC-PUFA biosynthesis regulation. These findings indicate that cultivating P. dumerilii in high salinity may enhance its n-3 LC-PUFA content. Similarly, the nutritional value of other polychaetes, considered sustainable alternatives for aquaculture feed, may be improved through salinity modulation.