Biosynthesis of long-chain polyunsaturated fatty acids in the nereid polychaete Hediste diversicolor: Molecular cloning and functional characterisation of three fatty acyl elongases and two front-end desaturases

Aquaculture of carnivorous fish species heavily relies on marine ingredients to guarantee the provision of essential nutrients, including long-chain (C20–24) polyunsaturated fatty acids (LC-PUFA) such as arachidonic acid (ARA, 20:4n-6), eicosapentaenoic acid (EPA, 20:5n-3), and docosahexaenoic acid (DHA, 22:6n-3). These nutrients are crucial for fish growth and neural development, offering significant health benefits to consumers. The rapid growth of global aquaculture production in recent decades, has driven the exploration of alternative and sustainable feeding sources that ensure n-3 LC-PUFA content. Within the context of circular economy, polychaete worms arise as promising candidates for aquafeed formulations, due to the enzymatic capacity for de novo LC-PUFA biosynthesis. The present study focused on the molecular and functional characterisation of elongases (Elovl) and front-end desaturases (Fed) involved in the LC-PUFA biosynthesis of the nereid polychaete Hediste diversicolor. The sequence and phylogenetic analysis established the orthology of three Elovl, identified as Elovl2/5, Elovl4 and Elovl1/7, that have putative functions in the LC-PUFA biosynthesis. Functional characterisation in yeast demonstrated their roles as PUFA elongases, exhibiting affinity towards C18 and C20 PUFA substrates (Elovl2/5) and towards C18, C20 and C22 (Elovl4 and Elovl1/7). Additionally, H. diversicolor possesses two front-end desaturases (Fed1 and Fed2) with Δ5 and Δ6/Δ8 desaturation activities, respectively, thus encompassing all the necessary desaturation reactions involved in the LC-PUFA biosynthesis pathways. These findings, along with the previously characterised methyl-end desaturases, show that H. diversicolor possesses the full enzymatic repertoire necessary to produce LC-PUFA like ARA and EPA. This highlights the potential of polychaetes and other annelids for producing nutrient-rich biomass suitable for aquaculture.