Many aquatic invertebrates, such as polychaetes, serve as excellent sources of the essential long-chain polyunsaturated fatty acids (LC-PUFA), namely eicosapentaenoic acid (EPA, 20:5n-3), docosahexaenoic acid (DHA, 22:6n-3) and arachidonic acid (ARA, 20:4n-6). Recent research highlights the presence of complete sets of LC-PUFA biosynthetic enzymes, including elongases (Elovl), front-end desaturases (Fed), and methyl-end desaturases (ω des), in marine polychaetes, with freshwater species being overlooked. We hypothesize that, compared to marine counterparts, freshwater-adapted polychaetes possess an enhanced capacity for LC-PUFA biosynthesis to compensate for the limited availability of these essential nutrients in their environment. The present study aims to test this hypothesis in the nereid polychaete Namalycastis rhodochorde, a species commonly found in freshwater, and highly valued as bait.
Searches were conducted in the N. rhodochorde transcriptome assembly to identify complete sequences of elongases and desaturases homologous to those previously characterized in marine polychaetes. Conserved motifs were identified, and phylogenetic relationships were established. Functional assays were performed in yeast. Conversions of the Elovl, Fed and ω des toward the exogenously supplemented polyunsaturated fatty acids (PUFA) substrates were assessed by analyzing the fatty acid profiles of transgenic yeast.
N. rhodochorde has 3 Elovl, 2 Fed, and 2 ω des involved in LC-PUFA biosynthesis. The elongases, identified as Elovl2/5, Elovl4, and Elovl1/7 based on their homology with vertebrate Elovl enzymes, extend PUFA substrates from C18 to C22. Functional assays showed Δ5 desaturase activity for Fed1 and dual Δ6/Δ8 activity for Fed2. Specifically, Fed1 is a Δ5 desaturase that synthesizes ARA and EPA from their precursors 20:3n-6 and 20:4n-3, respectively. The Δ6 desaturase activity of Fed2 allows the conversion of linoleic acid (LA, 18:2n-6) and α-linolenic acid (18:3n-3) into 18:3n-6 and 18:4n-3, respectively. Furthermore, Fed2 showed Δ8 activity on 20:2n-6 and 20:3n-3, converting them into 20:3n-6 and 20:4n-3, respectively. Two genes encoding ω des have been demonstrated to exist in N. rhodochorde; a Δ12 desaturase enabling the production of LA (de novo synthesis of PUFA), and an ω3 desaturase, with the ability to convert various ω6 PUFA into the corresponding ω3 PUFA products. These findings imply that N. rhodochorde possesses the requisite enzymatic machinery for synthesizing nutritionally valuable LC-PUFA, such as ARA and EPA. This capacity offers the possibility of cultivating these polychaetes using nutritionally poor substrates to create a new alternative and sustainable source of ω3 LC-PUFA-rich ingredients.
This work is part of the POLYPUFA project (PID2022-136234OB-C21), funded by MICIU/AEI /10.13039/501100011033 and by FEDER, UE. Additionally, this study is part of the ThinkInAzul program funded by the European Union NextGenerationEU (PRTR-C17. I1) and the Generalitat Valenciana (THINKINAZUL/2021/026)