Functional Characterization of Fad Genes from Two Chemosymbiotic Bivalves Inhabiting the Haima Cold Seep

Deep-sea cold seeps are chemosynthetically driven ecosystems deficient in essential PUFAs. However, the mechanisms by which seep-dwelling bivalves meet their physiological requirements for PUFAs remain poorly understood. Here, we investigated the fatty acid profiles and endogenous biosynthetic capacity of two dominant bivalves from the Haima Cold Seep—the mussel Gigantidas haimaensis and the clam Archivesica marissinica. Fatty acid analysis revealed their high proportions of bacterial-derived MUFAs, consistent with chemosynthetic nutrition. A. marissinica lacked detectable essential PUFAs, while G. haimaensis contained trace arachidonic acid (ARA) and eicosapentaenoic acid (EPA), suggesting partial dietary supplementation. Transcriptome assembly identified three fatty acid desaturase ( Fad ) genes per species, phylogenetically clustering into Δ5 and Δ6/8 clades, with lineage-specific duplications within the Δ5 clade. Functional assays in yeast demonstrated that Δ6/8-clade Fads possess Δ8-desaturase activity enabling LC-PUFA biosynthesis. Δ5-clade isoforms exhibited divergent substrate specificities: GhFads2 and AmFads1 functioned as classical Δ5-desaturases on PUFA substrates, whereas GhFads1 and AmFads2 specifically desaturated the bacterial MUFA C18:1n-7 to produce the non-methylene-interrupted (NMI) PUFA C18:2n-7—a precise nutritional adaptation to the n‑7 fatty acid-rich seep environment. Physiological assays using GhFads1-transformed yeast showed that NMI C18:2n-7 confers cold tolerance comparable to conventional PUFAs and provides superior protection under high hydrostatic pressure. Our results reveal that cold-seep bivalves retain endogenous LC-PUFA biosynthetic capacity and have evolved duplicated Δ5-desaturases with novel regioselectivity toward bacterial MUFAs. The resulting NMI fatty acids likely represent adaptive membrane modifications for survival under extreme deep-sea conditions.