Unique fatty acid desaturase capacities uncovered in Hediste diversicolor illustrate the roles of aquatic invertebrates in trophic upgrading

Omega-3 (ω3 or n-3) long-chain polyunsaturated fatty acids (PUFA), including eicosapentaenoic acid and docosahexaenoic acid (DHA), play physiologically important roles in vertebrates. These compounds have long been believed to have originated almost exclusively from aquatic (mostly marine) single-cell organisms. Yet, a recent study has discovered that many invertebrates possess a type of enzymes called methyl-end desaturases (ωx) that enables them to endogenously produce n-3 long-chain PUFA and could make a significant contribution to production of these compounds in the marine environment. Polychaetes aremajor componentsofbenthic fauna andthusimportant tomaintain a robust foodweb as a recycler of organic matter and a prey item for higher trophic level species like fish. In the present study, we investigated the ωx enzymes fromthe common ragworm, Hediste diversicolor, a common inhabitant in sedimentary littoral ecosystemsof theNorthAtlantic. Functional assays of the H. diversicolor ωx demonstrated unique desaturation capacities. An ω3 desaturasemediated the conversion of n-6 fatty acid substrates into their corresponding n-3products includingDHA.Afurther enzymepossessedunique regioselectivities combining both ?6 andω3 desaturase activities. These results illustrate that the long-chainPUFAbiosynthetic enzymaticmachineryof aquatic invertebrates such as polychaetes is highly diverse and clarify that invertebrates can be major contributors to fatty acid trophic upgrading in aquatic food webs. This article is part of the theme issue 'The next horizons for lipids as 'trophic biomarkers': evidence and significance of consumer modification of dietary fatty acids'.