A net gain in long-chain polyunsaturated fatty acids (PUFA) occurs in Arenicola marina bioturbated mesocosms in the light and dark when brewery yeast is the only food. Fatty acid profiles were determined by GC-MS and quantified for system units. Bacterial fatty acid biosynthesis is indicated by accumulation of the bacterial fatty acids anteiso 14:0, iso 15:0, iso 16:0, anteiso 16:0, iso 17:0, C16:1n-7 and C18:1n-7 (cis-vaccenic acid). De novo production of longer chain PUFA within the worm tissues is implicated by a chain elongation converting linoleic acid (C18:2n-6) to C20:2n-6 and subsequent desaturation, resulting in a gain of arachidonic acid (C20:4n-6) and eicosapentaenoic acid (C20:5n-3) via the intermediary C20:3n-6. To test for the presence of PUFA-synthesising bacteria in system units, 495 cloned 16S rDNA fragments were compared with the databases and degenerate PCR primers were designed for the ketoacyl acyl carrier protein synthase gene highly conserved in the polyketide synthase region of known PUFA-synthesising Gammaproteobacteria. Gammaproteobacteria related to known PUFA producers were not abundant (<1% of clones) and PCR primers, whose specificity was confirmed by conspecific amplification of DNA product from genomic DNA of Shewenella frigidimarina 12253 T and Colwellia psychrethraea 8813 T , did not amplify product from mesocosm DNA samples. PUFA production is therefore primarily due to processes within the lugworm A. marina. Biosynthesis by invertebrates may be significant in benthic ecosystems and their culture has the potential to contribute to non-fishery sources of essential fatty acids required for aquaculture feeds.KEY WORDS: Fatty acid biosynthesis · Elongase · PUFA · Polychaeta · Marine food web · Arenicola marina · Aquaculture nutrition · Aquaculture feeds Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 387: [223][224][225][226][227][228][229][230][231][232][233][234][235][236][237][238][239] 2009 head shaft) lying above the feeding pocket and contribute to the diet, but algae are not utilised effectively (Rijken 1979) and the presence of cellulases has not been conclusively demonstrated (Longbottom 1970, nor since). On the other hand feeding on bacteria associated with enriched sand is considered likely. The short residence time of the ingested sand in the gut (Hüttel 1990, Chen & Mayer 1999 makes it unlikely that internal fermentation (as in ruminants) occurs; rather, the lugworm-bioturbated burrow/sand-substrate system may act as an external rumen in which bacteria are cultivated, ingested and digested -a process of food enhancement referred to as gardening (Plante & Mayer 1994. Feeding at depth on anaerobic sediment is most characteristic of A. defodiens (Cadman & Nelson-Smith 1993) but it is also observed in A. marina when feeding on sediments rich in organic material in nature (Cadée 1976) and in the laboratory (Rijken 1979), and is routinely observed in commercially farmed populations 1 (S. Craig unpubl. data, P. J. W...