Ophryotrocha craigsmithi (Wiklund, Glower & Dahlgren, 2009) has a high capacity to modify lipids from aquaculture waste and synthesize long-chain polyunsaturated fatty acids

In Norway, Ophryotrocha craigsmithi is being evaluated as an extractive species in integrated multitrophic aquaculture systems (IMTA) and has been suggested as a suitable novel feed ingredient in aquatic feeds due to its high content of polyunsaturated fatty acids (PUFA). It has been hypothesized that O. craigsmithi is capable of trophic up-grading, and that bacterial mats play a crucial role in this species diet, enhancing growth rates and defining FA profiles. We studied the FA uptake and metabolism in O. craigsmithi reared on diets consisting of commercial fish pellets, bacteria incubated fish pellets or sludge collected in situ . We aimed to confirm whether O. craigsmithi performs trophic up-grading, and to elucidate the role of white bacterial mats for this species. FA profiles of diets and polychaetes were determined using GC-FID analysis, and lipid classes were separated by HPLC. Results indicate that O. craigsmithi performs trophic-upgrading and particularly produces high amounts of 20:4n-6 and 20:5n-3. We suggest that O. craigsmithi applies the Δ12 (methyl-end) enzyme to produce 18:2n-6 de novo and further use this precursor to produce 20:4n-6 before the Δ17 (methyl-end) enzyme is used for the final desaturation to 20:5n-3. Analysis of the phospholipids of polychaetes showed that FA profiles remained stable independent of diet, illustrating a high priority for cell membrane functionality. Bacterial FAs were mainly used as energy, although c18:1n-7 was selectively incorporated into cell membranes in polychaetes. Our results highlight this species' value in converting farm waste to high quality marine long-chain PUFA in IMTA systems. • Ophryotrocha craigsmithi appears to have the enzymes for de novo synthesis of long chain polyunsaturated fatty acids. • Bacterial fatty acids were mainly utilized as energy, although some were selectively retained in cell membranes. • Ability to synthesize essential fatty acids shows this species' value in aquaculture waste valorization.