Abstract
The rapid growth of aquaculture has prompted the exploration of alternative and sustainable feed ingredients. Polychaetes, such as Hediste diversicolor, offer promise in the circular economy due to their ability to recycle waste materials from bioindustries and convert low nutritional value fatty acids (FA) into eicosapentaenoic (EPA) and arachidonic (ARA) acids. This study explored the combined effect of temperature and salinity on growth and LC-PUFA biosynthesis in H. diversicolor fed on a side stream diet (33% sludge from salmon aquaculture (AS) and 66% solid-phase digestate from biogas production (SBD)). Polychaetes were cultured for 28 days along a 5-step temperature and salinity gradient ranging from 7.7 to 17.9 °C and 5 to 40 psu, respectively. Growth was highest at high temperatures (≥ 14.3 °C) and salinities (≥ 35 psu). The FA composition remained relatively stable across different culture temperatures, while salinity exhibited higher levels of n − 3 PUFA and n − 3 LC-PUFA at lower salinities (5 and 15 psu) compared to higher salinity (35 psu), mainly driven by EPA. This suggests that salinity may exert a more pronounced effect than temperature on the FA profiles of H. diversicolor. Despite low temperature and high salinity (40 psu) significantly upregulating the expression of elongase and desaturase genes, this was not translated into higher LC-PUFA. Remarkably, polychaetes exhibited high levels of n − 3 and n − 6 LC-PUFA, even when fed a diet deficient in such FA. These findings highlight the potential of H. diversicolor for LC-PUFA biosynthesis, positioning this species as a promising candidate for producing high-nutritional-value ingredients for aquafeeds.