Introduction
Biomass from microalgae has the potential to replace fish- and soybean-based ingredients in aquafeeds, improving the sustainability of the aquaculture sector. Nevertheless, the incorporation of microalgae can have an impact on the end cost of feeds. Furthermore, the rigid cell wall of microalgae can lower the nutrient bioavailability and digestibility of farmed fish. Through the present work, we show that by adopting a circular economy approach that relies on re-using nutrients and water from hydroponic drain water for microalgae growth, we can upgrade the produced biomass using a biorefinery process, thus developing innovative bioproducts for agriculture and aquaculture markets.
Material and methods
Tetradesmus obliquus and Raphidonema monicae were produced in an industrial 19-m3 tubular photobioreactor at Necton S.A. facilities , using hydroponic drain water as culture medium , reaching a dry weight of 1.8 and 1.6 g.L-1 , respectively. After harvesting the biomass, a simple biorefinery approach (high-pressure homogenization and centrifugation) was applied to generate two bioproducts: aqueous extracts for the agriculture sector and residual biomass for the aquaculture sector. The present study focused on t he residual biomass of T . obliquus and R. monicae that was incorporated at 10% into experimental aquafeeds as a substitute for fishmeal. Two experimental diets containing the respective microalgal biomass and a commercial-like diet (control diet with fishmeal) were offered to Atlantic salmon ( Salmo salar) post-smolts with an average start weight of 150.70±0.06 g. The experimental fish were reared in flow- through tanks in an indoor seawater research facility, and fed to satiation, twice a day, for 9 weeks. The feeding trial aimed to examine fish growth, feed performance, fillet quality as well as the intestinal health of the fish.
Results
At the end of the experiment, the final average weight of the fish was 368.70±1.20 g. We did not detect significant differences in specific growth rate (average 1.38±0.02 %.day-1) and feed conversion ratio (average 0.75±0.02) among the dietary treatments . The proximate composition of the whole body did not reveal treatment-linked differences: protein, fat, and ash content ranged between 52.69-54.55 % dry matter (DM), 13.83-14.22 % DM, and 1.28-1.45% DM , respectively. The fatty acid profile of the salmon fillet was determined as well, and the total polyunsaturated fatty acid content was in the range of 30.42-30.56%, with the sum of eicosapentaenoic and docosahexaenoic acids ranging between 10.74-11.09% of total fatty acids. With regards to digestibility, the diet containing R. monicae demonstrated a significantly higher protein digestibility when compared with that of T. obliquus and the control diet, while displaying a significantly higher lipid digestibility compared to that of T. obliquus . Histological analysis revealed that the fish fed T. obliquus had shorter villi and the fish fed R. monicae had significantly higher number of goblet cells per villi and significantly higher number of neutrophils. Nevertheless, analysis of the expression of genes connected to inflammation and tight junction proteins did not point to the adverse effects of the microalgal biomass.
Conclusion
The processed microalgal biomass of Tetradesmus obliquus and Raphidonema monicae can be effective substitutes of fish meal in diets for Atlantic salmon smolts. This study points to the potential value of the tested circular approach in developing new ingredients for aquafeeds.
Acknowledgments
Work funded by EEA Grant ALGACYCLE-PT-INNOVATION-0023, by the Portuguese national funds from FCT - Foundation for Science and Technology through projects UIDB/04326/2020, UIDP/04326/2020, LA/P/0101/2020 and through the doctoral research fellowship (2021.06332.BD).