Pre-treatment of Ulva rigida improves its nutritional value for European seabass (Dicentrarchus labrax) juveniles

Abstract

Macroalgae are promising ingredients for aquafeeds, but their recalcitrant polysaccharide structure limits their wide use. To disrupt this structure, different physical (ultra-sounds, autoclave, microwaves), chemical (acid, alkaline), and biotechnological (solid-state fermentation, SSF; SSF followed by sequential hydrolysis, SSF-SH) treatments were carried out in Ulva rigida. The chemical composition, morphological microstructure, release of reducing sugars, soluble protein, and phenolic compounds were evaluated in the final products.All treatments increased U. rigida protein content, while lipid content increased after autoclave, ultra-sound, microwave, and SSF treatments. SSF-SH treatment was more effective in disrupting the cellulose and hemicellulose fractions, resulting in a higher reducing sugars release. The alkaline treatment was more efficient in reducing lignin content and increasing phenolic compounds, antioxidant activity, and soluble protein of U. rigida.The most promising products were then included at 5 % in diets for European seabass (Dicentrarchus labrax) juveniles. Crude, ultra-sound, alkaline, and SSF-SH-treated U. rigida reduced fish growth, while SSF-treated U. rigida resulted in similar growth as the positive control (fish meal-based diet). Feed intake was reduced in all experimental groups, but the feed efficiency of fish fed SSF-U. rigida diet and the protein efficiency ratio of fish fed crude, alkaline, and SSF-treated U. rigida were higher than the positive control. The highest total alkaline proteases activity was achieved with the crude U. rigida diet, while no differences were observed in fish fed the positive control, ultra-sound, and SSF diets.Overall, of the processing methods tested, the most efficient was SSF, which improved feed efficiency of European seabass juveniles fed U. rigida-including diet without affecting growth performance.