From invasion to fish fodder: Inclusion of the brown algae Rugulopteryx okamurae in aquafeeds for European sea bass Dicentrarchus labrax (L., 1758)

In keeping with the premises of Blue Circular Economy in the European Union, the present study explored the possibility of using the invasive brown alga Rugulopteryx okamurae in aquafeeds for European sea bass (Dicen-trarchus labrax). Assuming the raw algae biomass could negatively impact animal performance, four experi-mental formulations were prepared, by including macroalgae material at 5%, using crude (CR), enzymatically hydrolysed and fermented (EF), enzymatically hydrolysed (E), or fermented (F) R. okamurae biomass, which we tested against a control feed (CT). To evaluate the effects of the experimental diets, besides animal growth performance and biometric parameters, we devised a toolbox focused on the intestine and intestinal function: i) ex-vivo epithelial resistance and permeability in Ussing chambers; ii) microbiota composition through NGS; iii) expression profiles of selected markers for epithelial integrity, transport, metabolism, and immune response, by qPCR. Our results show differentiated allometric growth among diets, coupled with intestinal epithelium al-terations in permeability, integrity, and amino acid transport. Additionally, evidence of microbiota dysbiosis and contrasting immune responses between experimental diets, i.e. pro-inflammatory vs. anti-inflammatory, are also described. In conclusion, we believe that R. okamurae could be a suitable resource for aquafeeds for the European sea bass, although its use requires a pre-treatment before inclusion. Otherwise, while the fish still have a positive growth performance, the gastrointestinal tract pays a toll on the integrity, transport, and inflammatory processes.