Effects of feeding European seabass (Dicentrarchus labrax) juveniles with crude, hydrolysed and fermented biomass of the invasive macroalga Rugulopteryx okamurae (Ochrophyta)

The increasing expansion of the invasive Asian alga Rugulopteryx okamurae along the Andalusian coasts is a major challenge to marine biodiversity, and urgent coordinated measures are required for its removal or elimination. Among the different actions, the biotechnological valorisation could be a strategy for the management of alien biomass within the frame of the Blue Economy. This work evaluates the potential of R. okamurae biomass, either raw or processed by biotechnological treatments based on enzymatic hydrolysis and fermentation processes, to be used as a dietary ingredient in juvenile seabass (Dicentrarchus labrax). These treatments, intended primarily to improve the nutrient availability, might also decrease the content of undesirable and potentially harmful algal metabolites, such as the diterpenoid dilkamural. To this end, a 90-day feeding trial was carried out using four experimental diets containing 5% (w/w) crude or processed R. okamurae biomass. The results obtained revealed that the inclusion in the diet of raw R. okamurae caused adverse effects on fish growth, yielding lower values than those of the control group (p < 0.05), although this detrimental effect was negligible when the algae was processed prior to its inclusion in the experimental feeds. In terms of muscle composition, diets supplemented with the algal biomass induced an increase in protein content, especially in fish fed with pre-treated biomass. Qualitative differences (p < 0.05) were also found in muscle fatty acid profile, with a significant increase in ARA and DHA contents in fish fed on algae-supplemented diets. A significant reduction (p < 0.05)in muscle and liver lipid oxidation was evidenced in fish fed the algae-enriched diets, especially with treated biomass. Finally, 5% R. okamurae raw biomass caused negative effects on gut functionality, although these effects were not observed when the algal biomass was previously processed. In conclusion, the results revealed the potential of R. okamurae as a functional ingredient for the feeding of juvenile sea bass, provided that the biomass is pre-treated, and corroborated the efficacy of a low-cost biotechnological treatment as a valorisation method for the biomass of this invasive algae