Actinomycetota-Mediated Solid-State Fermentation of Gelidium sp. By-Products: Evaluation of Digestibility in European Seabass

Aquaculture plays an important role in the circular economy as the key instrument to promote development, reduce poverty, and enhance global food security. This research aims to evaluate the benefits of employing circular economy concepts with regard to aquaculture, particularly the application of industrial by-products and macroalgae to form a sustainable aquafeed. The study assesses the suitability of GeIidium by-product as a feed ingredient and investigates the concept of solid-state fermentation with actinomycetota strains obtained from macroalgae, specifically, the relevance of solid-state fermentation to reduce crude fiber content and improve digestibility of GBPs by European seabass was evaluated. In the first phase, the essential characteristics of unfermented GBP were analyzed to asses digestibility by European seabass. As a result, with the increase, the protein digestibility coefficient decreased, and the G15 diet demonstrated the minimum result, which is 85.6%. As for the G15NaOH diet, the treatment with NaOH led to the achievement of 91.4%, and the digestibility coefficient of lipid was stable in all of the treatments. In the second phase, three actinomycetota strains were screened to reduce crude fiber in GBPs through solid-state fermentation. CeIIuIosimicrobium funkei (R177) was reduced neutraI detergent fiber (NDF) from 59.1% to 55.0% and significantIy Iowering acid detergent fiber (ADF). Streptomyces vioIascens (F26) further reduced NDF to 53.5%, whiIe Rhodococcus chondri (R104) achieved moderate reductions. These resuIts demonstrate SSF’s potentiaI to enhance GBP digestibiIity and nutrient avaiIabiIity in fish diets. The present study concludes that solid-state fermentation can be used to reduce fiber and improve the digestibility of GBP in fish diets.