Effects of solid-state fermentation of Gelidium corneum by-product on immune status and gut microbiota in European seabass

It was previously observed that dietary inclusion of 10 % Gelidium corneum by-product and sunflower cake mixture (1:1 ratio; Gmix), replacing corn gluten meal, reduced feed utilization efficiency in European sea bass juveniles, while the same mixture fermented with Aspergillus ibericus restored feed efficiency as well as nitrogen and energy utilization, achieving levels comparable to the control diet. The present study aimed to further evaluate this unfermented or A. ibericus fermented Gmix by assessing its potential to modulate the intestinal immune and oxidative status, as well as the microbiota profile of European seabass juveniles. At the end of the growth trial, intestinal antioxidant enzyme activities and lipid peroxidation levels were similar among groups, except for glutathione peroxidase activity, which was observed to be higher in fish fed the fermented Gmix diet than in the control diet. Total and reduced glutathione levels were similar among groups, but oxidized glutathione was higher with the fermented Gmix diet than in the other diets. Plasma peroxidase and lysozyme activities and immune-related gene expression in the distal intestine did not show significant differences. Compared to the control diet, dietary inclusion of Gmix, both unfermented and fermented, modulated the intestinal microbiota, particularly at the digesta level, where an increase in Streptococcus , Exiguobacterium , and Bacillus genera, and a decrease in Lactobacillus sp., was observed. Overall, including either unfermented or fermented Gmix in the diet of European seabass did not appear to adversely affect intestinal oxidative or immune status but modulated intestinal microbiota. However, further research is required to fully understand the interactions between European seabass well-being, gut microbiota, and Gmix (both unfermented and fermented), including long-term growth trials, inflammatory insult trials, and next-generation sequencing for intestinal microbiome analysis. • Higher glutathione peroxidase activity was found in fish fed the Gmix-ibericus diet. • Oxidized glutathione levels increased in fish fed the Gmix-ibericus diet compared to other diets. • Plasma peroxidase, lysozyme activities, and immune-related genes were unaffected by dietary treatments. • Both unfermented and fermented Gmix altered intestinal microbiota.