Aquafeed Enrichment with Dictyota Dichotoma and Bacillus Coagulans: A Synergistic Strategy to Promote Growth, Immune Function, and Antioxidant Defensive Pathways in Cyprinus Carpio

This study evaluated the effects of dietary supplementation with Bacillus coagulans and/or the brown macroalga Dictyota dichotoma on growth, blood biochemical indices, immune responses, redox status, and gene regulation in common carp (Cyprinus carpio). A total of 400 fingerlings (initially weighing, 3.0 ± 0.03 g) were randomly divided into 40 hapas (10 fish per hapa). These hapas were assigned into equal 8 experimental groups (each group contain five replicates). The groups included: a control (no supplementation), B. coagulans alone (2 g/kg), D. dichotoma at 2.5, 5, and 10 g/kg (D₁, D₂, D₃), and combinations of B. coagulans (2 g/kg) with each level of D. dichotoma (BC*D₁, BC*D₂, BC*D₃). After 8 weeks of feeding, the results indicated that the diet supplemented with 10 g/kg of D. dichotoma (D3), either alone or in combination with BC, significantly enhanced growth performance, feed conversion ratio (FCR), and feed efficiency ratio (FER). Meanwhile, the fish group fed 2 g/kg of BC showed a notable improvement in feed utilization parameters, although its impact on growth was limited. The whole-body chemical analysis results showed that either BC or D. dichotoma alone significantly increased ash content while decreasing protein and lipid levels in a dose-dependent manner. In contrast, the combined BC*D₃ group significantly enhanced protein retention levels while reducing ash and lipid content. Notably, D3-fed fish showed the highest serum protein content and reduced liver enzyme activities. In contrast, BC alone significantly lowered both liver and kidney health indicators. Additionally, significant interactions between BC and D₃ were observed for protein fraction levels, AST, and kidney function enzymes, with the combined BC and D3 treatment resulting in the most favorable profile. Furthermore, immunity measurements, including lysozyme, complement C3, IgM, and IgD, improved significantly with the combined supplementation of BC and a high level of D. dichotoma. Additionally, the dietary administration of both BC and 10 g/kg D. dichotoma significantly boosted antioxidant enzyme activities (glutathione peroxidase, catalase, and superoxide dismutase) while reducing MDA levels. Additionally, co-supplementation significantly upregulated hepatic expression of IL-6, IL-10, and TNF-2α. These findings highlight the synergistic potential of dietary supplementation with 2 g/kg Bacillus coagulans and 10 g/kg Dictyota dichotoma as effective functional feed additives for enhancing growth performance, immune responses, and oxidative stress resistance in common carp aquaculture.