Effects of Gallic Acid Supplementation on Intestinal Function and Gut Microbial Community Structure in Holothuria...

Gallic acid (GA) is a plant-derived polyphenol with antioxidant and antimicrobial activities, yet its efficacy as a functional feed additive in sea cucumbers remains unclear. Here, we systematically evaluated graded dietary GA supplementation in the tropical sea cucumber Holothuria leucospilota by integrating growth performance, body-wall nutritional composition, intestinal histomorphology, digestive and antioxidant enzyme activities, gut microbial community profiles, and intestinal transcriptomic responses. Sea cucumbers were randomly assigned to six dietary treatments containing 0, 200, 400, 800, 1600, or 3200 mg kg^-1 GA and fed for 75 days. GA elicited a pronounced nonlinear, dose-dependent response. The intermediate dose (GA2; 400 mg kg^-1) produced a relatively favorable response under the present feeding conditions, increasing final body weight and increasing the accumulation of crude protein and structural-protein-associated amino acids in the body wall, while preserving brush-border integrity and significantly enhancing α-amylase, lipase, cellulase, and superoxide dismutase (SOD) activities. Transcriptomic profiling revealed clear divergence between GA-treated groups and the control (Con), with GA2 characterized by upregulation of pathways associated with digestive hydrolysis, nutrient transport, lipid utilization, and redox/detoxification processes. Microbiome analyses showed progressive community restructuring with increasing GA, including reduced Proteobacteria and enrichment of Firmicutes/Bacilli at high doses, decreased α-diversity, and pronounced genus-level turnover. Correlation-based integration further resolved two host-microbe interaction modules that linked microbial taxa either to nutrient assimilation programs or to intestinal barrier and immune-response programs. Collectively, these findings support GA as a promising functional additive for H. leucospilota within a narrow optimal-dose window, while highlighting potential risks of dysbiosis and intestinal injury at excessive inclusion levels.