Intestinal Histology and Gut Microbiota Composition of Tilapia (Oreochromis sp.) Under Dietary Alginate Nanoparticles Derived From Sargassum polycystum.

This study evaluated the effects of alginate nanoparticles (NPs-A) extracted from Sargassum polycystum on gut bacterial community composition and intestinal histology in tilapia (Oreochromis sp.). NPs-A were prepared using ball milling to obtain nanoscale alginate with improved functional properties. Five dietary treatments were evaluated in triplicate: a negative control, a positive control (2 g alginate kg⁻¹ feed), and diets supplemented with 0.5, 1.5, or 2 g NPs-A kg⁻¹ feed. Tilapia (10.3 ± 0.11 cm) were stocked at a density of 125 fish m⁻³ in conical fiber tanks with a diameter of 100 cm and fed the experimental diets for 60 days. Intestinal samples were collected on days 0 and 60 for histological evaluation and bacterial community profiling based on the V3-V4 region of the 16 S rRNA gene. Dietary NPs-A supplementation was associated with differences in gut bacterial community composition and intestinal histological characteristics among treatments. Bacterial communities were dominated by Fusobacteriota and Bacteroidota across all treatments, indicating the presence of a relatively stable core microbiota, while Cetobacterium remained the dominant genus. Among the evaluated treatments, supplementation with 0.5 g kg⁻¹ NPs-A showed the most favorable overall response, characterized by the highest villus length and villus-to-intestinal diameter ratio, together with distinct bacterial community profiles. Higher NPs-A doses (1.5-2 g kg⁻¹) were associated with greater intestinal muscle thickness but lower villus development. Overall, dietary NPs-A derived from S. polycystum was associated with changes in gut microbiota composition and intestinal histology in tilapia. However, because gut microbiota analyses were based on pooled intestinal samples and microbial community differences were evaluated descriptively, further studies incorporating biological replication and functional analyses are required to validate these findings.