Background: Dietary algae have gained considerable attention as natural feed additives in aquaculture owing to their rich nutritional composition and bioactive compounds that can beneficially modulate the intestinal microbiome and improve fish health. However, information regarding the comparative effects of microalgae and macroalgae on the gut microbial ecology of ornamental fish remains limited. Therefore, the present study evaluated the influence of dietary Chlorella sp. and Porphyra sp. supplementation on the gut microbiota of goldfish (Carassius auratus). 
 
Methods: Two parallel feeding experiments were conducted for 75 days using seven isonitrogenous diets comprising a control diet and diets supplemented with Chlorella sp. (5, 7.5 and 10%) and Porphyra sp. (5, 10 and 15%). Gut microbial communities were characterized using 16S rRNA gene-based metagenomic sequencing to assess changes in microbial diversity and taxonomic composition following dietary supplementation. 
 
Result: Metagenomic analysis revealed significant treatment-dependent alterations in gut microbial diversity and community structure. Algal-supplemented groups exhibited greater species richness and alpha diversity than the control, with the highest diversity observed in the 10% Chlorella (FVCT3) and 10% Porphyra (FVPT2) groups. Dietary algae also enriched beneficial bacterial genera, Cetobacterium and other genera like Saccharothrix, Luteolibacter and Pseudoxanthomonas, which are associated with nutrient metabolism, vitamin synthesis and intestinal homeostasis. These findings indicate that dietary Chlorella sp. and Porphyra sp. effectively reshape the gut microbiota of C. auratus, highlighting their potential as sustainable functional feed additives for improving intestinal health in ornamental aquaculture. 
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Comparative Metagenomic Profiling of Gut Microbiome Remodeling Induced by Dietary Chlorella and Porphyra in Goldfish (Carassius auratus)