The early intestinal response of Mugil cephalus to fishmeal replacement with alternative protein sources under biofloc technology (BFT) was assessed. Juvenile mullet (82.68 ± 0.05 g) were distributed among three dietary treatments with decreasing fishmeal inclusion levels of 15%, 5%, and 0%, corresponding to FM15, FM5, and FM0, respectively. After 27 days of feeding, intestinal samples were analyzed for microbiota composition, transcriptomic responses and enzymatic activity. The FM0 group exhibited reduced intestinal enzyme activity, whereas no significant differences were observed in terms of gut microbiota among the diets. Furthermore, transcriptomic analysis revealed differentially expressed genes associated with enriched GO terms and KEGG pathways related to lipid metabolism. This response was consistent with the lower mesenteric fat indices and whole-body lipid content observed in fish fed the FM0 diet, suggesting an early metabolic adjustment associated with complete fishmeal replacement and the higher dietary protein content. All diets exhibited high protein efficiency ratios (9.98 ± 2.42) and low feed conversion ratios (0.40 ± 0.17), with no significant differences observed among them. However, complete fishmeal replacement negatively affected intestinal physiology and lipid storage, indicating that these alterations were not fully compensated for by the biofloc system and may not be physiologically sustainable over longer periods. Further studies over an extended period could be carried out to evaluate the long-term effects of these diets.

This study aimed to evaluate the effects of establishing a population of the green microalgae Chlorella sp. on different key aspects of a biofloc culture of Penaeus vannamei in comparison with those obtained with a commercial probiotic. The aspects evaluated were shrimp zootechnical performance, nutritional composition of biofloc and microbial populations of shrimp intestinal microbiota and biofloc. Four treatments were tested: two of them included Chlorella sp. alone (M) or in combination to a bacterial probiotic (MP), while the other two included only the probiotic (P) or none of them (C). The presence of Chlorella sp. enhanced protein and phosphorus bioavailability in the biofloc and contributed to nutrient bioremediation within the system. However, shrimp growth performance and survival were significantly reduced, likely due to greater and more prolonged nitrate exposure following microalgae inoculation. Although overall nitrate and phosphate concentrations were higher in microalgae treatment, their accumulation over the experimental period was lower in the presence of Chlorella sp. In addition, microalgal supplementation reduced the abundance of bacterial groups associated with oligotrophic lifestyles, complex organic matter degradation, and nitrogen cycling in the intestinal microbiota. In contrast, probiotic supplementation increased protein content in shrimp and phosphorus bioavailability in the biofloc, while zootechnical performance remained comparable to the control and was higher than in microalgae treatments. Furthermore, probiotic addition reduced the abundance of Flavobacteriaceae, a bacterial family proposed as a bioindicator of White Feces Syndrome (WFS) in P. vannamei.