Optimization of nitrogen removal and microbial communities in biofloc systems via carbon-to-nitrogen ratio regulation and Candida utilis.

Biofloc-based systems provide a sustainable strategy for nitrogen management in intensive aquaculture, but their efficiency depends strongly on carbon availability and microbial community regulation. In this study, we evaluated the combined effects of carbon-to-nitrogen (C: N) ratio regulation and Candida utilis supplementation on nitrogen removal and microbial community dynamics in a biofloc-based Chinese soft-shelled turtle culture system. Three C: N ratios (10, 15, and 20) were established using glucose as the carbon source, with or without C. utilis addition. Water quality parameters, feed conversion ratio (FCR), bacterial community composition (16 S rRNA sequencing), and antagonistic activity against Aeromonas hydrophila were analyzed. Increasing the C: N ratio significantly enhanced ammonia nitrogen removal through stimulated heterotrophic assimilation. Supplementation with C. utilis further reduced inorganic nitrogen concentrations across all C: N conditions and improved feed utilization efficiency. Microbial community analyses showed increased bacterial richness and diversity, accompanied by significant shifts in community composition. Notably, opportunistic pathogens such as Aeromonas were markedly suppressed in yeast-supplemented treatments. Co-culture experiments further demonstrated that live C. utilis inhibited A. hydrophila, suggesting a metabolism-dependent antagonistic mechanism. These findings indicate that integrating C: N regulation with C. utilis supplementation can enhance nitrogen transformation efficiency and microbial stability in biofloc systems, providing a promising strategy for sustainable aquaculture management.