Effects of biofloc-electrochemical system on water quality, fish performance, and microbial diversity

This study investigated the effects of the in-situ biofloc-electrochemical system (BES) on nitrogen (N) and phosphorus (P) removal, growth performance, health status of Nile tilapia (Oreochromis niloticus), and microbial composition in both biofloc and the intestinal tract. Tilapia were cultured in a biofloc system with microcurrent direct current electric fields at 0, 100, and 200 mA for 36 days. Results showed that electrolysis significantly enhanced the degradation of ammonia nitrogen (TAN), nitrite nitrogen (NO₂^--N), phosphate (PO₄^3--P), and total phosphorus (TP) (p < 0.05), but had no significant effect on nitrate nitrogen (NO₃^--N) removal (p > 0.05). In the 100 mA group, no significant differences were observed in feed conversion ratio (FCR) and specific growth rate (SGR) compared to the control group (p > 0.05). However, in the 200 mA group, FCR was significantly increased, and SGR was significantly decreased (p < 0.05), indicating that higher current levels suppressed tilapia growth performance. Enhanced digestive capacity in the intestinal tract was observed, with minimal oxidative stress effects in the gills and liver. Electrolysis also significantly altered the microbial composition, abundance, and diversity in both biofloc and intestinal microbiota (p < 0.05). Notably, increases in Actinobacteriota and Bacteroidota may improve tilapia immunity and nutrient cycling, influencing both growth and water quality. These findings suggest that while BES effectively improves water quality, further optimization is needed to enhance fish growth performance and nitrogen removal efficiency.