Thiobacillus denitrificans enhances nitrogen removal, growth performance, gut histomorphometry, and immunity in fish...

Recirculatory aquaculture systems (RAS) are one of the most sustainable solutions for Nile tilapia, Oreochromis niloticus, production. Nitrogenous waste accumulation has been a major constraint to its usage, especially on water quality and fish health. Effects of Thiobacillus denitrificans on nitrogen removal, water quality, microbial abundance, and growth performance as well as the immunity of Nile tilapia cultured in RAS were examined in this study. Three treatments were prepared that comprised a control (CTR = no inoculation, standard biofiltration, and sulfur pellets as electron donor), a conventional nitrifying biofilter (CNB = enriched with Nitrosomonas and Nitrobacter (1.0 × 10⁶ CFU mL^-1)), and a T. denitrificans-inoculated biofilter (TDB = enriched with T. denitrificans (1.0 × 10⁶ CFU mL^-1) for 56 days). Results showed that total ammonia nitrogen (P = 0.001), nitrite (P = 0.001), and nitrate (P = 0.001) were greatly lowered in TDB compared to the CTR, with the lowest levels found in TDB and the highest levels found in CTR. Also, total suspended solids (P = 0.001) and chemical oxygen demand (P = 0.001) were significantly lower in TDB than in the CTR group. It is observed that fish on TDB had a considerably increased final body weight (+44.4%) and body weight gain (+49.2%) compared to the control. The addition of TDB in RAS used to raise Nile tilapia greatly (P = 0.001), increasing TP level accompanied by marked reduction in GLU (P = 0.001) and CHO (P = 0.024) and ALT (P = 0.021) levels. Antioxidant profiles, acetic viscosity, and butyric acid (P < 0.05) were significantly elevated in fish raised in RAS inoculated with the respective microbial treatments compared to the fish in the control group, as was gut histomorphometry. Also, immune profiles (respiratory burst activity, lysozymes, protease, and esterase activities) were significantly (P < 0.05) elevated in fish raised in TDB, while the least were observed in the control group. This study demonstrated that T. denitrificans improves nitrogen cycling and removal; water stability in RAS, which results in indirectly induced growth; and enhanced immunity in Nile tilapia, thus offering a promising biotechnological approach for sustainable aquaculture.