Effect of the use of Bacillus spp. on the characteristics of dissolved fluorescent organic matter and the phytochemical quality of Stevia rebaudiana grown in a recirculating aquaponic system

The effect of the incorporation of mineralizing Bacillus spp. on the characteristics of fluorescent organic matter (FDOM) in a recirculating aquaculture system (Nile tilapia-Stevia rebaudiana) was evaluated. EEM-PARAFAC analysis was used to determine the composition of the dissolved organic matter and to study its relationship with nitrogen transformation. The composition and antioxidant activity of Stevia leaves were used as indicators of the benefits of bacterial supplementation on nutrient absorption. Two systems were used, each consisting of a circular fish tank (1.7 m3) and six units of the nutrient film (0.18 m3). One system was supplemented with bacteria (BS), while the other was used as control (NBS). The inclusion of Bacillus spp. facilitated mineralization and the availability of total phosphorus (TP), K+, and nitrogen, and also controlled the total ammonia nitrogen (TAN) for 56 days without water exchange. FDOM was modeled by four components (3-humic-like, 1-protein-like), which were good indicators of the process of mineralization. The fluorescence intensity in the biofilter was significantly correlated with TP, K+, temperature, and the absorption coefficient a254. The fluorescence index (FI) was a good indicator of the process of nitrification. Plants from BS required 46.4% less NO3– and 47.8% less K+ compared to the control, and absorbed 45.1% more TP. BS-Stevia leaves produced 38.6% more reducing sugars, 28.6% more flavonoids, and 35.9% more glycosylated flavonoids than the control. The fish in the BS system reached a higher final weight than NBS, resulting in a 1 kg/m3 higher gross yield. Even so, it will be necessary to reduce the pH of the water to increase the antioxidant scavenging capacity of the plants.