Effect of Exogenous Dietary Protease and a Humic Substance on Growth and Microbial Composition of Nile Tilapia (Oreochromis niloticus) and Romaine Lettuce (Lactuca sativa) in a Biofloc-Integrated Decoupled Aquaponics System

Aquaponics and biofloc-based aquaculture systems are environmentally sustainable food production systems. When biofloc technology is combined with hydroponic systems, it expands economic diversity by producing additional value-added products. Dietary additives were incorporated into fish feeds in this trial to improve food production in decoupled aquaponic systems. In this decoupled aquaponic system, the biofloc aquaculture system consisted of nine cylindrical tanks of 3750 L with 170 juvenile Nile tilapia (9.99 ± 0.54 g) stocked in each tank. The fish were randomly assigned to one of three treatment groups: fish fed a commercial feed (Com), commercial diet + protease complex (PC; AG175 by Jefo Nutrition; 175 g 1000 kg⁻¹), and commercial feed + humic substance (HS; MFG50 by Kent Nutrition; 2500 g 1000 kg⁻¹). The feeding trial was conducted for 60 days. A deep-water culture hydroponic system with nine 109 L troughs was used. Each trough corresponded to each tank and contained 15 romaine lettuce plants. The first 4 weeks were fish growth trials, and the last four were fish and plant growth trials. Fish and plant growth parameters showed no differences between treatments. However, the leaf greenness in romaine lettuce plants in the PC treatment had significantly higher chlorophyll content than the other treatments (p = 0.002). When microbial communities were analyzed, significant differences were found in the Shannon diversity indices (fecal, water, and root p < 0.001), Chao1 (fecal p < 0.002, water p < 0.001, and root p < 0.001), and observed species (feca; p = 0.046, water p = 0.017, and root p = 0.015). When beta diversity was analyzed through the Bray–Curtis dissimilarity index, fecal samples (p = 0.001) and root samples (p = 0.002) showed clear differences. The most abundant phyla found in all samples were Pseudomonadota. Overall, this study concluded that these additives did not affect the growth of Nile tilapia and romaine lettuce but influenced the bacterial composition of fish feces, water, and root samples and the chlorophyll content of the plants.