Effects of humic acid on the nitrogen utilization efficiency and microbial communities in aquaponic systems

Aquaponic systems consisting of recirculating aquaculture systems and hydroponic beds. Raft-floating aquaponics integrated with a moving-bed biofilm reactor (MBBR) was established. Humic acid (HA) was added to the aquaponic systems to promote nutrient recovery. An elementary analyzer was used to determine the nitrogen content of fish, plants, and sediments to calculate the NUE. High-throughput sequencing and quantitative polymerase chain reaction (qPCR) were conducted to reveal the influence of HA on microbial communities and nitrification genes. During an 80 day experimental period, the effects of water quality parameters, nitrogen utilization efficiency (NUE), and microbial communities in variable concentrations of HA-based aquaponic systems were compared. Among the HA-based systems, the one with an optimal concentration of 15 ppm indicated the best performance. In HA-based aquaponic systems with 15 ppm concentration, the maximum concentrations of total ammonia nitrogen and NO₂⁻–N were 6.02 mg/L and 12.693 mg/L, which were significantly lower than those in aquaponic systems with 0 ppm and 30 ppm. However, the accumulation of nitrate in the HA-based aquaponic systems with 15 ppm was greater than that in the others. The NUE of the 15 ppm HA-based aquaponic system was 58.34%, which was significantly higher than that of the 0 ppm (50.83%) and the 30 ppm (47.25%) systems. Consequently, the threshold concentration of HA was established as 15 ppm. The microbial communities of different HA-based systems revealed the presence of denitrifying and nitrifying microbial populations for nitrogen recovery in moving bed biofilm reactors (MBBRs); higher levels of these microbial populations thrived in the 15 ppm HA system than in the other systems. However, there was no significant difference in microbial communities from root samples. The highest copies of amoA and nxrB were found at 15 ppm for both MBBRs and roots, indicating that the addition of HA to the aquaponic systems yielded more products and improved water quality and NUE by changing the composition of microbial communities based on the threshold for HA input concentration. This study provided that HA show enormous potential for an interesting research and the development of green technologies for biological wastewater treatment and bioremediation.