Effects of common fishery drugs and water pH on nitrification performance and microbial community structure of biochar filler system

As a new type of filler, biochar membrane frame has a higher unit specific surface area, which is conducive to the attachment of denitrification microbial community and has a strong ability to purify sewage. Therefore, it has a broad prospect in the application of constructed wetlands in the future. However, it is not clear whether environmental changes in aquaculture and aquatic medicine will hinder the full use of fillers in constructed wetlands. In order to investigate the influence of external factors on the decontamination capacity of biochar membrane frame, different pH values (E1: 6.5, E2: 7.5 and E3: 8.5) and commonly used aquatic medicines (F1: florfenicol, 6 mg/L; F2: oxytetracycline, 20 mg/L) were applied in this study. The results showed that: ① Nitrospirae had the highest relative abundance among the nitrification bacteria; ② the ammonia-oxidizing rates of E1, E2 and E3 groups were 0.247, 0.249 and 0.305 mg/(L·h), respectively, which were lower than those of the control group (pH=8.2), 0.323 mg/(L·h). At low pH, the relative content of Nitrospira spp. was slightly decreased, but the relative content of Athiobacillus spp. nitrification had no significant difference; ③ the ammonia oxidizing rates in F1 and F2 groups were 0.172 mg/(L·h) and 0.172 mg/(L·h), respectively, which were lower than those in the control group (0.323 mg/(L·h). Compared with the control group, the relative contents of Helicobacter nitrificatus and Athiobacter denitrificatus had no significant change. It was inferred that the florfenicol and oxytetracycline inhibited their activities and reduced the ammonia oxidizing rate. In conclusion, slightly alkaline water is beneficial to the purification of water quality by biocar filler, and florfenicol and oxytetracycline can affect the abundance and activity of denitrification microorganisms on the biofilm and inhibit the ability of ammonia oxidizing rate. Therefore, when antibiotics are used for treatment, other water quality regulation measures should be taken to control the ammonia nitrogen content of aquaculture water, to ensure the safety of aquaculture objects, and maximize the purification effect of biochar fillers for aquaculture effluent.