Effect of Dissolved Oxygen on Poly(3-Hydroxybutyrate-co-3-Hydroxyvalerate) Solid-Phase Denitrification: Simultaneous Nitrification and Denitrification Performance and Microbial Community

Effective removal of ammonia, nitrate, and intermediate nitrite is a challenge faced by highly dissolved oxygen (DO) recirculating aquaculture systems (RAS). This study investigated the effect of DO concentrations (Group A, DO 3.15 ± 0.12 mg/L, Group B, DO 4.43 ± 0.15 mg/L, Group C, DO 6.52 ± 0.35 mg/L, Group D, and DO 7.86 ± 0.55 mg/L) on the simultaneous nitrification and denitrification (SND) of aquaculture seawater using poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) as a biofilm carrier and solid-phase denitrification (SPD) carbon source. Ammonia and nitrate were simultaneously removed in all the PHBV-supported SPD systems. Ammonia in Group C had the highest removal rate (5.72 mg/(L·h)), whereas nitrate and nitrite in Group A were completely removed at 4 and 5 h, respectively. The continuous release of PHBV provided carbon sources for denitrification. High throughout sequencing showed that Proteobacteria, Bacteroidetes, Desulfobacterota, and Chloroflexi at the phylum level, and Marinobacter, Shewanella, and Pseudomonas at the genus level exhibited unique relative abundances under varying DO concentrations. The nitrification genes in Group C showed the highest expression. Denitrification genes in Group A were enriched. The relative expression of nitrogen-transforming genes further demonstrated the results for water quality and the microbial community. DO concentration affected the efficiency of the nitrogen transformation pathway in the PHBV-supported SPD system.