Abstract:[Background] The second aerobic bioreactor O2 of the oxic-hydrolytic-oxic (O/H/O) system contributes to the biomineralization and complete nitrification of residual pollutants, which is important for the standard discharge of wastewater. [Objective] To elucidate the structure and functions of the microbial community in bioreactor O2. [Methods] The 16S rRNA gene was sequenced to investigate the microbial diversity and composition, predict the microbial functional pathways, and reveal the microbial co-occurrence and the environmental driving factors in bioreactor O2. [Results] Proteobacteria, Bacteroidetes, and Chlorobi were the dominant phyla in the bioreactor. Among the dominant genera, Rhodoplanes, Lysobacter, and Thiobacillus were involved in the degradation of residual pollutants, such as chemical oxygen demand (COD), phenols, and thiocyanate (SCN-), and Nitrosovibrio and Nitrospira were the ammonia-oxidizing bacteria (AOB) and the dominant nitrite-oxidizing bacteria (NOB), respectively. Functional profiling suggested that the benzoate degradation, aminobenzoate degradation, chloroalkane and chloroalkene degradation, eluorobenzoate degradation, and nitrotoluene degradation were the top five pathways in the xenobiotics biodegradation and metabolism. These major functional pathways were distributed widely in the dominant genera, implying their roles in biodegradation of residual pollutants. The pmoA / B / C-amoA / B / C, hao, and nxrA / B encoding related enzymes constituted the nitrification pathway. According to the result of microbial co-occurrence network, Lysobacter, Candidatus Solibacter, and Rhodoplanes occupied an important position in the O2 ecosystem. Redundancy analysis (RDA) suggested that the microbial community in the bioreactor was mainly affected by COD and NH3 [Conclusion] Rhodoplanes and Lysobacter were the key genera for biomineralization and ecological stability of the community. Nitrosovibrio and Nitrospira played an important part in nitrification. The metabolic pathways in O2 were dominated by biomineralization and complete nitrification of residual pollutants. COD and ammonia (NH3) were the main influencing environmental factors. This study illustrates the structure and functions of microorganisms in bioreactor O2, which is expected to lay a microbial basis for improving the treatment of coking wastewater by O/H/O system.
