[Comparison of the Microbial Community Structure in Nitrifying Processes Operating with Different Dissolved Oxygen...

High-throughput sequencing was applied to analyze the microbial community structure of nitrifying reactors operated with different dissolved oxygen (DO) levels. Results showed that the nitrifying reactor (R_L) run with low DO (0.2-0.3 mg·L^-1) exhibited greater microbial richness and diversity than the reactor run under the high DO condition[R_H, DO=(2.0±0.1) mg·L^-1]. In contrast, the microbial community in R_H was more highly functionally organized than that in R_L. Although the communities in R_H and R_L shared over 85% of the total sampled genetic information, the relative abundance of some individual species varied between the different DO conditions. Members of the Proteobacteria phylum, which accounted for 80.7% of the total microbes in R_H, were highly enriched, and the relative abundance of Nitrosomonas reached to 65.1%. However, the microbial community in R_L was dominated by Proteobacteria (43.8%), Firmicutes (20.0%), and Bacteroidetes (15.1%). In addition, a large fraction of bacteria possessing hydrolyzation and fermentation functions under anaerobic or anoxic conditions were also present in R_L including Lactococcus, Anaerolineaceae, and Rhodocyclaceae. As known ammonium-oxidizing bacteria, Nitrosomonas oligotropha and Nitrosomonas europaea were enriched in the R_H and R_L, respectively, while Nitrospira defluvii, being a nitrite-oxidizing bacteria, dominated both reactors. Rather than DO, ammonia and nitrite availability should be key factors in the selective enrichment of these nitrifiers.