Bacterial community dynamics and conversion of inorganic nitrogen under aerobic and micro-aerobic conditions

To investigate the inorganic nitrogen conversion in reactors that were operated under aerobic and micro-aerobic conditions and to identify populations that became acclimated in the reactors under those oxygen conditions, we operated two reactors with 72 h of hydraulic retention time and an artificial medium containing ammonium- and nitrite-nitrogen without any organic compound. We determined the concentration of inorganic nitrogen in samples from the reactors and the microbial community structure in the reactors by using PCR-DGGE of the partial 16S rRNA gene. The results showed that nitrification to nitrate fully progressed in the aerobic reactor within 24 h, but we could not find any obvious reaction in the micro-aerobic reactor. In a view of microbial community structure, the total number of microorganisms composing the communities in the reactors was dramatically decreased compared to those of the initial inoculated sludge that originated from the Jang-lim sewage treatment plant in Busan, Republic of Korea because of the limited nutrients present. One Nitrospira sp. was clearly detected under both aerobic (as DO > 2 mg l(-1)) and micro-aerobic (as DO < 0.7 mg l(-1)) conditions while no AOB-like bacterium was exactly matched among main bands. By the results, nitrite-oxidizing bacteria may be more tolerant to variations in oxygen, or the occupation of ammonia-oxidizing bacteria as dominant groups may have been inhibited by substrate starvation and high concentrations of nitrite-N in influent.