Effect of Zinc Oxide Nanoparticles on Physiological Activities and Gene Expression of Wastewater Nitrifying Bacteria

The effects of zinc oxide nanoparticles (nZnO) were examined for nitrifying bacterial enrichments by measuring substrate (ammonia) specific oxygen uptake rates (sOUR) in conjunction with the transcript level of functional genes involved in nitrification quantified by reverse transcriptase-quantitative polymerase chain reaction. Samples from nitrifying bioreactor were exposed in batch vessels to nZnO (1, 5 and 10 mg/L) for either 3 or 6 h. There was considerable increase in sOUR-based nitrification inhibition with increasing dosages of nZnO. At 10 mg/L nZnO, the inhibition was about 35% and 50% for 3 and 6 h exposure, respectively. As the nZnO dosage was increased, the transcript levels of amoA, hao, and nirK for 6 h exposure samples were decreased which corresponded well with sOUR data. In addition, an increase in intracellular reactive oxygen species content was observed for the cells of nitrifying bacteria when the total nZnO concentrations were 5 and 10 mg/L. The agreement between the relative expression of functional genes and sOUR shown in this study reveals the effectiveness of using transcriptional responses in conjunction with physiological activity for better understanding the impact of inhibitory compounds such as nanoparticles on nitrification process.