Short-term effects of Mn2O3 nanoparticles on physiological activities and gene expression of nitrifying bacteria under low and high dissolved oxygen conditions

Abstract

The short-term effects of Mn2O3 nanoparticles (NPs) were examined for nitrifying bacterial enrichments exposed under low and high dissolved oxygen (DO) conditions using substrate (ammonia) specific oxygen uptake rates (sOUR), reverse transcriptase – quantitative polymerase chain reaction (RT-qPCR) assays, and by analysis of 16S rRNA sequences. Samples from nitrifying bioreactor were exposed in batch vessels to Mn2O3 NPs (1, 5 and 10 mg/L) for either 1 or 3 h under no additional aeration or 0.25 L/min aeration. There was increase in nitrification inhibition as determined by sOUR with increasing dosages of Mn2O3 NPs for both low and high DO. At 10 mg/L Mn2O3 NPs, the inhibition was about 7–10% for 1 and 3 h exposure in both cases. There was notable reduction in the transcript levels of amoA, hao and nirK for 10 mg/L of Mn2O3 NPs under 3 h, high DO exposure, which corresponded well with sOUR. The 16S rRNA sequencing showed that there was an inhibitory effect on ammonia oxidizers activity upon exposure to 10 mg/L of Mn2O3 NPs. Collectively, the findings in this study advanced understanding of the different effects of Mn2O3 NPs on nitrifying bacteria.