Impacts of cerium oxide nanoparticles on bacterial community in activated sludge

Rapidly developing industry raises concerns about the environmental impacts of nanoparticles, but the effects of inorganic nanoparticles on bacterial community in wastewater treatment remain unclear. The present research assessed the impact of cerium oxide nanoparticles (nCeO) on the microbiome of activated sludge system. The results showed that 18,330 over 28,201 reads generated from control samples were assigned to Proteobacteria while 5527 reads (19.6%), 3260 reads (11.567%), and 719 reads (2.55%) were assigned to unclassified_Bacteria, Firmicutes and Actinobacteria, respectively. When stressed with nCeO₂ NPs, a decrease on reads was noted with 53, 48, 27.7 and 24% assigned to Proteobacteria. Gammaproteobacteria (80.57%) was found to be the most predominant Proteobacteria. The impact of nCeO₂ NPs was also observed on pollutants removal as only 1.83 and 35.15% of phosphate and nitrate could be removed in the bioreactor stressed with 40 mg-nCeO₂-NPs/L. This was confirmed by a drastic reduction of activities for enzymes catalysing denitrification (NaR and NiR) and degradation of polyphosphate (ADK and PPK). ADK appeared to be the most affected enzyme with activity decrease reaching over 90% when stressed with 10 mg-nCeO₂/L. Furthermore, bacterial diversity was not significantly different whereas their species richness showed significant difference between control and treated samples. A large number of reads from control samples could not be classified down to the lower taxonomic level "genera" suggesting hitherto vast untapped microbial diversity. The denitrification related genera including Trichococcus and Acinetobacter were found to alternatively dominating treated samples highlighting those nCeO₂ NPs could enhance the growth of some bacterial species while inhibiting those of others. Nevertheless, the study indicates that nCeO₂ NPs in wastewater at very high concentrations may have some adverse effects on activated sludge process as they inhibit the removal of phosphate.