Response of wastewater biofilm to CuO nanoparticle exposure in terms of extracellular polymeric substances and microbial community structure

The growing production and application of CuO nanoparticles increase the chance that these particles will be released into wastewater treatment plants (WWTPs) and interact with microorganisms. However, the toxicity response mechanism of biofilm to NP exposure may be different from that of activated sludge due to the denser and stronger microbial aggregate structure of biofilm. Thus, in this study, the response to CuO NPs of wastewater biofilm collected from a rotating biological contactor was investigated. Short-term exposure (24 h) to CuO NPs led to a great loss in cell viability, and SEM-EDS images revealed that the nano-CuO aggregates were not transformed to Cu-S species in the biofilm samples. In response, more extracellular polymeric substance (EPS) (especially loosely bound-EPS) was produced in wastewater biofilm exposed to CuO NPs, with a higher content of protein compared to polysaccharides. The shifts of fluorescence intensity and peak locations in 3D-EEM fluorescence spectra indicated chemical changes of the EPS components. FT-IR analysis revealed that exposure to nano-CuO had more distinct effects on the functional groups of proteins and polysaccharides in LB-EPS. Illumina sequencing of 16S rRNA gene amplicons revealed that CuO NPs enhanced bacterial diversity. The bacterial community structure significantly shifted, with a significantly increased abundance of Comamonas, a slight increase in Zoogloea, and a notable decrease in Flavobacterium. The shifts of these dominant genera may be associated with altered EPS production, which might result in microbial community function fluctuations. In conclusion, exposure to high concentrations of CuO NPs has the potential to shape wastewater biofilm bacterial community structure.