Individual and combined effects of polystyrene and divalent cadmium on nitrogen removal performance, microbial...

This study evaluated the effects of individual and combined polystyrene (PS) and divalent cadmium (Cd²⁺) on pollutant removal efficiencies, microbial enzyme activity, microbial toxicity, microbial community structure and nitrogen removal genes in four sequencing batch reactors (SBRs). The results showed that the removal efficiencies of chemical oxygen demand and NH₄⁺-N in the combined PS and Cd²⁺ treatment decreased by 12.37 % and 29.05 % compared to the control reactor, respectively. The nitrification and denitrification rates were significantly inhibited, which was correlated with the suppression of key microbial enzyme activities, particularly ammonia monooxygenase and nitrite oxidoreductase, leading to NH₄⁺ and NO₂^- accumulation. The combination of PS and Cd²⁺ exhibited synergistic toxicity, causing severe disruptions in the microbial community. The relative abundance of Gammaproteobacteria and Alphaproteobacteria declined, essentially for nitrification and denitrification. Additionally, the relative abundances of key nitrogen-cycling genera (Nitrosomonas, Nitrospira, Thauera and Dechloromonas) decreased, impairing nitrogen transformation processes. The relative abundances of nitrogen removal genes (amoA, napA, narG and nirS) were significantly reduced, further indicating functional deterioration. The findings highlighted that combined PS and Cd²⁺ induced compositional and functional shifts in microbial communities, exacerbating toxicity effects, disrupting nitrogen metabolism, and impairing SBR performance ultimately.