Metagenomic analyses reveal nitrogen metabolism responses to copper and chromium contamination in sludge-based microbial communities

In this study, we used metagenomic sequencing to explore the effects of chromium (Cr) and copper (Cu) on microbial communities and function by analyzing pathway of nitrogen metabolism based on KEGG annotation. High concentrations of Cr (VI) and Cu (II) reduced the removal efficiency of total nitrogen and ammonium in the system. Metagenomic analyses showed that the taxonomic composition of nitrogen metabolism-associated microorganisms (NMAM) was mainly composed of Betaproteobacteria, Alphaproteobacteria, Gammaproteobacteria, Sphingobacteria, Deltaproteobacteria, Flavobacteria and Nitrospira. Different concentrations of Cr (VI) and Cu (II) led to great changes in NMAM in activated sludge and enriched dominant microbes, respectively, suggesting specific response strategies in microbial communities. NMDS (Non-metric multidimensional scaling) analyses revealed the impact of different Cr (VI) and Cu (II) concentrations on NMAM β diversity, and indicated that Cr (VI) treatment induced greater variations in NMAM communities than Cu (II). We also identified a significant association (p < 0.05) between enzymes of nitrogen metabolism pathway and corresponding NMAM. This study revealed that under Cu (II) and Cr (VI) stress, the sludge nitrification pathway was inhibited, especially for ammonia monooxygenase and hydroxylamine dehydrogenase. These combined effects led to a decrease in sludge efficiency for nitrogen removal. These findings underscore the influence of heavy metals in water treatment.