Exposure to Cr(VI) affects partial denitrification process-nitrite accumulation, EPS characteristic and microbial community assembly

This study was aimed at assessing the effects of elevated Cr(VI) stresses on partial denitrification (PD) process. The results showed that 0.5-5 mg/L Cr(VI) exposure significantly decreased nitrite accumulation rate, while COD removal efficiencies increased due to the further conversion of NO2--N to N2. This suggested that Cr(VI) exposure shifted the biological process from partial denitrification to complete denitrification. The Cr(VI) removal effi-ciency decreased from 37.75 +/- 15.97% to 10.48 +/- 3.78% with increasing Cr(VI) concentration. Excitation -emission matrix fluorescence spectroscopy showed that Cr(VI) exposure significantly stimulated the synthesis of tryptophan protein-like, aromatic protein-like and humic acid-like substances in extracellular polymeric substances (EPS). In addition, significant changes in functional groups of EPS were found as revealed by Fourier transform infrared spectroscopy. Particularly, there was a considerable decrease in the alpha-Helix from 31.36% to 0% and a major increase in the antiparallel beta-Sheet from 4.81% to 9.03% with increasing Cr(VI) stresses. The biodiversity decreased significantly under Cr(VI) stresses, and the microbial community structures gradually shifted with increasing Cr(VI) dosage. Sloan model revealed that the relative importance of deterministic pro-cesses on the community assembly increased after Cr(VI) exposure, indicating that Cr(VI)-induced selective processes have a profound influence on microbial community of PD process. Network analysis revealed that Cr (VI) exposure significantly altered species-species associations, topological features of individual OTU, and keystone species, implying that substantial changes occurred in bacterial ecology after exposure to Cr(VI).