Response of activated sludge in sequencing batch reactor to stepwise increase in salinity: Performance evaluation, enzymatic activity, oxidative stress and metagenomic analysis

The nitrogen removal, enzymatic activity, oxidative stress, microbial community and functional gene response of sequencing batch reactor were investigated under stepwise increase in NaCl concentration. The COD and NH 4 + -N removal efficiencies gradually decreased from 0 to 60 g/L NaCl. Compared with 0 g/L NaCl, the rates of ammonia oxidation, nitrite oxidation, nitrite reduction and nitrate reduction decreased by 84.7%, 92.7%, 61.5% and 59.9% under 60 g/L NaCl, respectively. The activities of ammonia monooxygenase, nitrite oxidoreductase, nitrate reductase and nitrite reductase decreased by 84.6%, 91.4%, 98.5% and 91.8% under 60 g/L NaCl, respectively. Excessive accumulation of reactive oxygen species triggered antioxidant defense system under high salinity. The microbial diversity and richness obviously changed under different NaCl concentrations. The relative abundances of ammonia oxidation genes decreased with the increase in NaCl concentration. The relative abundance of some denitrification-related genes (e.g., nirK , nirS and norC ) increased under high salinity stress, whereas that of nosZ slightly decreased. The relative abundance of genes related to dissimilatory nitrate reduction (e.g., nrfA , nrfH and nirB ) decreased under high salinity, whereas those of other genes (e.g., napA and nrfA ) slightly increased. High salinity might limit the central carbon metabolism and cause potential effects on energy metabolism in TCA cycle. The research results will be helpful to understand the linkage among nitrogen removal, enzymatic activity, TCA cycle and microbial community under progressive salt stress.