Effects of salinity on anoxic–oxic system performance, microbial community dynamics and co-occurrence network during treating wastewater

Salinity is one of the great challenges in the biological treatment of high-salinity wastewater. Unraveling how microbial associations respond to salinity stress and how they affect ecosystem function and community stability are crucial for wastewater treatment. However, little is known about the relationships among microorganisms under salinity stress. Hence, an anoxic–oxic (A/O) reactor was adopted to investigate the effects of elevated salinity (20 g/L-60 g/L) on system performance, microbial community dynamics and co-occurrence network during treating urine wastewater. The results indicated that the reactor achieved over 90% removal of both urea and COD in a wide range of salinity. Microbial alpha diversity decreased with increased salinity, as well as beta diversity also significantly changed. Elevated salinity led to a greater proportion of negative correlations in both anoxic and oxic microbial co-occurrence networks, which acted as a survival strategy for microorganisms under high-salinity stress. Clostridiales and Oceanospirillales were the keystones and played important roles in maintaining system stability. Furthermore, the oxic communities were more stable than the anoxic communities under salinity stress. These findings offered new sights for understanding microbial relationships in response to salinity in anoxic and oxic systems, and also provided a reference basis for treating high-salinity wastewater efficiently.