Efficient biological nitrate removal from hypersaline streams: Denitrification performance and bacterial community...

Nitrate contamination in saline wastewater poses significant environmental risks, including eutrophication and groundwater contamination. Targeted nitrate removal from saline wastewater is of critical importance for environmental sustainability. In this study, we explored the response of microbial denitrification to varying salinity levels using a fixed-bed biofilm reactor (FBBR) operated under freshwater (0 g NaCl/L), mesosaline (50 g NaCl/L), and hypersaline (100 g NaCl/L) conditions. Despite the progressive increase in salinity, the system maintained exceptionally high nitrogen removal efficiency (>99.0 %). Elevated salinity resulted in reduced microbial community diversity. As a result, it promoted the enrichment of halophilic genera such as Halomonas, Virgibacillus, Ensifer, and Marinobacter. Notably, cooperative interactions between Halomonas and Marinobacter sustained denitrification under hypersaline conditions, whereas Halomonas exhibited a negative correlation with Thauera, likely due to competition for limited substrates. Although increased salinity simplified the microbial network, it enhanced connectivity and facilitated more efficient information and resource exchange, thereby promoting cooperative bacterial interactions that supported their adaptation to extreme environments. Furthermore, with increasing salinity, ecological drift emerged as the dominant force shaping community assembly. Collectively, these findings elucidate the underlying mechanisms and microbial interactions that drive efficient nitrogen removal in hypersaline conditions, offering valuable insights for optimizing saline wastewater treatment strategies.