Key microbes driving salinity-tolerant nitrogen removal in industrial wastewater: identification and functional...

The high salinity of industrial wastewater poses a serious challenge to the performance of biological nitrogen removal (BNR). At present, there is a knowledge gap in identifying key salinity-tolerant nitrogen-removal microorganisms and understanding the functional mechanisms driving their activity, which seriously limits the optimization of efficiency and BNR research. In this study, an integrated analytical workflow was established. This workflow enabled the identification of key microorganisms and the elucidation of their functional mechanisms in salinity-tolerant nitrogen removal. Based on 190 activated sludge samples collected from industrial wastewater treatment plants in eastern China, key functional modules related to salinity tolerance and nitrogen removal were resolved via co-occurrence network analysis. Subsequently, multiple methods, including network topology, co-expression patterns, and random forests, were integrated to comprehensively screen taxa within the modules, aiming to pinpoint microbial groups that possess both functional importance and topological centrality in the network, and to dissect their functional driving mechanisms under high salinity stress systematically. The results showed that salinity significantly influenced community structure and function, with 8-12 g/L identified as the critical salinity range. The screened core salinity-tolerant microorganisms included Thioalkalivibrio and Hyphomicrobium, while core nitrogen-removing microorganisms included Thauera and Comamonadaceae; Parvibaculum and other taxa exhibited dual functional potential. Under high salinity conditions, nitrogen metabolism was remodeled. Nitrification and denitrification gene abundances decreased by 20-56%, and the dissimilatory and assimilatory nitrate reduction (DNRA, ANRA) pathways increased by about 25% and 16%, respectively. The salinity-tolerance strategy also changes from ion accumulation to compatible solute synthesis or antioxidant stress responses. These gene groups were upregulated by about 10-59% and 17-106%, respectively. In a high salinity environment, key microorganisms mediate niche differentiation and energy redistribution within the microbial community, thereby facilitating collaborative remodeling of salinity tolerance and nitrogen removal. At the same time, although the microbial co-occurrence network became less dense, the roles of key microorganisms became more prominent. This study systematically established a framework for identifying key microorganisms and characterizing their functional mechanisms in the salinity-tolerant nitrogen removal process for industrial wastewater, providing a new theoretical basis for optimizing the BNR process for high salinity wastewater.