Understanding microbial responses and developing enhanced nitrogen removal of sulfur autotrophic denitrification in wastewater treatment under salinity stress

High salinity severely limits the efficacy of biological wastewater treatment, making it crucial to elucidate microbial response mechanisms to salinity stress for maintaining system functionality and stability. This study systematically examined the denitrification performance and microbial community dynamics of an autotrophic sulfur-based denitrification system by establishing salinity gradients of 0 %, 1 %, 3 %, and 5 %. Results indicate that salinity significantly inhibits denitrification efficiency, with nitrate removal rates fluctuating dramatically (dropping to a minimum of 19.46 %) at 5 % salinity. Nevertheless, the system retains some tolerance and recovery capacity. Salinity stress damages cell viability and reshapes biofilm structure: 5 % salinity causes cell membrane damage, suppresses EPS synthesis, reduces the proportion of viable cells to 45.6 %, and leads to loosening of the biofilm structure. Salinity addition increased community diversity and induced significant structural shifts. Under high-salinity (5 % NaCl) conditions, Thiobacillus relative abundance decreased to 16.89 %, while Vitellibacter increased to 36.45 %, reflecting shifts in microbial competitive dominance. Functional genes exhibited differential responses to salinity: narG and nirS abundance changes synchronized with the abrupt decline and recovery of denitrification efficiency, demonstrating high salinity sensitivity, while nirK showed greater tolerance. Ultimately, denitrification efficiency was confirmed to depend on the synergistic activity of the entire denitrification gene pathway (narG/napA → nirS/nirK → nosZ), rather than the abundance of individual genes. This study reveals the mechanisms of salinity stress effects across multiple levels-microbial community structure, functional genes, and biofilm morphology-providing theoretical foundations for regulating and optimizing biological treatment systems for saline wastewater.