Microbial Community Assembly Mechanisms of Groundwater Under Salinity–Oxygen Stress in the Golmud River Watershed, Northwest China

The mechanisms underlying groundwater microbial community assembly have long attracted attention in earth, environmental, and ecological studies. Nevertheless, limited knowledge is available regarding microbial community assembly within the intact groundwater flow systems in arid regions. In this study, long-term hydrochemical data and microbial community profiles were integrated to unravel the assembly processes and driving forces mediating microbial communities in the Golmud River watershed. Our results indicated that hydrochemical conditions gradually transitioned from oxidizing to reducing environments along the groundwater flow path, as evidenced by a 28.57% and 65.45% decrease in DO and ORP, respectively. Major ions, represented by TDS, displayed minimal variations in phreatic (519.72 ± 16.83 mg/L) and artesian groundwater (486.01 ± 27.71 mg/L), followed by pronounced enrichment in high-salinity groundwater (TDS: 316,112.74 ± 12,452.19 mg/L). <i>Gammaproteobacteria</i> and <i>Actinobacteria</i> declined markedly from phreatic (51.69 ± 6.83% and 9.54 ± 3.40%, respectively) to high-salinity groundwater (13.97 ± 3.70% and 4.77 ± 2.46%). Conversely, halophiles such as <i>Halobacteria</i> and <i>Parcubacteria</i> were rarely detected in low-TDS groundwater, but increased sharply in high-salinity groundwater, reaching 23.22 ± 10.42% and 8.34 ± 3.71%, respectively. Deterministic processes primarily controlled groundwater microbial communities across hydrochemical conditions (relative importance > 50%, NST index < 50%). Microbial co-occurrence networks revealed increasingly tight interactions and intensified competition among communities, driven by accumulated salinity-oxygen stress along the groundwater flow path. This study emphasizes the role of deterministic processes in shaping groundwater microbial community structure, particularly the impact of salinity-oxygen stress. Our findings advance the current understanding of the mechanisms by which hydrochemical processes shape groundwater microbial assemblages.