Abstract:

Shallow groundwater in the Yellow River Delta is characterized by high salinity due to historical seawater intrusion and evaporative concentration, which has led to complex hydrochemical evolution. However, the succession of microbial communities under these conditions remains poorly understood. In this study, 25 shallow groundwater samples, two Yellow River water samples, and one seawater sample were collected. Hydrochemical analysis, stable isotope tracing, and metagenomic sequencing were employed to identify salinity sources and to investigate the composition, succession patterns, and controlling factors of microbial communities across a salinity gradient. Results showed that groundwater total dissolved solids (TDS) ranged from 1.0-35.0 g/L, dominated by the Cl-Na water type. Combined evidence from δD and δ¹⁸O and Br^-/Cl^- ratios indicated the dual influence of seawater mixing and evaporative concentration. Microbial communities were dominated by bacteria (67.7%-98.6% relative abundance), followed by archaea (1.0%-32.0%), while eukaryotes and viruses were generally less abundant. Major bacterial groups included Pseudomonadota and Candidatus Omnitrophota, with the former being enriched in high-salinity samples and the latter preferring lower salinity environments. Among archaea, Candidatus Woesearchaeota was widespread, while Candidatus Lokiarchaeota and Candidatus Thermoprofundales showed ecological dominance under high-salinity conditions. Principal coordinates analysis (PCoA) and linear discriminant analysis effect size (LEfSe) revealed distinct microbial community structures across different salinity levels. Bacteria displayed greater sensitivity to salinity variation, whereas archaea exhibited higher salinity tolerance. Redundancy analysis (RDA) indicated that TDS, ${\mathrm{SO}}_{4}^{2-}$, Fe²⁺, and ${\mathrm{NH}}_{4}^{+}$ were the main factors shaping microbial communities in high-salinity groundwater, while ${\mathrm{NO}}_{3}^{-}$, pH, and ORP were more influential in low-salinity environments. This study reveals the succession patterns and environmental drivers of microbial communities in high-salinity groundwater, providing theoretical insight into microbial responses and ecological functions under salinity stress in coastal aquifer systems.

Key words: high-salinity groundwater, Yellow River Delta, microbial community structure, metagenomic sequencing, isotope tracing