Bacterial and archaeal community successions in high-salinity groundwater and their potential impact on arsenic cycling

Groundwater arsenic (As) contamination is a global issue involving complex biogeochemical processes. However, the arsenic cycling in high-salinity groundwater environments remain poorly understood. In this study, we used hydrogeochemical and microbial techniques to investigate the impact of salinity on bacterial and archaeal community structures and their functional evolution in the Yellow River Delta (YRD), China, and to explore how these dynamics influence arsenic enrichment. The results showed that bacterial richness and evenness decreased significantly with increasing salinity, especially in samples with TDS above 10 g/L, and the decrease was even more pronounced compared to archaea. Bacterial communities were dominated by Proteobacteria and Omnitrophica, while archaeal communities were predominantly composed of Halobacteria. Microbial communities actively mediate As-Fe-C-N-S redox cycling, exhibiting distinct cycling characteristics under varying salinity conditions. Microbe-mediated processes such as organic matter degradation, sulfate reduction, iron reduction, methanotrophy, and methanogenesis potentially contributed to As mobilization in low-salinity groundwater. In contrast, in high-salinity groundwater, sulfur respiration, iron respiration, and nitrate respiration were intensified, while methane oxidation and methanogenesis were inhibited, significantly affecting As cycling. This study highlights the critical role of salinity in shaping microbial community dynamics and their influence on arsenic biogeochemical cycling in the YRD aquifers, providing new insights into As mobilization in high-salinity groundwater.