Impact of discharge type on geothermal spring microbial community and carbon cycling: Insights from Chongqing, China

The influence of discharge type on hydrochemistry and its subsequent role in filtering geothermal microbial communities, particularly concerning the microbial carbon pump (MCP)-driven refractory dissolved organic carbon (RDOC), remains inadequately explored. This study investigated nine hot springs in Chongqing, China (comprising five natural hot springs, NHS, and four borehole hot springs, BHS), utilizing in situ physicochemical measurements, ion chemistry analyses, 16S rRNA gene (V3-V4 region) sequencing, beta-diversity assessment, redundancy analysis, co-occurrence network analysis, and PICRUSt2-based functional prediction. The springs were characterized as weakly acidic and of the SO4-Ca (Mg) type. BHS exhibited higher temperatures (T), EC, and SO42- concentrations, whereas NHS demonstrated slightly elevated total organic carbon (TOC) levels. Bacterial communities exhibited strong segregation according to discharge type (ANOSIM R = 0.93, p = 0.01) and were predominantly composed of Proteobacteria. Redundancy analysis identified T and TOC as the principal factors shaping microbial community composition (RDA1 explained 70.57% of the variance), with dissolved oxygen (DO), SO42-, pH, and HCO3- serving as additional contributors. Network analysis pinpointed keystone taxa associated with thermal and redox niches, while functional predictions indicated an enrichment of pathways related to carbohydrate metabolism, amino acid metabolism, energy metabolism, and carbon fixation. Under light-limited conditions, DOC served as a proxy for MCP-derived RDOC, with concentrations measuring 0.90 mg/L in NHS and 0.81 mg/L in BHS, corresponding to approximately 0.42% and 0.37% of HCO3- concentrations, respectively. These findings suggest the existence of a modest yet persistent carbon pool within these DIC-dominated waters. Collectively, this study establishes a connection between discharge-driven hydrochemical gradients and microbial community assembly as well as metabolic potential, offering novel insights into RDOC formation mechanisms. Our results enhance the understanding of geothermal ecosystem dynamics and provide valuable information for regional karst carbon sink assessments and geothermal ecosystem monitoring.