²²² Rn-Traced SGD Heterogeneity and Bacterial Assembly in the South China Mangrove Subterranean Estuary

This study aimed to determine how submarine groundwater discharge (SGD) types influence bacterial community assembly in a mangrove subterranean estuary. Groundwater and seawater samples collected from Qi’ao Island (Pearl River Estuary, China) in March-April 2024 were analyzed for hydrochemistry and bacterial communities. SGD was classified into fresh SGD (FSGD; meteoric-dominated) and recirculated SGD (RSGD; tide-driven seawater) using a radon-222 (222Rn) mixing model, with freshwater fractions calculated to distinguish SGD sources. Multivariate ordination and Mantel tests were used to assess community-environment relationships. FSGD occurred primarily at near-shore deeper sites (f ≥ 0.05), whereas RSGD dominated shallow offshore locations (f < 0.05). Although alpha diversity showed no significant differences among groups, beta diversity revealed distinct community structures. Proteobacteria were dominant overall, with group-specific enrichment of Campylobacterota and Alphaproteobacteria. Salinity-related ions and bicarbonate were key environmental drivers. These findings demonstrate that SGD types shape bacterial communities through environmental filtering and highlight their role in regulating coastal biogeochemical processes.