Longitudinal zonation and hydrothermal variation jointly influence the bacterioplankton community assembly in peatland rivers in the Tibetan plateau.

Hydrothermal variation reshapes bacterioplankton assembly processes, thereby altering carbon mineralization, sequestration, and overall dynamics across the climate-sensitive Tibetan Plateau. Yet the role of longitudinal zonation along the river continuum in influencing these processes remains poorly understood. Here, we analyzed 16 S rRNA gene sequences from 24 river sections in the Zoige Basin-the largest peatland on the Tibetan Plateau-under contrasting hydrothermal regimes (cold-dry vs. warm-wet). We examined taxonomic diversity, community composition, and assembly processes across different longitudinal zones (upstream vs. downstream). Upstream (stream orders 1-3) communities exhibited significantly higher α-diversity (Shannon-Wiener Index H' = 4.73 ± 1.19) and β-diversity (Bray-Curtis dissimilarity d_BC = 0.27 ± 0.07), and showed stronger sensitivity to precipitation and water temperature. In contrast, downstream (stream orders 4-6) communities displayed lower diversity (H' = 3.15 ± 0.64, d_BC = 0.44 ± 0.13), primarily influenced by total organic carbon. Stochastic processes dominated community assembly (57.5 %-91.9 %), with dispersal limitation prevailing upstream (37.9 %-60.4 %) and homogeneous dispersal being more pronounced downstream (54.5 %-71.2 %). Warmer and wetter conditions enhanced upstream network interactions and increased the contribution of deterministic processes from 24.4 % to 42.5 %. Among 90 core taxa identified, 64.4 %-predominantly Pseudomonadota-exhibited significant preferences for lower total organic carbon, higher mean monthly precipitation, and lower water temperature. These findings highlight the joint influence of bacterioplankton communities by longitudinal zones and hydrothermal regimes, and suggest that the pronounced sensitivity of upstream communities provides important insights into climate-driven variations in ecosystem functioning and informs biodiversity conservation in peatland river ecosystems.