Distribution patterns and assembly mechanisms of microbial communities in the riverine waters of southeastern Tibet

A comprehensive understanding of microbial community structure and assembly mechanisms in plateau rivers is crucial for the conservation and management of these ecosystems and their downstream environments. However, current knowledge of microbial communities in high-altitude rivers remains limited. In this study, we systematically investigated the distribution patterns, co-occurrence networks, and community assembly processes of fungal and bacterial communities across river systems in southeastern Tibet. The results revealed significant structural differences in microbial communities along the elevational gradient (p < 0.05), with species replacement accounting for 65.1 % and 50.5 % of β-diversity variations in fungal and bacterial communities, respectively. On the environmental stress gradient, both high and low environmental stress intensified microbial interactions. In addition, although homogeneous selection dominated community assembly across all regions for both fungi (67.9-70.0 %) and bacteria (89.5-90.0 %), deterministic processes exerted a weaker influence on fungal communities at higher elevations compared to lower elevations (p = 0.016), whereas the opposite pattern was observed for bacterial communities (p = 0.048). At the basin scale, annual mean temperature (fungi: R = 0.322; bacteria: R = 0.301), elevation (fungi: R = 0.341; bacteria: R = 0.275), longitude (fungi: R = 0.181; bacteria: R = 0.193), and latitude (fungi: R = 0.248; bacteria: R = 0.201) significantly influenced both fungal and bacterial community structures. Additionally, fungal communities were also shaped by water temperature (R = 0.183), annual mean precipitation (R = 0.299), and the positive (R = 0.207) and negative (R = 0.181) connectedness, while bacterial communities were also associated with total dissolved solids (R = 0.333) and pH (R = 0.125). These findings provide new insights into the biogeographic patterns and assembly mechanisms of microbial communities in plateau rivers and lay a scientific foundation for the protection and management of these water resources.