Microbial Community Characteristics and Underlying Drivers Along the Streams, Tributaries, and Main Stems of the Yangtze River Source Region.

Microorganisms are regarded as the key driver in regulating biogeochemical cycles. A comprehensive examination of the riverine microbial community composition and structure is crucial for understanding the functions of aquatic ecosystems. However, previous studies mainly focused on the spatial distribution of microorganisms in the main stems of rivers, ignoring the shifts in microbial communities along the streams, tributaries, and main stems of rivers. To address this gap, we collected water samples from 27 sites along the Yangtze River Source Region covering a 1000 km reach. Based on 16S amplicon sequencing, spectral and physicochemical analyses, we examined the microbial communities as well as their influencing factors along the stream orders. Our results revealed that the bacterial community composition in streams and tributaries was different from that in the main stems of rivers. Although no significant changes were observed in bacterial alpha diversity, the structure of bacterial communities significantly changed and their beta diversity increased along the streams, tributaries, and main stems of rivers, indicating the enhanced complexity of bacterial communities. This observation was further confirmed by network analysis revealing the increased links between bacterial communities along the stream orders. However, the predicted functional potential of bacteria engaged in carbon and nitrogen cycling gradually decreased from tributaries to main stems of rivers, illustrating a higher elemental cycling and associated ecological process in the headwater streams. Based on multiple statistical analyses, we found that the dominant driver affecting riverine bacterial communities was the environment, rather than substrate and nutrient variables. Moreover, deterministic processes exerted a dominant role in regulating bacterial community assembly, despite the increased relative contribution of stochastic processes in structuring the bacterial communities along the stream orders. This study revealed the variations of bacteria and their drivers from the perspective of stream orders, providing a new understanding of the microbial communities in high-altitude rivers.