Abstract:

Aims: The Yarlung Tsangpo River is a crucial river ecosystem on the Tibetan Plateau, and protozoa, as an essential component of eukaryotic microorganisms, play a key role in aquatic ecosystems. This study aimed to investigate the diversity patterns and geographical distribution of protozoan communities in the middle reaches of the Yarlung Tsangpo River during the dry season, focusing on their community structure, diversity, and environmental response mechanisms in both the mainstem and tributaries. 

Method: In the dry season of 2021, we conducted a comprehensive study of protozoan communities in the middle reaches of the Yarlung Tsangpo River. We analyzed community structure, diversity, and environmental drivers using high-throughput sequencing, generating ASVs (amplicon sequence variants). Statistical analyses included distance-decay, environmental-decay, neutral and null models, and co-occurrence network analysis to assess spatial and environmental influences on protozoan assemblages. 

Results: A total of 6,066 ASVs were detected, spanning 23 phyla, 55 classes, 86 orders, 114 families, 281 genera, and 323 species. Significant differences in community structure were observed between the mainstem and tributaries, with species turnover identified as the primary driver of β-diversity variations. Distance-decay and environmental-decay analyses revealed that protozoan communities in tributaries were more sensitive to geographical and environmental changes. Neutral and null model analyses indicated that community assembly in both habitats was primarily driven by homogenous selection, though stochastic processes played a greater role in tributaries. Co-occurrence networks showed distinct environmental influences on protozoan communities in the mainstem versus tributaries. 

Conclusion: These findings highlight the spatial heterogeneity and environmental response mechanisms of protozoan communities in river ecosystems. The study underscores the importance of tributaries as sensitive indicators of environmental and geographical changes, while emphasizing the dominant role of deterministic processes in community assembly. These insights contribute to a deeper understanding of microbial ecology in high-altitude river systems.

Key words: protozoan, community assembly, co-occurrence network, middle reaches of the Yarlung Zangbo River, Tibet