Coalescence characteristics of free-living and particle-attached bacteria in a cascade river-reservoir system: A case study of the Jinsha River

Microbial coalescence plays a crucial role in shaping aquatic ecosystems by facilitating the merging of neighboring microbial communities, thereby influencing ecosystem structure. Although this phenomenon is commonly observed in natural environments, comprehensive quantitative comparative studies on different lifestyle bacteria involved in this process are still lacking. The study focuses on 16S rRNA Amplicon Sequence Variants (ASVs) at the Jinsha River hydropower stations (Wudongde [WDD], Baihetan [BHT], Xiluodu [XLD], Xiangjiaba [XJB]), specifically examining free-living (FL) and particle-attached (PA) bacteria. Minimal differences in microbial composition were observed across water layers (surface, middle, and bottom). Analyses of overlapping ASVs, Bray-Curtis dissimilarity, and the SourceTracker algorithm revealed a significant difference in the coalescence ability of FL and PA bacteria, particularly in the surface water of XJB (FL: 31.1% ± 2.0%, PA: 27.6% ± 2.5%, p < 0.05). The coalescence of FL bacteria was primarily influenced by the mixing of adjacent water layers, while PA bacteria exhibited significant geographical variations across water layers (p < 0.05), displaying lower coalescence compared to FL bacteria. Using a cohesion metric, 12 keystone species in PA bacteria were identified and 7 in FL bacteria. Proteobacteria and Bacteroidetes were the most abundant phyla at the keystone species in PA and FL bacteria, respectively. The abundance of keystone ASVs decreased with distance in PA bacteria, whereas FL bacteria showed the opposite trend. At the genus level, Brevundimonas and Chryseobacterium were identified as keystone species in both lifestyles. Moreover, the impact of community coalescence on the stability tends to exhibit differences downstream in cascade stations. This study provides novel insights into the dynamic variations of microbial communities with diverse lifestyles in stratified aquatic environments and assesses the impact of dam construction on microbial coalescence and the alteration of keystone species.