4.1 Different diversity patterns of abundant and rare bacteria
In this study, we found that bacterial α-diversity was significantly higher in the rare subcommunity than in the abundant subcommunity (Fig. S2). This result supports the notion that rare taxa play the role of “seed bank” in maintaining microbial diversity (Lynch and Neufeld, 2015; Shade et al., 2014). Our results also showed that the rare subcommunity displayed higher β-diversity than the abundant subcommunity in fluvial sediments (Fig. 2). This finding suggests that differentiation of the rare subcommunity along environmental gradients is higher than the abundant subcommunity. Variations in β-diversity generally depend on the interaction of species traits (e.g., ability to disperse and niche width) and characteristics of the environmental conditions (e.g., environmental heterogeneity and topographic complexity) (Maloufi et al., 2016; McKnight et al., 2007). In virtue of the rare and abundant subcommunities being subjected to the same environmental conditions, the higher β-diversity of the rare subcommunity may partly be attributable to the potential of some rare taxa could be actively growing or waking up from dormancy and present in the currently active community (Jiao et al., 2017; Pedrós-Alió, 2012).