Contrasting community assembly processes structure lotic bacteria metacommunities along the river continuum

The heterogeneous nature of lotic habitats (i.e. flowing freshwaters such as rivers and streams) facilitate the complex ecological and evolutionary processes that govern the microbial communities within them. Due to such complexity, our understanding of lotic microbial ecology still lacks a unified conceptual framework for the ecological processes that shape these communities. We explored how bacterial community composition and underlying ecological assembly processes differ between lotic habitats by examining community composition and inferring community assembly processes across four major habitat types (free-living, particle-associated, biofilm on benthic stones and rocks, and sediment) across a single river catchment. Using high throughout sequencing of the 16S rRNA encoding gene, we inferred community composition in a total of 195 bacterial communities across 12 river sites from headwater streams to the main river. Our results indicate that there are remarkably distinct differences in the bacterial communities between four major habitat types, with substantial contrast in the ecological processes shaping their community assembly processes also being observed. In particular, we show that the relative influence of deterministic environmental filtering is elevated in sediment and biofilm communities compared to free-living and particle-associated communities, where stochastic processes play a larger role.