Global patterns in planktonic river microbiomes

Microbes are dominant players in the ecosystem functioning of river drainage networks, driving the cycling of biogeochemically active elements (e.g., C, N, P) and pollutants (e.g., Hg, As) and serving as the base of river food webs. River microbiome composition undergoes a transition within river networks from dispersed groundwater and soil microbes in headwaters to planktonic microbes in large rivers. The ecosystem functions of river microbiomes are likely linked to the formation of these planktonic communities. We hypothesized that, across a broad range of river systems, this transition in microbiome diversity and function is controlled by temperature and water residence time, and is co-incident and interacting with transitions in river chemistry, consistent with the river continuum concept. We tested this hypothesis with several datasets from rivers around the globe. We found microbial diversity (16S rRNA) across individual river networks to be correlated with temperature and water residence time in the Congo, Connecticut, Willamette, and Deschutes Rivers. We found a similar pattern in a 4-year time series from the mouths of 16 large rivers spanning the continental U.S., demonstrating the influence of seasonality in temperature and residence time. In several rivers, genomic potential (metagenomes) correlated with seasonal conditions common to all rivers including temperature and inorganic chemistry (e.g., conductivity, phosphate). However, gene expression (metatranscriptomes) tracked variation in organic matter among rivers. This suggests that while river microbiome composition is controlled by physical factors, active ecological functions are linked to the chemical and biological conditions of individual rivers.