Linking hydrological, biochemical and microbial community patterns in urban rivers: insights from the River Tevere (Rome, Italy).

River waters at lowland reaches experience a direct disturbance owing to increasing water resource exploitation and urbanization, with important implications for ecosystem functioning and services. Despite decreasing water quality and altered hydrological regime may dramatically affect the natural processes as a whole, little is known on the effects on aquatic microbial communities, whose dynamics play a key role in organic matter decomposition and nutrient cycling along the river continuum. The objective of this study was to explore the links between hydrology, water chemistry and microbial community structure in a highly impacted Mediterranean river. To this aim, the catchment of the River Tevere (including the main stem, the stream Cremera, and the major tributary Aniene) was sampled at the closing section in differently urbanized areas at two contrasting seasons (winter/summer). The major hydrological, physical and chemical characteristics of river waters were measured directly or retrieved within datasets from monitoring agencies. The microbial community structure was analyzed by NGS Illumina profiles and flow cytometry to identify and quantify the aquatic prokaryotes and picoeukaryotes (i.e., heterotrophs and photoautotrophs). Our results outlined recurrent patterns and quantitative changes of interacting microbial assemblages across the urbanization gradient at different hydrological settings. Alphaproteobacteria largely dominate the microbial community, followed by Verrucomicrobia and Betaproteobacteria. The total prokaryotic cell abundance increased toward the river mouth, with higher values registered downstream the city of Rome (4*10^6 cells/ml). The per-cell nucleic acid content, intended as a proxy of the cell metabolic activity, increased accordingly, while the ratio between photoautotrops and heterotrophs decreased downstream the confluence with main tributary. Given the links between hydrological and microbial community patterns, river microbes could provide valuable indications on the ecological effects of urbanization and altered environmental conditions. Moreover, flow cytometry seems an appropriate tool to rapidly provide multi-parametric data for a better understanding of the biogeochemical processes at the microscale level in river systems.