Background
More than a half (i.e. 55 %) of the current world population lives in urban areas, and this figure is expected to push towards 68 % by 2050, according to data from the United Nations [1]. The most urbanized region nowadays is North America, with 82 % of its population living in cities, while the figure for Europe is slightly lower; 74 % [1].
Urbanization is recognized to pose several threats to the ecosystems present in the watershed, among which, streams and rivers are particularly touched [2,3]. The overall physical-chemical effects of urbanization on rivers and streams include flow/channel alterations, increased water temperature, and accumulation of nutrients, and toxicants in river water [2]. Most of these effects result from increased impervious surface areas and the resultant increased runoff after storm events, leading to discharge regime shifts and greater transport of chemicals [4]. On the other hand, biotic responses include increased abundance of tolerant species while sensitive ones are decimated [5].
At the ecosystem level, general trends point towards decreased nutrient uptake capacity, but there is still much work to do, for example concerning stream metabolism, before concluding whether ecosystem responses to urbanization are consistent or no 2. Stream metabolism (including primary production and community respiration), in turn, has recently being recognized as a reliable marker for the ecological status of rivers and streams, as it encompasses responses at multiple levels of biological organization [6,7].
Among river biota, microbial organisms participate in virtually all major ecosystem processes, including nutrient spiraling, pollutant transformation and net metabolism [8–11]. Because of a lack of appropriate methodologies, classically relying on information gained through culture-dependent techniques, the impacts of urbanization have rarely been assessed on stream ecosystems from a microbial community perspective. These methodological constraints are now being overtaken, thanks to the current revolution in DNA-sequencing technologies, which are being accessible and cost-effective for the scientific community [12]. In recent years, these DNA-based technologies have revealed that urbanization severely impacts microbial communities within stream ecosystems [13–17], mostly in urban rivers across North America, one of the leading regions in terms of urban development. Unfortunately, there is still a knowledge gap in terms of the impacts of urbanization on microbial organisms across Europe. Furthermore, how this translates into altered stream metabolism is virtually unknown, despite this relationship is crucial to infer the impacts of urbanization on higher trophic levels.
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