Structure, Diversity, and Metabolic Potential of Bacterioplankton in an Urban River at a Long Low-Level Water Period.

A wide range of pollutants enter rivers from cities located on their banks, causing structural and functional changes in the riverine microbial communities. Although microorganisms play important roles in aquatic environments and are sensitive indicators of water pollution, the anthropogenic transformations of riverine bacterioplankton remain insufficiently studied. Using microscopy and 16S rRNA gene metabarcoding, we investigated the distribution, size-morphological structure, taxonomic composition, and potential metabolic functions of bacterioplankton in the Tura River near a major regional center (Tyumen city, Western Siberia, Russia). The river has experienced progressive shallowing and prolonged low water conditions for several consecutive years, substantially exacerbating pollution impact. Bacterial abundance, structure, diversity, and potential metabolic functions differed markedly among river sections located upstream, within, and downstream of the city. The greatest number of bacterial pathogens and pollution indicators was detected downstream of the city. Under anthropogenic impact, rare and moderate taxa benefit, whereas typically dominant freshwater taxa are more sensitive to pollution and eutrophication. Within the city, increases in bacterial abundance, taxonomic diversity, and predicted metabolic functions, including pollutant biodegradation, suggest both community adaptation and bioremediation potential, as well as functional instability and a shift toward allochthonous or opportunistic taxa. These results highlight the important role of bacterioplankton in urban river ecosystems along pollution gradients.