Microbial Taxonomic and Functional Features Involved in Self-Purification of a Brazilian Polluted River

Summary The Tietê River is an anthropogenically disturbed urban water body polluted by a combination of untreated domestic sewage releasing (carbon, nitrogen and phosphorus pollution) and diffuse pollution that cross São Paulo State in Southeast of Brazil. Along its course, it presents contrasting sites showing elevated levels of nutrients and contaminants (eutrophic sites) and oligotrophic environments, in both water and sediments. In this study, we investigated how pollution influences the taxonomic and functional diversity of microbial communities in the Tietê River watershed, with the aim of understanding their role in pollutant transformation during downstream transport. Four sampling sites along a pollution gradient—from São Paulo city to a relatively pristine area near the river mouth—were evaluated. Results indicated that diversity differences were primarily linked to water quality, with higher diversity observed in less contaminated sites. Heterotrophic metabolism was more prominent in polluted regions, whereas photoautotrophic and lithotrophic microorganisms were more abundant in clean areas. Additionally, genes associated with the metabolism of aromatic compounds and virulence factors were more prevalent in environments with higher anthropogenic influence, suggesting a functional shift geared toward environmental adaptation and bioremediation. We propose that, in areas with high organic matter concentrations, microbial communities tend to adopt an r-strategy lifestyle, characterized by rapid growth and reproduction, while in oligotrophic, less polluted sites, more competitive k-strategists predominate. Although the following hypothesis was not extensively studied, the lower abundance of genes involved in secondary metabolic synthesis in eutrophic sites suggests that pollution may reduce the availability of novel species or traits relevant for biotechnological applications. Additionally, community shifts appear to be influenced by "conditionally rare taxa," which temporarily alter their activity and abundance in response to environmental constraints, playing a critical role in water self-purification processes. Overall, these findings offer new insights into the environmental factors driving self-purification in the Tietê River and shed light on the ecological mechanisms underpinning river resilience.