Extreme pollution reshapes microbial community assembly in a tropical urban river.

Anthropogenic pollution profoundly alters urban rivers, yet its effects on microbial community assembly remain poorly understood. Here, we investigated spatial patterns of bacterial community structure and assembly along the Tietê River (Brazil), a heavily polluted tropical urban river. We combined 16S rRNA gene amplicon sequencing with physicochemical characterization across a Water Quality Index (WQI) gradient and quantified ecological assembly processes using the iCAMP framework. Contrary to the expected pollution-driven loss of diversity, alpha diversity was significantly higher in severely degraded stretches. Community composition shifted from freshwater-associated taxa, such as Comamonadaceae, in less impacted sites to pollution-tolerant groups, particularly Arcobacteraceae, in degraded reaches. Homogeneous selection predominated in Good and Regular WQI sites but declined markedly under Poor WQI conditions, where drift and dispersal limitation became the dominant assembly processes. These results indicate that extreme pollution alters the nature of environmental filtering, favoring disturbance-tolerant functional traits distributed across phylogenetically diverse taxa and increasing the influence of stochastic assembly. Consequently, taxonomic composition becomes less predictable from local environmental conditions. Our findings provide a mechanistic framework for understanding how chronic anthropogenic disturbance reshapes microbial community assembly in tropical urban rivers.