Polymetallic Contamination Along an Urbanization Gradient in the Funa River (Kinshasa, DRC): Structural Reordering of Cultivable Bacterial Communities

Introduction Urban rivers in sub-Saharan Africa are increasingly affected by untreated domestic, industrial, and artisanal effluents, leading to polymetallic contamination that may alter microbial community structure and ecosystem functioning. However, the influence of chronic metal pollution on bacterial phenotypic composition remains poorly documented in tropical urban freshwater ecosystems. Purpose This study aimed to assess dissolved trace metal contamination along the Funa River (Kinshasa, Democratic Republic of the Congo) and investigate its relationship with the phenotypic distribution of cultivable bacterial communities. Methods A longitudinal survey was conducted at six sampling stations. Eighteen water samples were collected for physicochemical analyses, and nine composite samples were prepared for microbiological assessment. Ten trace metals were quantified using energy-dispersive X-ray fluorescence spectrometry (ED-XRF). Cultivable heterotrophic bacteria were enumerated on Mueller–Hinton and MacConkey media and characterized by Gram staining. Spatial differences were evaluated using the Kruskal–Wallis test, and associations between metal concentrations and bacterial phenotypes were examined using bootstrapped Spearman rank correlations (1,000 resamples). Results The Funa River exhibited severe polymetallic contamination, with several metals exceeding guideline values. Cadmium showed extremely high contamination factors (CF = 2,889--5,789), while lead reached 1,433 μg L-1 downstream. Dissolved iron varied significantly among stations (H = 6.14, p = .046). Total heterotrophic bacterial abundance ranged from (3.23 ± 1.53) x 106 to (3.73 ± 2.97) x 106 CFU mL-1. A marked shift in bacterial phenotype was observed, shifting from Gram-positive dominance upstream to exclusively Gram-negative populations downstream. Bootstrapped correlations revealed significant negative associations between most metal metrics and Gram-positive bacteria (ρ = -.50, p = .041), alongside corresponding positive associations with Gram-negative bacteria (ρ = +.50, p = .041). Conclusion Chronic polymetallic contamination acts as a strong ecological filter shaping bacterial community structure in the Funa River, favoring Gram-negative populations adapted to metal stress. These findings underscore the need for improved urban wastewater management and molecular investigations to resolve microbial adaptation pathways and public health risks.