Heavy metal contamination and associated microbial community responses in sediments of a mining-impacted watershed in southeastern Brazil

Mining is a major source of heavy metal (HM) release into aquatic environments, yet integrated catchment-scale assessments of both geochemical and microbiological dynamics in tropical fluvial systems remain scarce. This study investigated heavy metal contamination and associated microbial communities in sediments and water treatment plant sludge collected from 13 sites within a mining-impacted watershed in the Conceição do Mato Dentro region (Minas Gerais, Brazil) during rainy and dry seasons. Heavy metals were quantified by inductively coupled plasma mass spectrometry (ICP-MS), though a necessary 50-fold sample dilution required by the digestion protocol restricted quantification to four metals (Cr, Fe, Mn, and Zn), leaving six target elements below detection limits. Microbial communities were assessed using culture-based methods and 16S rRNA metabarcoding. Chromium, iron, manganese, and zinc were consistently detected at elevated concentrations, frequently exceeding sediment quality guidelines. Iron and chromium were particularly enriched near mining structures, revealing clear spatial gradients of metal accumulation across the watershed. Seasonal patterns indicated that hydrological variability influences metal mobilization and redistribution within the sedimentary system. Culture-based analyses recovered 116 bacterial isolates dominated by metal-tolerant genera, while metabarcoding of targeted samples revealed distinct taxonomic profiles, highlighting the enrichment of Arthrobacter and other taxa at the most contaminated site (PB2). These results indicate that chronic exposure to mining-derived metals is associated with distinct sediment-associated microbial community profiles along geochemical gradients in this mining-impacted fluvial environment