Soil pollution by heavy metals (HMs) poses a major threat to soil quality and human health, with mining and smelting industries identified as key sources. Soils around smelters are often considered polluted hotspots, being generally unsuitable for agricultural activities. Although many studies have identified microbial taxa able to survive in such environments, most have focused on relatively low HM concentrations. The purpose of the study was to assess the ecological risk and to evaluate the diversity and structural shifts in microbial communities, as well as to predict key metabolic pathways associated with HM resistance in soils near Pb–Zn smelter in Bulgaria. The soils ranged from low-risk to disastrous, with cadmium (Cd) identified as the primary contributor to soil toxicity. High-throughput sequencing of 16S rRNA and ITS amplicons revealed widespread dominance of the phyla Proteobacteria, Actinobacteriota and Acidobacteriota, and Ascomycota, with the prevailing classes Acidobacteriae, Chloroflexia, and Eurotiomycetes, indicating their high tolerance to HMs. Functional predictions suggested enrichment of key pathways in the most polluted soils related to HM resistance, including efflux systems and detoxifying enzymes. These results highlight the necessity of integrating soil microbial indicators into agricultural management strategies to ensure safe food production.

Heavy metal/loid (HM) pollution of soils, primarily as a consequence of mining and ore-processing activities, poses significant risks to ecosystems and human health. Soil microbial communities play essential roles in maintaining key ecosystem functions, including nutrient cycling, carbon sequestration, and soil stability. The purpose of this study was to characterize the taxonomic composition and diversity of bacterial communities and evaluate their functional adaptation to heavy metal stress in soils affected by long-term gold–copper mining activities in Bulgaria. Ten soil samples representing a Cu pollution gradient (53–860 mg kg⁻¹) were categorized into five pollution classes. High-throughput sequencing of 16S rRNA gene amplicons revealed the dominance of the phyla Pseudomonadota (mean relative abundance 32%), Acidobacteriota (22%), and Actinomycetota (16%). At the class level, Alphaproteobacteria (18%), Terriglobia (16%), and Gammaproteobacteria (14%) were the most abundant taxa, indicating their adaptation to long-term heavy metal contamination. The genus Z2-YC6860 exhibited significant tolerance to Cu, whereas Bradyrhizobium_503372 was negatively associated with As and Zn concentrations. Functional predictions suggested enrichment of key pathways related to heavy metal resistance, including efflux systems and detoxification. The study design spans a broad Cu pollution gradient across river-associated and industrially impacted sites, providing an ecologically relevant framework for evaluating microbial responses to long-term metal stress.

The aim of the study was to evaluate the long-term impact of heavy metals on soil bacterial metabolism by assessing their capacity to utilize a set of carbon sources. The study area is in the vicinity of the Pb/Zn smelter KCM 2000, South-Central Bulgaria, with considerable Pb, Zn, Cd, Cu, and As pollution. The Biolog Ecoplate approach was used to evaluate the bacterial metabolic activity, expressed by the average well colour development; the bacterial metabolic fingerprints expressed by community-level physiological profiling, and Shannon bacterial functional diversity. The results revealed reduced metabolic activity recorded for bacterial communities inhabiting the heavily polluted soils. The most preferred carbon groups utilized by the bacterial communities were the polymers, followed by carbohydrates and amino acids. Local variability of soil properties, such as inorganic ions, total organic carbon, pH, soil moisture, and soil texture, could modify the effects of heavy metals. This study highlighted that Biolog EcoplateTM is a suitable approach for assessing bacterial functional shifts in relation to long-term soil pollution stress.