A survey of bacterial and fungal community structure and functions in two long-term metalliferous soil habitats.

Mercury contamination at legacy nuclear sites such as the Savannah River Site and Oak Ridge Reservation poses persistent ecological risks, yet its impact on soil microbiomes remains incompletely understood. This study integrates qPCR, 16S/ITS amplicon sequencing, and shotgun metagenomics to assess bacterial and fungal community structure, diversity, and functional potential across gradients of total mercury, methylmercury, and bioavailable mercury. Bacterial α-diversity declined with increasing Hg levels, while fungal diversity remained stable and highest in low-contamination soils. Dominant bacterial phyla included Pseudomonadota, Bacteroidota, Bacillota, Acidobacteriota, and Actinomycetota; fungal communities were primarily Ascomycota and Basidiomycota. Canonical correspondence analysis revealed distinct taxon-Hg speciation linkages, and functional gene profiling showed enrichment of stress-response genes, membrane transporters, and phosphate metabolism pathways in contaminated soils. Notably, bioavailable Hg did not correlate directly with total Hg, underscoring the importance of speciation in microbial exposure. These findings highlight the adaptive plasticity of native microbiomes and identify microbial taxa and pathways relevant to bioremediation and can guide ecosystem restoration activities in Hg-impacted soil habitats.