Microbial diversity in soils from antimony mining sites: geochemical control promotes species enrichment

Elevated soil concentrations of antimony (Sb) and co-contaminants are frequently encountered where antimony has been mined on a large scale. For instance, the Xikuangshan antimony mine in central South China has sustained, over many centuries, dispersed and spatially variable input of toxic elements into the soil ecosystem. We utilized this unique environment to assess the impact of geochemical conditions on soil microbiology. Geochemical conditions were assessed by monitoring absolute and available fractions of toxic elements and disrupted soil properties. Soil microbiology was studied by high-throughput sequencing and statistical analysis, including principle component analysis and canonical correspondence analysis. Results show that Sb concentrations were ranged from 970 to more than 24,000 mg/kg. As concentrations were three times higher than the regional background values and ten times higher for Pb, 590 times higher for Cd and 30 times higher for Hg. About 5-10% of the total soil Sb was environmentally mobile. Microbial diversity was high, and soil properties such as pH, organic matter, iron and sulfate controlled the absolute microbial activity. We identified strong positive and negative correlations with specific bacterial taxonomic groups which show: (1) an intolerance of available fractions for all elements, e.g., Gemmatimonas, Pirellula, Spartobacteria; (2) a good tolerance of available fractions for all elements, e.g., Povalibacter, Spartobacteria; and (3) a mixed response, tolerating available Sb, Hg and Cd and inhibition by As, Pb, e.g., Escherichia/Shigella and Arthrobacter, and in reverse, e.g., Gemmatimonas and Sphingomonas. The site hosts great diversity dominated by Gram-negative organisms, many with rod (bacillus) morphologies but also some filamentous forms, and a wide range of metabolic capabilities: anaerobes, e.g., Saccharibacteria, metal oxidizing, e.g., Geobacter, chemoautotrophs, e.g., Gemmata, and sulfur reducing, e.g., Desulfuromonas. The bioremediation potential of Arthrobacter and Escherichia/Shigella for Sb control is highlighted.