Antimony Causes Mortality and Induces Mutagenesis in the Soil Functional Bacterium Azospirillum brasilense Sp7

Antimony (Sb) is increasing in the environment but effects of exposure in ecosystems are not well understood. The aim of this work was to examine effects of antimony exposure on the multifunctional, plant growth promoting, ubiquitous soil bacterium Azospirillum brasilense Sp7. Contaminated mine water with high Sb concentrations (0.13 ± 0.09 mg L⁻¹) was lethal to A. brasilense Sp7 in laboratory experiments. Exposure-dose- and time-dependent incubation toxicity assays on A. brasilense Sp7 with Sb(III) and Sb(V) at different concentrations (0.05–5 mg L⁻¹) also resulted in cell mortality which was dose and time dependent. Median effect concentrations of 0.004–0.049 and 0.019–0.467 mg L⁻¹ were estimated for Sb(III) and Sb(V), respectively. Exposure to Sb(III) resulted in greater cell mortality than Sb(V) at all concentrations tested. Exposure also resulted in the emergence of phenotypic variants that were more frequent with exposure to Sb(III). The toxicity assays demonstrated that Sb alone could have been responsible for the mortality observed with exposure to the contaminated mine water even without any other contaminants present. A. brasilense Sp7 was highly sensitive to Sb exposure and the observed effects suggest possible consequences for microbial function, plant-bacterial symbioses and ecosystem health with Sb contamination.