Molybdenum tolerance of strains screened from molybdenum tailings and their potential application in...

Mining and smelting activities of molybdenum (Mo) have led to increasingly severe Mo pollution in the environment, posing potential threats to ecosystems and human health. Microbial remediation technology has emerged as an effective approach for reducing Mo pollution due to its environmental friendliness and cost-effectiveness. In this study, two Mo (Ⅵ) tolerant strains (MoTB 2 and MoTB 79) with exceptional Mo reduction capacities were isolated and identified from the rhizosphere soil of pioneer plants in Mo-contaminated tailings. Systematic studies were conducted to evaluate their Mo tolerance, reduction efficiency, and underlying mechanisms in both controlled and soil environments. The results showed that both strains exhibited robust growth in high-Mo (VI) conditions (6000 mg L^-1), comparable to Mo-free environments, with optimal growth observed at pH 5.0 and 30°C. Notably, under conditions of 5.0 mmol L^-1 phosphate and 60.0 mmol L^-1 molybdate, the strains demonstrated significant Mo (Ⅵ) removal capabilities (14.8%-22.5% within 24 h) via bioconversion to molybdate. Further mechanistic analysis using a multi-technique approach revealed that FTIR spectroscopy identified phosphate groups, amide bonds, C-O-C, and -CH groups as key functional entities for extracellular Mo (Ⅵ) adsorption, while TEM-EDX confirmed intracellular bioaccumulation. Critically, XPS quantification demonstrated valence state transformations, with 79.8%-86.3% of toxic Mo (Ⅵ) reduced to less toxic Mo (Ⅳ) (23.4%-39.7%) and Mo (Ⅴ) (40.1%-63.0%). To bridge laboratory findings with environmental applications, soil remediation experiments revealed an 11.8%-19.6% conversion of mobile Mo (Ⅵ) to immobilized Mo (0) and Mo (Ⅳ) fractions via XANES, effectively passivating bioavailable Mo. Notably, the strains also exhibited phosphate/potassium solubilization capabilities, suggesting dual roles in metal detoxification and nutrient cycling. This integrated study elucidates the potential of MoTB strains as eco-engineers for bioremediation in Mo-contaminated ecosystems through adsorption, bioaccumulation, multi-valence reduction, and soil fertility enhancement.