Isolation and Characterization of Plant Growth-Promoting Microorganisms from Heavy Metal Contaminated Soil

Abstract

The study aimed to isolate and characterize plant growth-promoting microorganisms (PGPMs) from heavy metal-contaminated soils to explore their potential in bioremediation and sustainable agriculture. Soil samples were collected from industrial, mining, and agricultural sites with heavy metal exposure, and microorganisms were isolated using selective media. Morphological, biochemical, and molecular analyses identified diverse bacterial and fungal genera, including Bacillus, Pseudomonas, and Trichoderma. These isolates exhibited significant plant growth-promoting traits, such as indole-3-acetic acid (IAA) production, phosphate solubilization, and siderophore production. Notably, Bacillus subtilis strains produced up to 40 µg/mL of IAA, and Pseudomonas fluorescens showed efficient phosphate solubilization. Heavy metal tolerance was assessed through minimum inhibitory concentration (MIC) tests, revealing several isolates with high tolerance to lead (Pb), cadmium (Cd), and zinc (Zn). For instance, Bacillus megaterium and Pseudomonas aeruginosa maintained growth at metal concentrations exceeding 500 mg/L. Mechanisms of tolerance included exopolysaccharide (EPS) production and enhanced antioxidant enzyme activities (superoxide dismutase, catalase, and peroxidase), which were particularly notable in Bacillus subtilis and Aspergillus niger. These traits enable the microorganisms to bind heavy metals, reduce their bioavailability, and neutralize reactive oxygen species (ROS), thereby enhancing their survival and function in contaminated environments. The findings suggest that these PGPMs can be utilized for bioremediation by detoxifying and stabilizing heavy metals in soils, reducing their ecological risk. Additionally, their application in agriculture as biofertilizers can promote plant growth and soil health, reducing reliance on chemical fertilizers. Future research should focus on field trials to validate laboratory findings, exploring microbial consortia for synergistic effects, and developing formulations for large-scale application. Overall, the successful isolation and characterization of PGPMs from heavy metal-contaminated soils provide a promising approach to addressing soil contamination and enhancing agricultural productivity. These microorganisms offer a sustainable solution to improve plant growth in challenging environments, contributing to environmental conservation and food security.