In Vivo Studies to Elucidate the Role of Extracellular Polymeric Substances from Azotobacter in Immobilization of Heavy Metals

The role of extracellular polymeric substances (EPS) produced by the heavy metal-resistant strain of Azotobacter spp. in restricting the uptake of cadmium (Cd) and chromium (Cr) by wheat plants cultivated in soils contaminated with the respective heavy metals has been demonstrated. A heavy metal-resistant strain of Azotobacter spp. was isolated and identified. Minimum inhibitory concentrations (MIC) of Cd2+ and CrO4 2− were determined to be 20 and 10 mg L−1, respectively. Under in vitro conditions, the EPS produced by the strain could bind 15.17 ± 0.58 mg g−1 of Cd2+ and 21.9 ± 0.08 mg g−1 of CrO4 2−. Fourier transform infrared spectra of the EPS revealed the presence of functional groups like carboxyl (−COOH) and hydroxyl (−OH), primarily involved in metal ion binding. Under pot culture experiments, the isolated strain of Azotobacter was added to the metal-contaminated soils in the form of free cells and immobilized cells. The total Azotobacter count and plant metal concentrations under different treatments showed a negative coefficient between the Azotobacter population and plant Cd (−0.496) and Cr (−0.455). Thus it could be inferred that Azotobacter spp. is involved in metal ion complexation either through EPS or through cell wall lipopolysaccharides (LPS).