Metal-Tolerating ability of some selected Rhizobia strains

Abstract

Heavy-metal pollution from many industrial processes is a major threat to human health and the environment resulting in loss of farming and grazing land. Many bacterial strains have been reported to tolerate metals but there is a dearth of research on the metal-tolerating ability of rhizobia strains, hence the need for this study to screen selected Rhizobial strains for their ability to tolerate varying concentrations of selected heavy metals. Ten Rhizobia strains including Bradhyrhizobium japonicum strains FA3, UDSA136, USDA 9032, USDA110, RANI 22, USDA 4675, RAUG and Bradhyrhizobium sp. strains B574, R25B and USDA 3541 obtained from the culture collection of the International Institute of Tropical Agriculture (IITA) were used for this study. They were screened for their ability to tolerate varying concentrations (10-150 µg/mL) of six selected metals (Copper, Cobalt, Cadmium, Lead, Zinc and Iron) on metal-incorporated Congo-red medium. FA3 (Bradhyrhizobium japonicum) showed the highest tolerance to iron (100 µg/mL), while FA3, USDA110 and USDA 4675 showed highest resistance to zinc with Minimum Inhibitory Concentration (MIC) of 150 µg/mL. Strains USDA 3451, USDA 4675, B547 were unable to grow on the cobaltincorporated medium, while strains RAUG 1, USDA 4675, USDA136 and R25B had the highest MIC of 150 µg/mL for lead. Copper was the most toxic to the Rhizobial strains as the MIC recorded was between 10-20 µg/mL, while all the strains were able to tolerate 150 µg/mL concentration of Cadmium. Rhizobial strains could find a use in the bioremediation and recovery of soils contaminated with heavy metals as shown by their potentials to tolerate certain degree of metal concentration in this study.