Heavy metals multi-tolerant Bradyrhizobium isolated from mercury mining region in Algeria

Abstract

Heavy metals accumulation in the environment has led to a decrease in the capacity of ecosystems to sustain life as human, animal and plant health is threatened. To remedy this problem, rhizoremediation has been suggested as a solution. Legumes and rhizobia symbiotic association has captivated attention due to its involvement in the restoration of heavy-metal-contaminated sites. Thus, the aim of this study was to isolate and characterize the strains nodulating Calicotome spinosa plant that naturally occurred in two Algerian mercury mines. Fifty-four bacterial strains were isolated, then grouped into sixteen distinct BOX-PCR patterns and were genetically identified as belonging to the Bradyrhizobium genus. The studied strains were able to induce nodules on Retama monosperma, R. reatam, Lupinus albus, while no nodulation was observed in Glycine max, their symbiotic capacity was confirmed by amplifying the nodC gene. The phylogenetic analysis based on the nodC has grouped this Bradyrhizobium strains to either symbiovar genistearum or retamae. The isolates revealed diversity in terms of NaCl; pH tolerance, and phosphate solubilization. Production of siderophores was negative for these strains. All the isolated Bradyrhizobium were tolerant to both Zn and Pb in contrast they were sensitive to Cu and Cd. Interestingly, 43% of strains were tolerant to high Hg levels. Hence, some strains displayed multiple tolerances to heavy metals. Therefore, this is the first time we identify Bradyrhizobium strains originating from a North African mercury mine. This study could help to select mercury and other heavy metal-tolerant rhizobia showing an interesting potential to be used as inoculants to remediate the heavy metal soil accumulation.