Screening and identification of plant growth promoting bacteria and their influence on soil heavy metal forms

The unregulated dismantling of electronic products has led to heavy metal contamination of soil at electronic waste dismantling sites, causing serious environmental problems and not conducive to environmentally sustainable development. Therefore, we screened four bacterial strains from soil collected from a contaminated e-waste site. The objectives of the study were: to identify plant growth-promoting strains with useful characteristics; to determine the behavior of the strains by various qualitative and quantitative tests; to determine the effect of each strain on the migration of different forms of heavy metals from the soil by shake flask tests; and to determine the mechanisms by which this migration occurs. This study investigated the nitrogen fixation, inorganic phosphorus solubilization, iron carrier production, indole-3-acetic acid secretion, and metal tolerance of each bacterial strain, and used 16S rDNA analysis to determine its taxonomic status and growth characteristics. The 16S rDNA analysis revealed that strains RH1 and RH3 belonged to the genus Rhizobium, and strains MO2 and MO4 belonged to the genus Microbacterium. MO4 had the highest indole-3-acetic acid production capacity of 26.98 mg/L; RH3 had the highest inorganic phosphorus solubilization capacity of 1.33; RH1, MO2, RH3, and MO4 were capable of fixing nitrogen; and MO2 and MO4 were capable of producing iron carriers. A series of tests showed that the strains affected the levels of different forms of heavy metals and played a mobilizing role. In conclusion, RH3 and MO4 are excellent plant growth-promoting bacteria and have considerable potential for the green remediation of soils contaminated with heavy metals.