Heavy metals removal and reduction of heavy metal phytotoxicity by Arthrobacter, Bacillus, and Pseudomonas strains harboring genes encoding metal resistance

An experiment was conducted to assess the potential of bacteria isolated from wastewater-irrigated soils to remediate heavy metals and mitigate heavy metal-induced phytotoxicity in wheat. Four bacterial isolates, i.e., M50-3/1, wwCd-5/1, wwCd-5/2, and wwCd-5/3 were isolated from wastewater-irrigated soils and tested for metal tolerance by MIC assay. Metal removal efficiency was evaluated in a mini-pilot assay involving both wastewater and contaminated soil. The bacterial isolates showed tolerance to all the levels (50–2000 mg L⁻¹) of the heavy metals (Cd, Pb, and Zn). Whereas, for As the MIC values of the isolates M50-3/1, wwCd-5/2, and wwCd-5/3 were 100, 2000, and 500 mg L⁻¹, respectively. All the bacterial isolates significantly solubilized higher amounts of phosphate, produced IAA and antioxidants (APX, CAT, and POX) in the growth medium supplemented with 500 mg L⁻¹ each of As, Cd, Pb, Zn, individually and in combined application as compared to control. In the assay, the lowest amounts of As (45.0 μg L⁻¹), Cd (0.21 μg L⁻¹), Pb (0.77 μg L⁻¹), and Zn (0.3 μg L⁻¹) were detected in the supernatant of treatment T4. The isolates wwCd-5/1, wwCd-5/2, wwCd-5/3, and M50-3/1 were identified as Arthrobacter sp., Bacillus thuringiensis, Bacillus aequororis, and Pseudomonas putida, respectively, through 16S rRNA sequencing. The amplified sequences of heavy metal resistance-encoding genes (CzcA, CzcD, and pbrA) from four isolates revealed significant sequence homology with those found in Bacillus and Pseudomonas strains. The application of metal-tolerant bacteria improved plant growth and reduced shoot metal contents and phytotoxicity under metal stress of 500 mg kg⁻¹.