Arsenic and other heavy metals resistant bacteria in rice ecosystem: Potential role in promoting plant growth and tolerance to heavy metal stress

The role of microorganisms in arsenic (As) uptake in association with terrestrial plants is identified, though their rhizosphere specific association and function in rice and barnyard grass in As contaminated soil are not elucidated. We investigated the abundance and diversity of arsenic and other heavy metal tolerant bacteria isolated from barnyard grass and rice rhizospheres, and the bulk soil from an As-contaminated paddy field. Among 83 isolated As resistant bacteria, 22 strains were screened and characterized based on their growth potential in 100 mM arsenate. One strain oxidized and reduced arsenic, whereas five isolates oxidized arsenite to arsenate and seven isolates reduced arsenate to arsenite. All the isolates were tolerant to the highest levels of Zn, Cd, Cr, Cu, Co, and Hg tested and except one, all also survived at the highest level of Pb. Molecular characterization by 16s rRNA gene analysis revealed that the selected bacteria belong to Sphingomonadaceae (13), Moraxellaceae (2), Enterobacteriaceae (2), Planococcaceae (2), Pseudomonadaceae (1), Xanthomonadaceae (1) and Cyanobacteria (1). Bacterial abundances were more in the rhizosphere than in bulk soil and diversity was more in barnyard grass rhizosphere than in rice. Among the isolates, 21 strains solubilized phosphate, produced indole acetic acid and grew in N-free medium. All the isolates that were tested, enhanced the germination and root fresh weight of rice seedlings. These isolates, based on their As and other metals resistance, As detoxification capacity, and plant growth promotion ability, especially the members of Sphingomonadaceae, could be conclusively used for plant growth promotion and microbe assisted remediation of the As and other heavy metal contaminated soil employing barnyard grass and rice.