Characterization of a symbiotically effective Rhizobium resistant to arsenic: Isolated from the root nodules of Vigna mungo (L.) Hepper grown in an arsenic-contaminated field.

Bacteria were isolated from the root nodules of Vigna mungo (L.) Hepper, grown in an arsenic-contaminated field and the strain was selected by its nodulation ability as well as better arsenic tolerant capacity compared to others. The selected strain was identified as Rhizobium by 16S rDNA sequencing and designated as VMA301. Phylogenetic analysis of the gene sequences showed its close relatedness with Sinorhizobium fredii. LC(50) value of arsenate for the bacteria as determined by flow cytometry was found to be 2.8 mM and arsenic uptake was measured by atomic absorption spectrometry as 0.048 mg g(-1) biomass. The high amount of arsenic was toxic to the cell, which changed the morphology of the bacteria to an elongated shape. Presence of a transcriptional regulatory gene (ArsR) of the ars genetic system was confirmed by amplification and sequencing. The symbiotic property of the isolate was also confirmed by amplification and sequencing of the NodC gene. These results indicate that the isolated Rhizobium bacteria may exert dual roles in the environment, arsenic bioremediation from the soil as well as increase of soil fertility through nitrogen fixation.