Survival of introduced phosphate-solubilizing bacteria (PSB) and their impact on microbial community structure during the phytoextraction of Cd-contaminated soil

This study was conducted to investigate whether or not phosphate-solubilizing bacteria (PSB) as a kind of plant growth promoting rhizobacteria enhance the uptake of Cd by plants. In addition, the effect of PSB augmentation during phytoextraction on the microbial community of indigenous soil bacteria was also studied. In the initial Cd-contaminated soil, the major phyla were Proteobacteria (35%), Actinobacteria (38%) and Firmicutes (8%). While Proteobacteria were dominant at the second and sixth week (41 and 54%, respectively) in inoculated soil, Firmicutes (mainly belonging to the Bacilli class-61%), dramatically increased in the eight-week soil. For the uninoculated soil, the proportion of α-Proteobacteria increased after eight weeks (32%). Interestingly, Actinobacteria class, which was originally present in the soil (37%), seemed to disappear during phytoremediation, irrespective of whether PSB was inoculated or not. Cluster analysis and Principal Component Analysis revealed that the microbial community of eight-week inoculated soil was completely separated from the other soil samples, due to the dramatic increase of Bacillus aryabhattai. These findings revealed that it took at least eight weeks for the inoculated Bacillus sp. to functionally adapt to the introduced soil, against competition with indigenous microorganisms in soil. An ecological understanding of interaction among augmented bacteria, plant and indigenous soil bacteria is needed, for proper management of phytoextraction.