Cadmium tolerant microbial strains possess different mechanisms for cadmium biosorption and immobilization in rice seedlings

Heavy metal remediation, such as cadmium (Cd2+) by microbial strains is efficient and environment-friendly. In this current study, we exploited the potential of Bacillus strains (Cd2+-tolerant; NMTD17, GBSW22, and LLTC96) to regulate Cd2+ biosorption mechanisms and improve rice seedling growth. The results showed that initial concentration and contact time affected Cd2+ biosorption, and the kinetic models of pseudo orders were effective in the elaborate biosorption process. Mainly, the bacterial cell wall had the potential for Cd2+ biosorption, and we found non-significant biosorption alterations among bacterial strains' inner and outer surfaces of cell membranes. Furthermore, the Fourier transform infrared (FTIR) spectroscopy analysis identified the differences in functional groups, such as C-N, PO2, -SO3, CO, COOH, C-O, C-N, -OH, and -NH that interact in biosorption by Bacillus strains. The scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) examination revealed that the binding of Cd2+ to microbes was mostly based on ion exchange pathways. Moreover, the Bacillus strains responded to Cd2+ stress in rice under pot experiment at various concentrations (0, 0.25, and 0.50 mg kg-1), and they also influenced the chlorophyll contents and antioxidants activities were studied. The analysis of physio-morphological parameters was observed to be increased, which indicated that all Bacillus strains showed significant effects on rice growth under Cd2+ stress. These results revealed that the selected strains had the capability for additional use in the development of Cd2+ bioremediation methods. These strains also provided plant growth-promoting (PGP) traits that can alleviate the harmful effects of Cd2+ in rice plants.