Two plant growth promoting bacterial Bacillus strains possess different mechanisms in adsorption and resistance to cadmium

Microorganisms are promising biosorbents for decontaminating cadmium-polluted soil or water systems, but the underlying remediation mechanisms are still unclear. In this study, the cadmium biosorption mechanisms and capabilities of plant growth-promoting microorganisms (Bacillus megaterium NCT-2 and Bacillus paranthracis NT1) were investigated. Batch biosorption experiments showed that the optimal biosorption conditions for B. megaterium NCT-2 and B. paranthracis NT1 were pH 6.0, a biomass dosage of 1.0 g L-1, and an initial Cd2+ concentration of 10 mg L-1, and pH 8.0, a biomass dosage of 1.0 g L-1, and an initial Cd2+ concentration of 10 mg L-1, respectively. The biosorption processes of both biosorbents were well described by the pseudo-second order kinetic model, which indicated that the biosorption of Cd2+ was mainly chemisorption. The intracellular accumulation portion of adsorbed Cd2+ in B. megaterium NCT-2 was much higher than in B. paranthracis NT1 (43.11% and 3.25%, respectively), which resulted in the lower cadmium tolerance (14 mg L-1 and 280 mg L-1, respectively) and higher cadmium removal efficiency (46.79% and 20.45%, respectively) of B. megaterium NCT-2 compared to B. paranthracis NT1. SEM-EDS and FTIR analysis suggested the probable interactions of Cd2+ with the biosorbent surface ligands, such as -OH, -NH, -SO3, CO and -COOH during surface adsorption. Results of qRT-PCR illustrated that the difference in cadmium resistant mechanism and adsorption performance between B. megaterium NCT-2 and B. paranthracis NT1 may be regulated by the genes cadA, zitB, khtT, and bshA and cadA, trkA, czcD, and bshA, respectively. Our results revealed that these two biosorbents have the potential for further use in the development of cadmium remediation technologies and could provide insight into the mechanisms of cadmium biosorption.