Heavy metal bioaccumulation and cation release by growing Bacillus cereus RC-1 under culture conditions

In an effort to explore the detoxifying mechanisms of B. cereus RC-1 under heavy metal stress, the bioaccumulation by growing cells under varying range of pH, culture time and initial metal concentration were investigated from a perspective of cation release. The maximum removal efficiencies were 16.7%, 38.3%, 81.4% and 40.3% for Cu2+, Zn2+, Cd2+ and Pb2+, respectively, with initial concentrations of 10 mg/L at pH 7.0. In presence of Cu2+ or Zn2+, large quantities of cations were released into the medium in descending order of Na+ > K+ > Ca2+ > Mg2+ while bioremoval of the two essential metals Cd2+ and Pb2+ was accompanied with cellular Na+ and Mg2+ uptake from the medium' respectively. The relative mean contributions of intracellular accumulation to the total removal were approximately 19.6% for Cu2+, 12.8% for Zn2+, 51.1% for Cd2+, and only 4.6% for Pb2+. Following exposure at high concentration, B. cereus RC-1 could keep intracellular Cd2+ concentrations constant, possibly by means of a Cd-efflux system whose activity coincided with uptake of Na+, and reduce intracellular Pb2+ concentration due to the effect of Mg2+ on limiting Pb2+ access to the cells. Cellular morphology, surface functional groups and intracellular trace elements were further investigated by SEM-EDX, TEM-EDX, FTIR and ICP-MS analysis. The phenomena that removal of Cd2+ and Pb2+ coincided with uptake of Na2+ and Mg2+, respectively, inspires a novel research perspective towards the study of protective mechanism of bacterial cells against the toxicity of heavy metals.