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 Cu, Zn, Cd and Pb, respectively, with initial concentrations of 10 mg/L at pH 7.0. In presence of Cu or Zn, large quantities of cations were released into the medium in descending order of Na>K>Ca>Mg while bioremoval of the two essential metals Cd and Pb was accompanied with cellular Na and Mg uptake from the medium respectively. The relative mean contributions of intracellular accumulation to the total removal were approximately 19.6% for Cu, 12.8% for Zn, 51.1% for Cd, and only 4.6% for Pb. Following exposure at high concentration, B. cereus RC-1 could keep intracellular Cd concentrations constant, possibly by means of a Cd-efflux system whose activity coincided with uptake of Na, and reduce intracellular Pb concentration due to the effect of Mg on limiting Pb 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 Cd and Pb coincided with uptake of Na and Mg, respectively, inspires a novel research perspective towards the study of protective mechanism of bacterial cells against the toxicity of heavy metals.