Investigation of the toxic effect of cadmium on Candida humicola and Bacillus subtilis using a microcalorimetric method

In this study, the technique of microcalorimetry based on heat-output by aerobic bacterial respiration was explored to evaluate the toxic effect of cadmium on Candida humicola, Bacillus subtilis, singularly or in a mixture of both. Power-time curves of the growth metabolism of C. humicola and B. subtilis and the effect of Cd2+ were studied using the TAM III (the third generation thermal activity monitor) multi-channel microcalorimetric system, isothermal mode, at 28 °C. The differences in shape of the power-time curves and the thermodynamic and kinetic characteristics of microorganisms growth were compared. The effect of cadmium added into microorganism would significantly reduce the life cycle and change the thermal effect of microbial metabolic process with different concentrations of Cd2+. The experimental results revealed that at the same concentration, the sequence of inhibitory ratio (I) and maximum thermal power (Pmax) of the Cd2+ was: mixed microorganisms > C. humicola > B. subtilis. The sequence of total thermal effect (Qtotal) and growth rate constant (k) is mixed microorganisms > B. subtilis > C. humicola. These results are important to further studies of the physiology and pharmacology of C. humicola and B. subtilis and may support the theory of restoring contaminated soil.