The toxic effect of cadmium on pure microbes using a microcalorimetric method and a biosensor technique

A microcalorimetric technique based on microbes heat-output was explored to evaluate the effect of Cd (II) on Bacillus subtilis and Candida humicola. The power-time curves of the growth metabolism of Bacillus subtilis and Candida humicola and the effect of Cd (II) on it were studied by using a TAM III microcalorimeter, ampoules method at 28 degrees C. For the evaluation of toxic effect on pure micro-organisms, the maximum peak-heat output power (P(max)) in the growth phase, the growth rate constants (k), the log phase heat effects (Q(log)), and the total heat effect (Q(T)) for Bacillus subtilis and Candida humicola were determined. Dissolved oxygen and biochemical oxygen demand (BOD) were evaluated by a biosensor. Cadmium has been regarded as the essential biological trace element. Cd (II) solutions of different concentration have different effects on Bacillus subtilis and Candida humicola growth metabolism. The higher concentrations of Cd (II) inhibit the growth of Candida humicola (1600-3200 microg.mL(-1)), Bacillus subtilis (240-480 microg.mL(-1)); the lower concentrations can promote the growth of both micro-organism. The values of cell dry weight is also showed in conformity in the cell dry weight changes to the micro-organisms' growth time. Comparison of growth curves of two micro-organisms showed that both the trends of biochemical oxygen demand were exhibiting regressive changes with the passage of time during their generation times (t(G)). Results from ultraviolet spectrophotometer and precision pH meter all showed that the control growth curves were visioning same trends with the thermodynamic curves of micro-organisms measured by microcalorimeter.