Microcalorimetric measurements of the microbial activities of single- and mixed-species with trivalent iron in soil

A microcalorimetric technique was applied to a series of experiments to follow the toxic effect caused by the trivalent iron on the single and mixed microbes in sterilized soil that was inoculated with the single Bacillus subtilis (B. subtilis) (prokaryotic bacterium), single Candida humicola (C. humicola) (eukaryotic fungus) and the mixed-species. The microbial activity was stimulated by the addition of 5.0 mg glucose and 5.0 mg ammonium sulfate under a 35% controlled humidity in the studied soil samples of 1.2 g. The power–time curves from every experiment were analyzed, and from these analyses characteristic parameters, such as growth rate constant (k) and total thermal effect (Q) which can reflect the biochemical reactions were determined. The mixed-species have moderate tolerance to the iron overload, comparing with single species, and exhibit synergistic interaction in exponential growth phase (0–400.0 μg mL−1). Meanwhile, there is no much difference in the thermal effect (Q) per gram soil sample for the single and mixed culture. This also validates that the nutrient substances in natural environment determine the organisms’ metabolic activities. Ultraviolet–visible spectrophotometry and dissolved oxygen sensor also were successfully applied to reflect the activities of B. subtilis and C. humicola in the pure culture. The investigation could provide insight into the microbial ecology of bacteria and fungi in ecological niches.