Microbial metal toxicity had been attributed to metal concentration and bioavailability, the later factor being dependent on speciation; the distribution of metals into various physicochemical forms. This research was aimed at establishing a correlation between the nature of predominating ligands in natural environments which can associate with metals and influence toxicities to biodegrading microbes. Cadmium, a known inducer of oxidative stress, was chosen because of the nature of the oil-degrading microorganisms used. The organisms were chosen because of their robust accumulation of biomass in crude oil/mineral salts medium under aerobic cultivation conditions. The forms of cadmium used included acetate, chloride and sulfate at 0, 1, 10, 100, and 1000 mg cadmium/L concentrations. Duration of study was 24 h with agitation. At the end of the study, cadmium toxicity was found to positively correlate (r2 = 0.94, P < 0.01) with chemical species. Acetate was the most toxic of the ligands tested, but not necessarily at the highest cadmium concentration. Attempts at reducing cadmium toxicity with divalent cations like zinc, magnesium and barium; cations found in the study area, revealed that only zinc and magnesium could mitigate cadmium toxicity (P < 0.05) to the bacterium. These findings underscore the unpredictability of metal toxicity in natural environments, and call for better understanding of the physiology and ecology of bioremediating microbes.
Key words: Chemical species, divalent cations, crude oil, bioremediation.
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