Heavy metal stress response in the diazotroph, Rhizobium petrolearium

Abstract

Heavy metals occur at elevated concentrations in the environment mainly due to anthropogenic activities. The response of the hydrocarbon-degrading, diazotroph, Rhizobium petrolearium, to varying concentrations of the heavy metals – lead, cadmium and chromium(VI) – was investigated by establishing the effect of loading concentrations of 1 mM to 15 mM on its growth and enzymatic activity with increasing exposure time. The findings revealed a decline in growth and enzymatic activity with increasing heavy metal concentration and increasing exposure period. Mean reductions in abundance of 42.0% – 100.0%, 52.1% – 100.0% and 54.4% – 100.0% for different concentrations of lead, cadmium and chromium(VI) respectively were obtained. For lead and cadmium, no growth (100% decline) was seen by the end of the study at 10mM and 15mM exposure concentrations. There was no growth at the Cr(VI) concentrations of 5mM – 15mM by the end of the study. Statistically significant differences (p≤0.05) were seen in growth reduction between the different application concentrations. The results inferred the order of toxicity against R. petrolearium based on growth response to be chromium(VI) > cadmium > lead. At higher concentrations of 10 mM and 15 mM, all three heavy metals inhibited α-amylase and protease activity but had only minimal impact on oxidase, catalase and cellulase activities. Only Pb and Cr(VI) inhibited protease activity at higher concentrations of 10mM and 15mM. Based on the enzymatic activity inhibition tests, the order of toxicity was Cd > Cr(VI) > Pb. The findings suggest that at high concentrations, the heavy metals, lead, cadmium and chromium(VI) could impact negatively on abundance and interrupt enzymatic activity in soil bacteria like R. petrolearium.