Background: : Lead (Pb) is an important environmental pollutants that play a significant role in increasing the stability of some other pollutants by changing the microbial profile of the soil.  Bacillus subtilis WPI is an abundant bacteria existing in wastewater. Because of laccase enzyme in this bacterium, the decomposition process of aromatic pollutants in wastewaters can be facilitated. We aimed to investigate the effect of different Pb concentrations on B. subtilis growth and biological activity of laccase enzyme in B. subtilis WPI.
Material and Methods: B. subtilis WPI was isolated from the paper mill industrial wastewater of Hormozgan, Iran, from March to August 2017. After purification, the growth trend of B. subtilis WPI as well as the activity of laccase enzyme in different concentrations of Pb was investigated based on kinetic method.

Results: Bacterial growth at Pb concentration of 400 mg/L reduced in a dose-dependent manner, and this decrease was significant at concentrations of 300 and 400 mg/L (P < 0.001). The level of laccase enzyme activity in the lead concentration range of 20-160 mg/L also reduced in a dose-dependent manner, which implied that the highest decrease was observed at lead concentration of 160 mg/L (P < 0.01). Our findings showed that there was no significant change in bacterial growth in lead concentration range of 20-200 mg/L, while a significant change was observed in the activity of laccase enzyme in the mentioned concentration range.
Conclusion: Therefore, it seems that this reduction in enzyme activity can indirectly increase the stability of aromatic oil pollutants in the environment.