The inhibitory mechanism of natural soil colloids on the biodegradation of polychlorinated biphenyls by a degrading bacterium

Highlights: • Natural soil colloids greatly inhibited the biodegradation of PCB. • The adsorption by soil colloids restricted the bioaccessibility of PCB to the bacterium. • PCB reduction in biodegradation was not ROS affected. • Ex-DLVO theory proved that Ultisol colloids could suppress the effective contact between cells and adsorbed-PCB. In spite of extensive studies of soil model components, the role of natural soil colloids in the biodegradation of organic pollutants remain poorly understood. Accordingly, the present study selected Mollisol colloids (MCs) and Ultisol colloids (UCs) to investigate their effects on the biodegradation of 3, 3′, 4, 4′-tetrachlorobiphenyl (PCB77) by Bradyrhizobium diazoefficiens USDA 110. Results demonstrated that both natural soil colloids significantly decreased the biodegradation of PCB77, which partly resulted from the significant decrease in the bioaccessibility of PCB77. Furthermore, the activity of Bradyrhizobium diazoefficiens USDA 110 was remarkably inhibited under the exposure to the two types of soil colloids, which was mainly ascribed to the inhibition of cell reproduction but not the lethal effect of reactive oxygen species. The calculated results from Ex-DLVO theory further indicated that the repulsion between UCs and biodegrading bacteria retarded the effective contact of cells with adsorbed PCB77 from UCs, resulting in the decline of the rate of cell reproduction. In general, the inhibition of MCs was limited to PCB77 bioaccessibility, whereas the negative effect of UCs was controlled by PCB77 bioaccessibility and the effective contact of cells with colloids. This study could provide implication for the enhancement of microbial remediation in contaminated soil.