Heavy metal tolerance of bacterial isolates associated with overburden strata of an opencast coal mine of Assam (India)

Coal overburden strata (OBS) vary in thickness, geochemical composition, and physical properties from stratum to stratum. Here, we enumerated the cultivable bacterial diversity and their distribution in different OBS taken from the opencast mining of Tikok colliery, Assam. The pH of the coal OBS ranged from 2.46 to 7.93, but 73% of the OBS was acidic. The OBS samples were mostly of shale types except for a few that were sandstone, mudstone, and red soil. The bacterial CFUs per gram OBS samples were highly diverse ranging from 52 to 57.4x10(4). A total of 79 bacterial pure culture isolates belonging to 19 genera, 12 family, and 3 phyla (Actinobacteria, Firmicutes, and Proteobacteria) were recovered in nutrient agar plates. Firmicutes appeared dominant over the others. All the isolates were screened for heavy metal tolerance in broth culture augmented with five different metals (Ni2+, Cu2+, Cr6+, As3+, and Cd2+) separately. The number of isolates that showed tolerance was 95% for Cr6+, 69.6% for Ni2+, 50.6% each for As3+ and Cu2+, and 7.6% for Cd2+. The bacterial isolates with high metal tolerance, i.e., 5 to 12 mM could be promising for bioremediation of Ni2+, Cu2+, Cr6+, and As3+ from the sites contaminated with these heavy metals.