Molecular Identification of Potent Chromium Reducing Bacteria Isolated from Hydrocarbon-Contaminated Soil within Sokoto Metropolis

Abstract

Hydrocarbon-contaminated soils are recognized as reservoirs for heavy metal-utilizing bacteria due to the phenomenon of co-selection. These bacteria can have potential in the biosorption of chromium heavy metal. This research aimed to screen the chromium reduction potential of bacteria isolated from hydrocarbon-contaminated soils. The soil samples used in this study were collected from hydrocarbon-affected sites in the Sokoto metropolis; additionally, a control sample was collected from non-polluted soil. Bacteria were isolated using standard protocols. Variable amounts of chromium were prepared using potassium monochromate (K₂CrO₄) and then incorporated into a nutrient broth medium. The most potent, molecularly identified hydrocarbonoclastic bacteria were screened for chromium tolerance, and the percentage reduction in chromium content was also measured. Mean colony counts from the hydrocarbon-contaminated soil ranged from 1.00×10⁶ to 1.30×10⁶ CFU/g, while the control soil had 2.30×10⁵ CFU/g. From the 14 strains, two, molecularly identified using NCBI BLAST as Brucella intermedia and Bacillus sp., were shown to be the most potent chromium-tolerant isolates. B. intermedia reduced Cr from an initial value of 350 mg/L to 198 mg/L within 72 hours (44% removal efficiency). At the lowest concentration used in this study (50 mg/L), a removal efficacy of 96% was achieved. Bacillus sp. recorded the highest chromium reduction compared to Brucella intermedia at the tested concentrations (50, 150, 250, and 350 mg/L). A 100% reduction in Cr was obtained at the 50 mg/L concentration. This study demonstrated that Bacillus sp. and Brucella intermedia are particularly effective at reducing chromium from chromium metal solutions of different concentrations. These isolates can be used for bioremediation of chromium-polluted soils or water bodies.