Isolation of Aerobic Heterotrophic Bacteria from a Microbial Mat with the Ability to Grow on and Remove Hexavalent Chromium through Biosorption and Bioreduction

Water pollution with toxic hexavalent chromium, Cr(VI), is an environmental threat that has a direct impact on living organisms. The use of microorganisms from microbial mats to remove Cr(VI) has scarcely been investigated. Here, we isolated aerobic heterotrophic bacteria from a Cr-polluted microbial mat found in a mining site in Oman, and investigated their ability to remove Cr(VI), and the underlying mechanism(s) of removal. All isolates fell phylogenetically into the genera Enterobacter, Bacillus, and Cupriavidus, and could completely remove 1 mg L⁻¹ Cr(VI) in 6 days. The strains could tolerate up to 2000 mg L⁻¹ Cr(VI), and exhibited the highest Cr(VI) removal rate at 100 ± 9 mg L⁻¹ d⁻¹. Using scanning electron microscopy (SEM) coupled with elemental analysis, the strains were shown to adsorb Cr(VI) at their cell surfaces. The functional groups OH, NH₂, Alkyl, Metal-O, and Cr(VI)-O were involved in the biosorption process. In addition, the strains were shown to reduce Cr(VI) to Cr(III) with the involvement of chromate reductase enzyme. We conclude that the aerobic heterotrophic bacteria isolated from Cr-polluted microbial mats use biosorption and bioreduction processes to remove Cr(VI) from wastewater.