Abstract

There has been increased interest in the isolation of bacteria with a potential role in bioremediation from extreme environments such as phosphate mines containing various metals and radionuclides. This paper describes the isolation and characterization of a novelMicrococcus species from a phosphate mining region in the South of Tunisia, designated as strain BRM7. Colonies of bacterial strain BRM7 produced on Tryptone-Glucose-Yeast Extract (TGY) agar plates were yellow, smooth, circular and 0.5–1.5 mm in diameter. Cells of strain BRM7 were Gram-positive cocci, with a diameter of 0.7–1.0 µm. The most abundant cellular fatty acids of strain BRM7 were found to be anteiso-C15: 0 (49.42%) and iso-C15: 0 (32.63%). On the basis of its morphological attributes, biochemical characteristics, and partial sequencing of 16S rRNA gene (rDNA), the strain BRM7 belongs to the genus Micrococcus with 99% identity to Micrococcus luteus. Strain BRM7 grew well in tryptone, glucose and yeast extract (TGY) medium, and tolerated (1) high salt concentrations (up to 20%), (2) a wide range of pH (5.0–12.0), and (3) high temperatures (up to 45°C). The bacterial isolate Micrococus sp. BRM7 showed a high tolerance to strontium (Sr, D₁₀ (dose for 90% reduction in Colony Forming Units (CFUs)) = 350 mM) with a similar tolerance curve to Cupriavidus metallidurans CH34, best known for its high tolerance to a wide range of heavy metals. Interestingly, Micrococus sp. BRM7 has an ionizing-radiation (IR) resistance D₁₀ (~800 Gy) four times higher than that of C. metallidurans. Immobilization into alginate beads indicated that Micrococus sp. BRM7 cells have the potential to adsorb 17 and 34% of Sr following an incubation time of 3 and 24 h, respectively. Overall, the results of this study suggest that BRM7 can be valorized to bioremediate Sr in radioactive residues like phosphogypsum (PG), an industrial concentrator of this toxic metal.

Key words: Bioremediation, immobilization, ionizing radiation, Micrococcus, phosphate, strontium.