Optimization of exopolysaccharide production and diesel oil emulsifying properties in root nodulating bacteria

Abstract

Bioremediation, a strategy mediated by microorganisms, is a promising way used in the degradation or removal of organic contaminants from soil or aquatic system. Exopolysaccharide (EPS) which was produced by a variety of Gram-negative bacteria has been demonstrated to be a potential bioemulsifier used in the degradation of hydrocarbons. In the present study, attempts were made to optimize the production of EPS from our newly isolates by adjusting the culture conditions and medium components. Besides, the performance of diesel oil emulsification using partially purified EPS derived from different conditions was also demonstrated. Out of 40 root nodulating bacteria the better emulsifying abilities were recorded from three strains namely Rhizobium miluonense CC-B-L1, Burkholderia seminalis CC-IDD2w and Ensifer adhaerens CC-GSB4, as can be seen from their emulsification index (E₂₄) 66, 64 and 60%, respectively. These three strains produced 212, 203 and 198 mg l⁻¹ of EPS, respectively, in yeast extract mannitol (YEM) medium. After modifying culture conditions, better performances can be achieved from these three strains, with increases of 21.7, 21.4, 16.7% in the EPS production and 12.1, 10.9, 8.3% in E₂₄, respectively. When considered for strain CC-B-L1 and CC-IDD2w, the addition of 1.5% (v/v) of mannitol and 0.1% (v/v) of asparagine in YEM enhanced 42.9 and 34.7% in EPS production along with 28.8 and 37.5% higher in E₂₄. The supplement of 2.0% (v/v) glucose and 0.2% (v/v) asparagine in YEM increased 65.2% of EPS and 38.3% of E₂₄ in strain CC-GSB4. This is the first report demonstrating the optimization of diesel emulsification by EPS from root nodulating isolates, and these microbial agents might be used in the remediation of hydrocarbon contaminated soils in a near future.