Abstract

In the context that Rhizobium species are likely to be involved in a broad range of functions in diverse ecosystems and the commercial and ecological importance of the rhizobial exopolysaccharides (EPS), the present investigation addresses the isolation, purification and physico-chemical characterization of EPS from the wild-type and mutant strains of Rhizobium tropici SEMIA 4080. Moreover, the stability of emulsions formed with soybean oil, diesel oil and toluene using different concentrations of purified EPS derived from different strains was also demonstrated to elucidate the potential of EPS as soil-stabilizing agents or as bio-emulsifiers for use in the degradation of hydrocarbons. Cold 96% ethanol was added to the supernatant obtained from the centrifugation at a 3:1 (v/v) ethanol:supernatant ratio to precipitate the EPS. The emulsion forming and stability capacity was evaluated for two different EPS concentrations. Briefly, an EPS aqueous solution (1 and 5 g/L) was mixed with each hydrophobic compound (3:2, v/v ratio), stirred in the vortex and left stand for 24 h. The test was also performed with xanthan gum (Fluka), with the same concentration in deionized water. Emulsifying activity was expressed as the percentage of the total height occupied by the emulsion. During growing on diesel oil and sucrose as the sole carbon sources, the strains SEMIA 4080 and mutant M03 extracellularly released an exopolysaccharide exhibiting bioemulsifying activity into the surrounding medium. Yield production varied from 4.39 to 5.52 g/L of EPS according to the carbon source added, in the same way chemical composition, viscosity and emulsifying activity of EPS varied with the culture conditions. The EPS produced by the wild-type Rhizobium tropici SEMIA 4080 resulted in more stable emulsions with the tested toluene than xanthan gum, and the emulsification indexes with hydrocarbons and soybean oil were higher than 50 %, indicating strong emulsion-stabilizing capacity. The capacity of strains SEMIA 4080 and mutant M03 to grow and produce bioemulsifier in presence of oil hydrocarbons together with the high emulsifying activity and high viscosity power of the biopolymers synthesized in hydrocarbons media could be considered highly beneficial for application of both bioemulsifier and producing strain in bioremediation of oil pollutants.