Bioremediation of heavy metal-polluted environments by non-living cells from rhizobial isolates.

Rhizosphere bacteria, for example, rhizobia, can play several roles, and one of the most important, the protection of plant roots against toxic conditions and other environmental stresses. In this work, the action of Cu2+ and Cr6+ on cell growth and EPS production of four strains of rhizobia, Rhizobium tropici (LBMP-C01), Ensifer sp. (LBMP-C02 and LBMP-C03), and Rhizobium sp. LBMP-C04, were tested. The results confirmed the strong effect of Cu2+ and Cr6+ on bacterial exopolysaccharides (EPS) synthesis, and how cells can adsorb these metals, which may be a key factor in the interactions between rhizosphere bacteria and host plants in heavy metal-contaminated soils. Here, we emphasize the importance of proving the potential of treating bacterial cells and their extracellular EPS to promote the bio-detoxification of terrestrial and aquatic systems contaminated by heavy metals in a highly sustainable, economic, and ecological way.

Chemical and rheological properties of exopolysaccharides produced by four isolates of rhizobia.

The rheological, physicochemical properties, emulsification and stability of exopolysaccharides (EPSs) from four rhizobia isolates (LBMP-C01, LBMP-C02, LBMP-C03 and LBMP-C04) were studied. The EPS yields of isolates under these experimental conditions were in the range of 1.5-6.63gL(-1). The LBMP-C04 isolate, which presented the highest EPS production (6.63gL(-1)), was isolated from Arachis pintoi and was identified as a Rhizobium sp. strain that could be explored as a possible potential source for the production of extracellular heteropolysaccharides. All polymers showed a pseudoplastic non-Newtonian fluid behavior or shear thinning property in aqueous solutions. Among the four EPS tested against hydrocarbons, EPS LBMP-C01 was found to be more effective against hexane, olive and soybean oils (89.94%, 82.75% and 81.15%, respectively). Importantly, we found that changes in pH (2-11) and salinity (0-30%) influenced the emulsification of diesel oil by the EPSs. EPSLBMP-C04 presented optimal emulsification capacity at pH 10 (E24=53%) and 30% salinity (E24=27%). These findings contribute to the understanding of the influence of the chemical composition, physical properties and biotechnology applications of rhizobial EPS solutions their bioemulsifying properties.