Ce3+-induced exopolysaccharide production by Bradyrhizobium sp. MAFF211645

Abstract Ce 3+ , a rare earth element (REE), has been widely used in high-technology industries. Despite the importance of Ce 3+ in the fields of chemistry and physics, the role of Ce 3+ in biology has been ignored. To investigate physiological effects of Ce 3+ on microorganisms, we screened microorganisms that showed peculiar growth in the presence of Ce 3+ . We isolated a free-living soil bacterium that produced exopolysaccharide (EPS) around its colonies on 1/100 nutrient agar with 30 μM CeCl 3 or 1.0% d -mannitol. The bacterium was identified as Bradyrhizobium sp. by morphological, biochemical, and physiological tests as well as 16S rDNA sequence analysis. La 3+ , Pr 3+ , and Nd 3+ also induced EPS production in large quantities, while Sm 3+ did in small amounts. However, other heavier REEs from Eu 3+ to Lu 3+ , and metals such as Na + , Al 3+ , K + , Ca 2+ , V 3+ , Cr 3+ , Co 2+ , Ni 2+ , Sr 2+ , Ba 2+ , and Pb 2+ did not induce EPS production. The mean molecular weight of EPS was estimated to be approximately 1 × 10 6 by Sepharose CL-4B column chromatography. TLC revealed that EPS was composed of l -rhamnose. Quantitative analysis of alditol acetate derivatives of acid hydrolyzate of EPS by GLC revealed that EPS was composed of more than 95% l -rhamnose, indicating that this EPS was a rhamnan. The spectrum of FT-IR of the rhamnan demonstrated that l -rhamnose residues in the rhamnan were α-linked. GC/MS analysis of methylated alditol acetate derivatives of the rhamnan demonstrated that it was composed of main chain α-(1→4)-linked l -rhamnopyranosyl residues. From spectral analyses of 1 H-NMR and FT-IR, EPS produced in the presence of 1.0% d -mannitol was found to be structurally similar to rhamnans.