Ce³+, a rare earth element (REE), has been widely used in high-technology industries. Despite the importance of Ce³+ in the fields of chemistry and physics, the role of Ce³+ in biology has been ignored. To investigate physiological effects of Ce³+ on microorganisms, we screened microorganisms that showed peculiar growth in the presence of Ce³+. We isolated a free-living soil bacterium that produced exopolysaccharide (EPS) around its colonies on 1/100 nutrient agar with 30 μM CeCl₃ 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. La³+, Pr³+, and Nd³+ also induced EPS production in large quantities, while Sm³+ did in small amounts. However, other heavier REEs from Eu³+ to Lu³+, and metals such as Na+, Al³+, K+, Ca²+, V³+, Cr³+, Co²+, Ni²+, Sr²+, Ba²+, and Pb²+ did not induce EPS production. The mean molecular weight of EPS was estimated to be approximately 1 x 10⁶ 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 ¹H-NMR and FT-IR, EPS produced in the presence of 1.0% D-mannitol was found to be structurally similar to rhamnans.
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