Ce^<3+>-induced exopolysaccharide production by Bradyrhizobium sp. MAFF211645(MICROBIAL PHYSIOLOGY AND BIOTECHNOLOGY)

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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 nutriellt agar with 30μM 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 demonstmted that L-rhamnose residues in the rhamnan wereα-linked. GC/MS analysis of methylated alditol acetate derivatives of the rhamnan demonstrated that it was composed of main chainα-(1→4)-1inked 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.

Ce^<3+>-induced exopolysaccharide production by Bradyrhizobium sp. MAFF211645(MICROBIAL PHYSIOLOGY AND BIOTECHNOLOGY)

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