Ce3+-induced exopolysaccharide production by Bradyrhizobium sp MAFF211645

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 mu M CeCl3 or 1.0% D-mannitol. The bacterium was identified as Bradyrhizobium sp. by morphological, biochemical, and physiological tests as well as 165 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 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 alpha-linked. GC/MS analysis of methylated alditol acetate derivatives of the rhamnan demonstrated that it was composed of main chain alpha-(1 -> 4)-linked L-rhamnopyranosyl residues. From spectral analyses of H-1-NMR and FT-IR, EPS produced in the presence of 1.0% D-mannitol was found to be structurally similar to rhamnans. (C) 2010, The Society for Biotechnology, japan. All rights reserved.