Adsorption of rare earth elements onto bacterial cell walls and its implication for REE sorption onto natural microbial mats

Adsorption of rare earth elements (REE) onto the cell walls of Bacillus subtilis (a gram-positive bacterium) and Escherichia coli (a gram-negative bacterium) was studied between pH 2.5 and 4.5 and at various bacterial concentrations. The distribution coefficients of REE between the bacterial cell surface and water showed a pattern with a prominent enrichment of heavy REE (HREE), including a maximum around Sm and Eu. There was also an enrichment around Pr accompanied by a decline for Nd, which was attributed to the tetrad effect. The occurrence of M-type tetrad effect suggests that REE form inner sphere complexes during their adsorption onto bacteria. The enrichment of the distribution coefficients in the HREE region was more enhanced at higher bacterial concentrations, which could not be explained by one type of binding sites on the bacterial surface. Instead, the data are consistent with two ligand types for the sorption of REE. The pattern of bacterial distribution coefficients can be explained by the stability constants of REE with carboxylate and phosphate groups, suggesting that they are most likely responsible for the adsorption of REE on the bacterial cell surface. Microbial mats and thermal waters at the Nakafusa hot spring (Nagano Prefecture, Japan) were also examined to evaluate whether the REE patterns of natural samples could be used as indicators of the presence of bacteria. The apparent distribution coefficients of REE displayed a pattern similar to that obtained in the laboratory experiments using pure bacterial strains. The results suggest that the REE pattern of chemical sedimentary rocks may be used to identify the bacterial contribution to the deposition of the rocks in the geological record.