A rare earth element signature of bacteria in natural waters?

Abstract

This study shows that rare earth element (REE) distribution patterns between bacteria and water for five different bacterial strains ( Bacillus subtilis, Escherichia coli, Alcaligenes faecalis, Shewanella putrefaciens, and Pseudomonas fluorescens) exhibit a steep increase in the heavy REE (HREE) part of the distribution pattern and a weak peak around the middle REE (MREE) part. Though the REE patterns for B. subtilis and E. coli were already reported in our previous paper [Takahashi, Y., Châtellier, X., Hattori, K.H., Kato, K., Fortin, D., 2005. Adsorption of rare earth elements onto bacterial cell walls and its implication for REE sorption onto natural microbial mats. Chem. Geol., 219, 53–67.], the present results clearly show that the characteristic REE pattern is commonly observed for various bacterial species. In addition, similar REE patterns were observed for bioflims (containing neutrophilic iron-oxidizers and other bacteria) formed in a groundwater discharge area in the Budo pond in Hiroshima (Japan), suggesting that the REE distribution pattern found in the laboratory experiments was reproduced in naturally occurring bacteria in fresh waters. In addition, the REE pattern, characterized by a maximum at MREE and an increase at HREE, was also observed in Fe oxyhydroxide precipitates formed below the same biofilm, which contained bacterial remnants. These results show that the presence of bacteria in natural Fe oxyhydroxides, and possibly in other natural minerals can be reflected in the shape of REE patterns. Such results could potentially be extended to geological materials, such as banded-iron formation (BIF), and used to discuss the effect of microbial activity on the formation of BIF. Although there are still many problems related to the extrapolation of the results obtained for Fe oxyhydroxides in modern water systems to BIF, such as abiotic processes and post-depositional alteration of the REE patterns, our study suggests that the REE pattern can be used as another tool to assess the role of bacteria in mineral formation in natural waters.