Characterization of U(VI) sequestration by Acidovorax facilis - a spectroscopic and microscopic approach

To improve bioremediation strategies based on a better understanding of binding mechanisms on the molecular level, we applied U(VI) interaction experiments with Acidovorax facilis. This is a facultative aerobic, chemoorganotrophic gram-negative betaproteobacterium, ubiquitously distributed in the nature including uranium-contaminated sites. Our study combines a variety of microscopic and spectroscopic techniques in order to elucidate the interaction process of U(VI) with A. facilis. Kinetic U(VI) sorption experiments were performed under aerobic conditions at 30°C by adjusting an initial U(VI) concentration to 0.1 mM at a pH of 5 by adding UO2(NO3)2 to the batch culture.
The results showed that different cell compartments play a major role in the sequestration of U(VI). Our findings clearly point out that the local coordination of U(VI) on functional groups of the cell membrane components of A. facilis depends upon time incubation. U(VI) biosorption by outer membrane lipopolysaccharide (LPS) containing phosphoryl residues was observed by TRLFS within the first hours of contact between the cells and U(VI). By increasing the incubation time up to 24 h the implication of carboxyl groups within the cell wall peptidoglycan (PGN) was proved in addition to phosphoryl groups. These results support those obtained by EXAFS, where a relative short average U-Oeq bond length of 2.35 Å was observed, indicating a binding of the U(VI) via organic phosphate groups (from LPS) in a monodentate fashion. The strong interaction of U(VI) with phosphorylic and carboxylic groups was reinforced by ATR FT-IR spectroscopic studies due to the presence of characteristic phosphoryl vibrations. Most of the bound U(VI) presumably remained on the cells, more precisely on the phosphorylic functionalities at the cell membrane. In addition to these functional groups located at the cell surfaces, U is coordinated also, but with low degree, to phosphoryl groups of the intracellular polyphosphate granules as was indicated by STEM analysis.

U. Gerber et al., Journal of Hazardous Materials 317(2016), 127-134.
E. Krawczyk-Bärsch, U. Gerber et al., Journal of Hazardous Materials (2017), under review.