Sorption of uranium(VI) by selected bacteria, algae and fungi and characterization of uranyl biomass species using different microscopic and spectroscopic methods

Microorganisms like bacteria, algae and fungi have a significant influence on the immobilization, mobilization and transport of radionuclides like uranium and other heavy metals in the biological and geological environment via the soil and water path. To understand the mechanisms of uptake, transport, deposition, degradation and the behavior of actinides in different biological and geological systems structural knowledge about the formed actinides species are of great importance and are essential for a reliable assessment of these processes.
Arthrobacter (bacteria), Chlorella vulgaris (green algae) and Schizophyllum commune (fungi) interact in different ways with uranium and are therefore used as model organisms to study different interaction mechanisms and binding forms of uranium, including possibly also so far unknown uptake processes into living cells. Hence the aim of this study was to localize the uranium(VI) on/inside the cells using microscopic methods. Furthermore, experiments with a combination of different spectroscopic techniques were performed to obtain first information about the kind and structure of the formed uranium species and to identify the functionalities, which are responsible for the binding of uranium on/inside the living cells.
All investigated organisms bind significant amounts of uranium(VI) in the pH range from 4 to 7 and contact time of 48 hours. Arthrobacter oxygen and A. nicotinae have binding capacities of 50 – 95 mg U/g biomass dry weight at a uranium concentration of 1∙10-4 M and are thus comparable with that of Schizophyllum cells binding 75 mg U/g bio dry weight independent of the pH value. In contrast to this, metabolic active Chlorella cells bind only up to 30 mg U/g dry biomass under the same experimental conditions. At a more environmentally relevant uranium concentration of 5∙10-6 M in mineral medium and during ongoing cultivation, a mobilization of the algae-bound uranium occurred. At uranium concentrations higher than 1∙10-4 M algal cells died, whereas fungal cells tolerated even high uranium concentrations and were able to accumulate up to 280 mg U/ g biomass dry weigth at pH 5 and 6. At pH 4 and 7, Schizophyllum cells bound only 130 to 150 mg U/ g biomass dry weight. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) combined with EDX analyses were used for the localization of formed uranyl species in the biomass of all investigated organisms. TEM micrographs of thin sections of uranium containing Arthrobacter and Chlorella cells show uranium mainly bound on parts of the cell wall. In some cases uranium containing accumulates were found inside the cells. However, it remains to be resolved if these cells are still alive and possess intact cell walls. In contrast, SEM pictures of transparent fungal cells clearly show uranium containing accumulates inside originally living cells and on their cell surface. Therefore, further research is needed to identify involved uptake mechanisms.
For the determination of the functionalities, which are important for the immobilization of uranium, the interaction of uranium(VI) with metabolic active bacterial, algal and fungal cells was investigated by means of time-resolved laser-induced fluorescence spectroscopy (TRLFS) and X-ray absorption fine structure spectroscopy (EXAFS). The measured luminescence spectra of uranyl containing cell species of all investigated organisms show bathochromic shifts of the uranyl emission bands in comparison to the corresponding emission signals of the uranyl species in the initial solution independent of the uranium concentration and the pH value of the solution (Fig. 1). Obtained results demonstrated a change of the uranyl speciation during the sorption processes. The carboxylic and organic/inorganic phosphate groups are responsible for uranium binding on the biomass with varying contributions dependent on the microbial biomass, cell status and uranium concentration in the initial sorption solution. The dominant interaction of uranium(VI) with organic/inorganic phosphate groups could be verified by EXAFS investigations.