Mohamed L. Merroun

Complexation of Uranium by Cells and S-Layer Sheets of Bacillus sphaericus JG-A12

ABSTRACT Bacillus sphaericus JG-A12 is a natural isolate recovered from a uranium mining waste pile near the town of Johanngeorgenstadt in Saxony, Germany. The cells of this strain are enveloped by a highly ordered crystalline proteinaceous surface layer (S-layer) possessing an ability to bind uranium and other heavy metals. Purified and recrystallized S-layer proteins were shown to be phosphorylated by phosphoprotein-specific staining, inductive coupled plasma mass spectrometry analysis, and a colorimetric method. We used extended X-ray absorption fine-structure (EXAFS) spectroscopy to determine the structural parameters of the uranium complexes formed by purified and recrystallized S-layer sheets of B. sphaericus JG-A12. In addition, we investigated the complexation of uranium by the vegetative bacterial cells. The EXAFS analysis demonstrated that in all samples studied, the U(VI) is coordinated to carboxyl groups in a bidentate fashion with an average distance between the U atom and the C atom of 2.88 ± 0.02 Å and to phosphate groups in a monodentate fashion with an average distance between the U atom and the P atom of 3.62 ± 0.02 Å. Transmission electron microscopy showed that the uranium accumulated by the cells of this strain is located in dense deposits at the cell surface.

Bacterial interactions with uranium: An environmental perspective

The presence of actinides in radioactive wastes is of major concern because of their potential for migration from the waste repositories and long-term contamination of the environment. Studies have been and are being made on inorganic processes affecting the migration of radionuclides from these repositories to the environment but it is becoming increasingly evident that microbial processes are of importance as well. Bacteria interact with uranium through different mechanisms including, biosorption at the cell surface, intracellular accumulation, precipitation, and redox transformations (oxidation/reduction). The present study is intended to give a brief overview of the key processes responsible for the interaction of actinides e.g. uranium with bacterial strains isolated from different extreme environments relevant to radioactive repositories. Fundamental understanding of the interaction of these bacteria with U will be useful for developing appropriate radioactive waste treatments, remediation and long-term management strategies as well as for predicting the microbial impacts on the performance of the radioactive waste repositories.

Complexation of uranium (VI) by three eco-types of Acidithiobacillus ferrooxidans studied using time-resolved laser-induced fluorescence spectroscopy and infrared spectroscopy

Time-resolved laser-induced fluorescence spectroscopy (TRLFS) was used to study the properties of uranium complexes (emission spectra and fluorescence lifetimes) formed by the cells of the three recently described eco-types of Acidithiobacillus ferrooxidans. The results demonstrated that these complexes have different lifetimes which increase in the same order as the capability of the strains to accumulate uranium. The complexes built by the cells of the eco-type II were the strongest, whereas, those of the eco-types I and III were significantly weaker. The emission spectra of all A. ferrooxidans complexes were almost identical to those of the uranyl organic phosphate compounds. The latter finding was confirmed by infrared spectroscopic analysis.

Microbial synthesis of core/shell gold/palladium nanoparticles for applications in green chemistry

We report a novel biochemical method based on the sacrificial hydrogen strategy to synthesize bimetallic gold (Au)–palladium (Pd) nanoparticles (NPs) with a core/shell configuration. The ability of Escherichia coli cells supplied with H2 as electron donor to rapidly precipitate Pd(II) ions from solution is used to promote the reduction of soluble Au(III). Pre-coating cells with Pd(0) (bioPd) dramatically accelerated Au(III) reduction, with the Au(III) reduction rate being dependent upon the initial Pd loading by mass on the cells. Following Au(III) addition, the bioPd–Au(III) mixture rapidly turned purple, indicating the formation of colloidal gold. Mapping of bio-NPs by energy dispersive X-ray microanalysis suggested Au-dense core regions and peripheral Pd but only Au was detected by X-ray diffraction (XRD) analysis. However, surface analysis of cleaned NPs by cyclic voltammetry revealed large Pd surface sites, suggesting, since XRD shows no crystalline Pd component, that layers of Pd atoms surround Au NPs. Characterization of the bimetallic particles using X-ray absorption spectroscopy confirmed the existence of Au-rich core and Pd-rich shell type bimetallic biogenic NPs. These showed comparable catalytic activity to chemical counterparts with respect to the oxidation of benzyl alcohol, in air, and at a low temperature (90°C).

Spectroscopic and Microscopic Characterization of Uranium Biomineralization in Myxococcus xanthus

In this work, synchrotron-based X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM) studies were carried out to elucidate at molecular scale the interaction mechanisms of Myxococcus xanthus with uranium at different pH values. Extended X-ray absorption fine structure (EXAFS) spectroscopic measurements showed that there are significant differences in the structural parameters of the U complexes formed by this bacterium at pH 2 and 4.5. At very low acidic pH of 2, the cells accumulated U(VI) as organic phosphate-metal complexes. At pH 4.5, however, the cells of this bacterium precipitated U(VI) as meta-autunite-like phase. TEM analyses demonstrated that at pH 2 the uranium accumulates were located mainly at the cell surface, whereas at pH 4.5 a uranium precipitation occurred on the cell wall and within the extracellular polysaccharides (EPS) characteristic of this bacterium. Dead/live staining studies showed that 30% and 50% of the uranium treated cell populations were alive at pH 2 and 4.5, respectively. The precipitation of U(VI) as mineral meta-autunite-like phase is possibly due to the bacterial acidic phosphatase activity. The precipitation of uranium as mineral phases may lead to more stable U(VI) sequestration that may be suitable for remediation purposes. These observations, combined with the very high uranium accumulation capability of the studied bacterial cells indicate that M. xanthus may significantly influence the fate of uranium in soil environments where these bacterial species are mainly found.

Lanthanum fixation by Myxococcus xanthus: cellular location and extracellular polysaccharide observation.

Myxococcus xanthus is a soil bacterium of the myxobacteria group and is abundant in almost all soils. Its role in soil ecology is considered significant. One noteworthy characteristic of the bacterium is that it produces large quantities of extracellular polymeric substances (EPS). It is also known that its biomass has the capacity to fix heavy metals. Here it is reported that M. xanthus was able to accumulate 0.6 mmol of La per g of wet biomass and/or 0.99 mmol per g of dry biomass. Transmission Electron Microscopy (TEM) observation of M. xanthus cells treated with La showed that a substantial amount of this cation was fixed in the EPS and in the cell wall. Smaller amounts were also observed in the cytoplasm. Fixed La appeared as phosphate in all cellular locations. The results given here also show that the use of La enables TEM observation of the M. xanthus EPS as a dense fibrillar net surrounding the cells. This technique is relatively easy and prevents EPS collapse, which occurs frequently during the fixation and dehydration procedures commonly used in preparations for TEM observations. Since antibodies are no longer required, the La stain can be carried out without delaying bacterial cell cultivation or isolation. In addition, the presence of La in cell cytoplasm without cell degeneration suggests that this microorganism could be used as a model in the study of bacteria-lanthanide interactions.

Bio-precipitation of uranium by two bacterial isolates recovered from extreme environments as estimated by potentiometric titration, TEM and X-ray absorption spectroscopic analyses.

This work describes the mechanisms of uranium biomineralization at acidic conditions by Bacillus sphaericus JG-7B and Sphingomonas sp. S15-S1 both recovered from extreme environments. The U-bacterial interaction experiments were performed at low pH values (2.0-4.5) where the uranium aqueous speciation is dominated by highly mobile uranyl ions. X-ray absorption spectroscopy (XAS) showed that the cells of the studied strains precipitated uranium at pH 3.0 and 4.5 as a uranium phosphate mineral phase belonging to the meta-autunite group. Transmission electron microscopic (TEM) analyses showed strain-specific localization of the uranium precipitates. In the case of B. sphaericus JG-7B, the U(VI) precipitate was bound to the cell wall. Whereas for Sphingomonas sp. S15-S1, the U(VI) precipitates were observed both on the cell surface and intracellularly. The observed U(VI) biomineralization was associated with the activity of indigenous acid phosphatase detected at these pH values in the absence of an organic phosphate substrate. The biomineralization of uranium was not observed at pH 2.0, and U(VI) formed complexes with organophosphate ligands from the cells. This study increases the number of bacterial strains that have been demonstrated to precipitate uranium phosphates at acidic conditions via the activity of acid phosphatase.

Comparative heavy metal biosorption study of brewery yeast and Myxococcus xanthus biomass

The biosorption for La2+, Co2+, Mn2+, UO22+, Pb2+, Ag+, Zn2+, Cd2+ and Cr2+ by wet and dry biomass form Myxococcus xanthus obtained from laboratory cultures and Saccharomyces cerevisiae from the brewing industry has been studied. M. xanthus biomass was found to be the most efficient biosorbent for all of the metals assayed. However, due to the fact that S. cerevisiae is a low cost residual by-product from the brewing industry, and at the same time yields good levels of biosorption, it is considered in this work to be of great interest for use as a detoxifier of heavy metals contaminated waters. In addition, the use of sodium carbonate as a desorbent agent is discussed where it was possible to recover up to 94,53 % of UO22+ by both M. xanthus and S. cerevisiae biomass.

Bacterial biomineralization: new insights from Myxococcus-induced mineral precipitation

Abstract Bacteria have contributed to the formation of minerals since the advent of life on Earth. Bacterial biomineralization plays a critical role on biogeochemical cycles and has important technological and environmental applications. Despite the numerous efforts to better understand how bacteria induce/mediate or control mineralization, our current knowledge is far from complete. Considering that the number of recent publications on bacterial biomineralization has been overwhelming, here we attempt to show the importance of bacteria–mineral interactions by focusing in a single bacterial genus, Myxococcus, which displays an unusual capacity of producing minerals of varying compositions and morphologies. First, an overview of the recent history of bacterial mineralization, the most common bacteriogenic minerals and current models on bacterial biomineralization is presented. Afterwards a description of myxobacteria is presented, followed by a section where Myxococcus-induced precipitation of a number of phosphates, carbonates, sulphates, chlorides, oxalates and silicates is described and discussed in lieu of the information presented in the first part. As concluding remarks, implications of bacterial mineralization and perspectives for future research are outlined. This review strives to show that the mechanisms which control bacterial biomineralization are not mineral- or bacterial-specific. On the contrary, they appear to be universal and depend on the environment in which bacteria dwell.

BIOSORPTION OF URANIUM BY MYXOCOCCUS XANTHUS

This paper deals with uranium biosorption by M~.YO~OL.~US xanthus biomass in which dry biomass, accumulating up to 2.4 mM of uranium gP ‘. is demonstrated to be a more efficient biosorbent than wet biomass. For uranium concentrations of 0.1-0.3 mM, between 95.79% and 95.99% of the uranium was taken up from the solution. Dry biomass biosorption was found to be relatively rapid, reaching equilibrium after 5-l0min. In addition, the pH influenced biosorption, pH 4.5 promoting maximum uptake. It was also established that the biosorbed uranium is located on the cellular wall and within the extracellular mucopolysaccharide of this microorganism. Furthermore, using sodium carbonate as a desorbent agent, 80.82% of the biosorbed uranium could be recovered. The results obtained indicate the possible utilization of M. xunthus biomass to solve some problems of the water contaminated by uranium. En este trabajo se estudia la bioadsorcion de uranio por la biomasa de M_t.~ococcu.s santhus, y se demuestra que la biomasa seca es mejor bioadsorbente que la himeda. siendo capaz de acumular hasta 2.4mM de uranio g- ’ Se ha comprobado que para concentraciones de uranio comprendidas entre 0. I y 0.3 mM se puede eliminar de la solution entre el 95.79% y el 95.99%. Ademis. se ha puesto de manifiesto que la bioadsorcion por la biomasa seca es un proceso relativamente rapido, alcanzandose el equilibrio entre 5 y IO minutos. Esta bioadsorcion se afecta por el pH, siendo mas efectiva a pH 4.5. El estudio de la localization celular de1 uranio captado por la biomasa indica que se deposita a nivel de la pared celular y del mucopolisadrido extracelular. De otra parte. utilizando carbonato sodico, puede recuperarse el 80.82% del uranio acumulado por la biomasa seca. En resumen, 10s resultados obtenidos indican que la biomasa de M. xanthus podria tener una posible utilization coma bioadsorbente para la resolution de algunos problemas de aguas contaminadas con uranio. ‘(‘I 1998 Elsevier Science Ltd. All rights reserved