Electron donor | EPFL Graph Search

In chemistry, an electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process. Typical reducing agents undergo permanent chemical alteration through covalent or ionic reaction chemistry. This results in the complete and irreversible transfer of one or more electrons. In many chemical circumstances, however, the transfer of electronic charge to an electron acceptor may be only fractional, meaning an electron is not completely transferred, but results in an electron resonance between the donor and acceptor. This leads to the formation of charge transfer complexes in which the components largely retain their chemical identities. The electron donating power of a donor molecule is measured by its ionization potential which is the energy required to remove an electron from the highest occupied molecular orbital (HOMO). The overall energy balance (ΔE), i.e., energy gaine

Insights into the microbial reduction of pentavalent and hexavalent uranium species by Shewanella oneidensis MR-1

Margaux Camille Andréa Molinas

Decades of uranium-related activities such as ore mining, nuclear power generation, weapon manufacture, storage of nuclear wastes, have contributed to the release of U in the environment. U occurrence is concerning, since in surface environments, U is typically found in the U(VI) oxidation state, as uranyl, complexed with various ligands, and is highly soluble. Therefore, this toxic metal is likely to leech into soils and contaminate surface waters and groundwaters. In addition, owing to the long half-lives of the predominant isotopes 238U and 235U, U is considered as a persistent contaminant over geological timescales. For several decades, efforts have focused on the development of in situ remediation solutions to tackle U pollution in the subsurface, and limit its mobility.With this aim, interest in the metabolic potential for U(VI)-respiration by dissimilatory metal-reducing bacteria (DMRB), such as Shewanella oneidensis MR-1, has seen a significant increase. This metabolically-versatile bacterium has been reported to reduce mobile U(VI) to typically insoluble crystalline and amorphous U(IV). The reduction of U(VI) in S. oneidensis MR-1 is coupled to the oxidation of an electron donor, which feeds electrons into an electron transport chain, extending from the cytoplasm to the outer-membrane of the cells. The microbially mediated reduction of U(VI) to U(IV) is the result of a two-step process. It is assumed that one electron is first transferred to U(VI) to form a pentavalent U(V) intermediate, followed by the abiotic disproportionation of two U(V) atoms into U(IV) and U(VI). However, evidence for this mechanism is limited to experimental systems rich in carbonate, which permits the rapid disproportionation of U(V). Thus, it remains unclear whether a second, biologically-mediated, electron transfer to U(V) is possible under conditions in which disproportionation is limited. To explore this, a novel U(V)-dpaea complex, that is stable in water at pH 7, was utilized to investigate the second step of the reduction mechanism. Here, we observed that U(V) can be biologically reduced by an additional one-electron transfer, resulting in the accumulation of U(IV) without the need for disproportionation. To improve our understanding of the molecular mechanism of U-dpaea reduction, we incubated mutant strains of S. oneidensis MR-1, lacking (i) only outer-membrane c-type cytochromes or (ii) all c-type cytochromes, with solid phase U(VI)-dpaea and aqueous U(V)-dpaea. We determined that U(VI)-dpaea reduction proceeds via the initial dissolution of the solid phase and that U(V)-dpaea reduction is mediated by outer-membrane c-type cytochromes. In particular, in vitro reactions between the purified outer-membrane c-type cytochrome MtrC and U(V)-dpaea demonstrated that MtrC can directly transfer electrons to U(V)-dpaea.Finally, we sought to determine the factors that influence electron transfer kinetics between DMRB and U. To this end, we reacted U(VI) coordinated by various aminocarboxylate ligands with purified MtrC. Here, U speciation significantly impacted reduction rates and appeared to be related to the binding strength of the U-MtrC interaction, i.e., hydrogen bonding versus electrostatic.All together, these findings provide further insights in the reduction mechanism of U by DMRB, and underline the importance of U speciation in controlling the pathway and rate of electron transfer.

EPFL2021

Behaviour of carbon-14 containing low molecular weight organic compounds in contaminated groundwater under aerobic conditions

Aislinn Ann Boylan

Short chain carbon-14 (C-14) containing organic compounds can be formed by abiotic oxidation of carbides and impurities within nuclear fuel cladding. During fuel reprocessing and subsequent waste storage there is potential for these organic compounds to enter shallow subsurface environments due to accidental discharges. Currently there is little data on the persistence of these compounds in such environments. Four C-14-labelled compounds (acetate; formate; formaldehyde and methanol) were added to aerobic microcosm experiments that contained glacial outwash sediments and groundwater simulant representative of the Sellafield nuclear reprocessing site, UK. Two concentrations of each electron donor were used, low concentration (10(-5) M) to replicate predicted concentrations from an accidental release and high concentration (10(-2) M) to study the impact of the individual electron donor on the indigenous microbial community in the sediment. In the low concentration system only similar to 5% of initial C-14 remained in solution at the end of experiments in contact with atmosphere (250-350 h). The production of (CO2)-C-14(g) (measured after 48 h) suggests microbially mediated breakdown is the primary removal mechanism for these organic compounds, although methanol loss may have been partially by volatilisation. Highest retention of C-14 by the solid fractions was found in the acetate experiment, with 12% being associated with the inorganic fraction, suggesting modest precipitation as solid carbonate. In the high concentration systems only similar to 5% of initial C-14 remains in solution at the end of the experiments for acetate, formate and methanol. In the formaldehyde experiment only limited loss from solution was observed (76% remained in solution). The microbial populations of unaltered sediment and those in the low concentration experiments were broadly similar, with highly diverse bacterial phyla present. Under high concentrations of the organic compounds the abundance of common operational taxonomic units was reduced by 66% and the community structure was dominated by Proteobacteria (particularly Betaproteobacteria) signifying a shift in community structure in response to the electron donor available. The results of this study suggest that many bacterial phyla that are ubiquitous in near surface soils are able to utilise a range of C-14-containing low molecular weight organic substances very rapidly, and thus such substances are unlikely to persist In aerobic shallow subsurface environments.

ELSEVIER SCI LTD2018

Metal reduction and sporulation in Desulfotomaculum reducens

Elena Cristina Dalla Vecchia

The genera Desulfotomaculum and Clostridium, belonging to the phylum Firmicutes, comprise Gram-positive, low G+C genomic content, anaerobic, spore- forming bacteria. Desulfotomaculum is a metabolically and environmentally versatile genus capable of growing fermentatively as well as by respiring sulfate. D. reducens is also capable of metal reduction and has been reported to conserve energy from this process. In this thesis, iron (Fe(III)) reduction by D. reducens in the presence of pyruvate or lactate as electron donors was investigated. When pyruvate is present, D. reducens grows fermentatively. If Fe(III) is present, it can act as an electron sink alternative to protons and consume excess reducing equivalents accumulated during pyruvate oxidation. Electron transfer to Fe(III) by D. reducens in the presence of pyruvate is mediated by a soluble electron carrier, likely riboflavin, released by cells during fermentation. This mechanism allows the microorganism to reduce extracellular, solid phase Fe(III) even if it is not directly accessible by physical contact. In the presence of lactate, D. reducens is unable to conserve significant energy from electron donor oxidation and Fe(III) reduction. In contrast to pyruvate, during lactate oxidation, no redox-active compound is released by the cells, and extracellular, solid phase Fe(III) can only be reduced by direct contact. This implies that a surface exposed reductase must be present in D. reducens cells. The investigation of the surface proteome of this organism led to the identification of three proteins, most likely localized to the cell surface, potentially involved in Fe(III) reduction. One is a heterodisulfide reductase subunit A, a putative intermediate in the Fe(III) reduction chain. The second is a membrane-associated hydrogenase. The third protein is annotated as alkyl hydroperoxide reductase although its actual function may be thiol-disulfide oxidoreduction; several lines of evidence suggest the involvement of the latter in metal reduction by D. reducens. Clostridium species are widespread in the environment and are capable of utilizing a great variety of organic substrates for fermentative growth, while they are unable to conserve energy through respiration. However, they can reduce terminal electron acceptors to eliminate excess reducing equivalents from fermentation. Among the electron acceptors used by Clostridium spp., are metals, such as iron and uranium (U(VI)). One species whose vegetative cells are capable of U(VI) reduction is C. acetobutylicum, although little information is available on the mechanism underlying this process. In this thesis, the potential involvement of the spores of C. acetobutylicum in U(VI) reduction is probed. It was found that the spores of this species can reduce the radionuclide, and that to do so they require the presence of an unidentified soluble compound released by growing cells and H2 as the electron donor. The involvement of spores in metal reduction is very interesting because generally spores are considered not to interact directly with the surrounding environment. In fact, spores are dormant, thus metabolically inactive, cells. Spore formation is a cell differentiation process activated in response to environmental stress and conditions that are hostile to vegetative growth. Sporulation has been extensively studied and characterized in Bacillus subtilis, for which a model has been developed. Some information is also available on sporulation in the clostridia. In this thesis, a genomic investigation of sporulation in the Desulfotomaculum genus was undertaken. The B. subtilis model was used as a reference and the information on Clostridium as a comparison. Significant similarities were found in the core regulatory process across genera, while some differences were identified in the initiation of sporulation. Substantial differences were highlighted in the spore structural proteins.

EPFL2014