Alternative Electron Transfer Research Articles

Alternating electron transfer mechanism in the case of high-performance tetrathiafulvalene–tetracyanoquinodimethane enzymatic electrodes

The electron transfer mechanism in enzymatic electrodes employing tetrathiafulvalene–tetracyanoquinodimethane (TTF–TCNQ) complex is despite of numerous publications, still a matter of controversy. To clarify this issue, enzymatic electrodes based on TTF, TCNQ and TTF–TCNQ have been prepared in an identical manner and their electrochemical behavior and activity have been tested. The enzymatic electrodes containing the respective mediator, glucose oxidase and Vulcan nanoparticles dispersed in a gelatin matrix show high activity for glucose oxidation in terms of both currents and oxidation onset potential. The observed electrochemical features, supported by infrared spectroscopy measurements, indicate that the activity of the TTF–TCNQ electrodes can be ascribed to TTF and TCNQ species released from the organic salt. The two mediators are active in different potential regions, implying an alternating electron transfer mechanism. TTF is active at more negative potentials and generates higher current densities, while TCNQ exhibits activity only at potentials more positive than 0V vs. SCE. This work analyzes and summarizes different aspects of the most recent electron transfer hypotheses by providing the respective experimental evidences in an effort for better understanding of this puzzling bioelectrochemical system.

Neuroprotective Actions of Methylene Blue and Its Derivatives

Methylene blue (MB), the first lead chemical structure of phenothiazine and other derivatives, is commonly used in diagnostic procedures and as a treatment for methemoglobinemia. We have previously demonstrated that MB could function as an alternative mitochondrial electron transfer carrier, enhance cellular oxygen consumption, and provide protection in vitro and in rodent models of Parkinson’s disease and stroke. In the present study, we investigated the structure-activity relationships of MB in vitro using MB and six structurally related compounds. MB reduces mitochondrial superoxide production via alternative electron transfer that bypasses mitochondrial complexes I-III. MB mitigates reactive free radical production and provides neuroprotection in HT-22 cells against glutamate, IAA and rotenone toxicity. Distinctly, MB provides no protection against direct oxidative stress induced by glucose oxidase. Substitution of a side chain at MB’s 10-nitrogen rendered a 1000-fold reduction of the protective potency against glutamate neurototoxicity. Compounds without side chains at positions 3 and 7, chlorophenothiazine and phenothiazine, have distinct redox potentials compared to MB and are incapable of enhancing mitochondrial electron transfer, while obtaining direct antioxidant actions against glutamate, IAA, and rotenone insults. Chlorophenothiazine exhibited direct antioxidant actions in mitochondria lysate assay compared to MB, which required reduction by NADH and mitochondria. MB increased complex IV expression and activity, while 2-chlorphenothiazine had no effect. Our study indicated that MB could attenuate superoxide production by functioning as an alternative mitochondrial electron transfer carrier and as a regenerable anti-oxidant in mitochondria.

Probing the interplay between factors determining reaction rates on silica gel using termolecular systems

In this study we have compared energy and electron transfer reactions in termolecular systems using a nanosecond diffuse reflectance laser flash photolysis technique. We have previously investigated these processes on silica gel surfaces for bimolecular systems and electron transfer in termolecular systems. The latter systems involved electron transfer between three arene molecules with azulene acting as a molecular shuttle. In this study we present an alternative electron transfer system using trans β-carotene as an electron donor in order to effectively immobilise all species except the shuttle, providing the first unambiguous evidence for radical ion mobility. In the energy transfer system we use naphthalene, a structural isomer of azulene, as the shuttle, facilitating energy transfer from a selectively excited benzophenone sensitiser to 9-cyanoanthracene. Bimolecular rate constants for all of these processes have been measured and new insights into the factors determining the rates of these reactions on silica gel have been obtained.

Экспрессия гена gdh2 арабидопсиса зависит от активности альтернативного пути переноса электронов в митохондриях

Экспрессия гена gdh2 арабидопсиса зависит от активности альтернативного пути переноса электронов в митохондриях

The Antisense RNA As1_flv4 in the Cyanobacterium Synechocystis sp. PCC 6803 Prevents Premature Expression of the flv4-2 Operon upon Shift in Inorganic Carbon Supply

The functional relevance of natural cis-antisense transcripts is mostly unknown. Here we have characterized the association of three antisense RNAs and one intergenically encoded noncoding RNA with an operon that plays a crucial role in photoprotection of photosystem II under low carbon conditions in the cyanobacterium Synechocystis sp. PCC 6803. Cyanobacteria show strong gene expression dynamics in response to a shift of cells from high carbon to low levels of inorganic carbon (C(i)), but the regulatory mechanisms are poorly understood. Among the most up-regulated genes in Synechocystis are flv4, sll0218, and flv2, which are organized in the flv4-2 operon. The flavodiiron proteins encoded by this operon open up an alternative electron transfer route, likely starting from the Q(B) site in photosystem II, under photooxidative stress conditions. Our expression analysis of cells shifted from high carbon to low carbon demonstrated an inversely correlated transcript accumulation of the flv4-2 operon mRNA and one antisense RNA to flv4, designated as As1_flv4. Overexpression of As1_flv4 led to a decrease in flv4-2 mRNA. The promoter activity of as1_flv4 was transiently stimulated by C(i) limitation and negatively regulated by the AbrB-like transcription regulator Sll0822, whereas the flv4-2 operon was positively regulated by the transcription factor NdhR. The results indicate that the tightly regulated antisense RNA As1_flv4 establishes a transient threshold for flv4-2 expression in the early phase after a change in C(i) conditions. Thus, it prevents unfavorable synthesis of the proteins from the flv4-2 operon.

Identification of Disulfide Bonds in Wheat Gluten Proteins by Means of Mass Spectrometry/Electron Transfer Dissociation

Disulfide bonds within gluten proteins play a key role in the breadmaking performance of wheat flour. In the present study, disulfide bonds of wheat gluten proteins were identified by using a new liquid chromatography-mass spectrometry (LC-MS) technique with alternating electron transfer dissociation (ETD)/collision-induced dissociation (CID). Wheat flour was partially hydrolyzed with thermolysin (pH 6.5, 37 °C, 16 h), and the digest was subjected to LC-MS with alternating ETD/CID fragmentation. Whereas CID provided peptide fragments with intact disulfide bonds, cleavage of disulfide bonds was preferred over peptide backbone fragmentations in ETD. The simultaneous observation of disulfide-linked and disulfide-cleaved peptide ions in the mass spectra not only provided distinct interpretation with high confidence but also simplified the conventional approach for determination of disulfide bonds, which often requires two separate experiments with and without chemical reduction. By application of the new method 14 cystine peptides were identified. Eight peptides confirmed previously established disulfide bonds within gluten proteins, and the other six cystine peptides were identified for the first time. One of the newly identified cystine peptides represented a "head-to-tail" cross-link between high molecular weight glutenin subunits. This type of cross-link, which has been postulated as an integral part of glutenin models published previously, has now been proven experimentally for the first time. From the six remaining cystine peptides interchain disulfide bonds between α-gliadins, γ-gliadins, and low molecular weight glutenin subunits were established.

Gate bias polarity dependent H migration and O vacancy generation through Si=O–H complex formation in SiO2/Si(100)

We studied H-atom migration in SiO${}_{2}$/Si systems triggering O vacancy generation based on first-principles calculations. The migrating H atoms in SiO${}_{2}$ have been found to be mostly negatively charged irrespective of the charge-state polarity in the systems because strong charge transfer from the Si to the O atoms substantially increases the O 2$p$ energy levels. Although the SiO${}_{2}$ systems with no defect are not degraded by the migrating H atoms, 2Si$=$O--H complex formation in the SiO${}_{2}$/Si systems has been found to generate an O vacancy via H${}_{2}$O molecule desorption after alternate electron transfer between the 2Si$=$O--H complex and the Si substrate, leading to the final electrical breakdown of SiO${}_{2}$ films. The migration of negatively charged H atoms and their degradation agree well with the recently observed gate bias polarity dependent SiO${}_{2}$ film degradation.

Peptide molecular junctions: Distance dependent electron transmission through oligoprolines

We have investigated the efficiency of electron transmission through thiolated oligoproline derivatives of general formula: Cys–(Pro)n–CSA, where CSA is a cystamine linker and n=1–4. The conductance measurements were performed using STM-based molecular junction approach. We have noted that the conductance of the oligoprolines decays exponentially with increasing length of the molecules and the decay constant was 4.3nm−1. This indicates that electron transfer is dominated by superexchange mechanism. Based on this observation, we have concluded that the height of the barrier is affected by the specific conformation of the peptide backbone. Such conclusion is supported by the fact that the oligoprolines do not form intramolecular hydrogen bonds, which could provide alternative electron transfer pathways.

Sulphur dioxide evokes a large scale reprogramming of the grape berry transcriptome associated with oxidative signalling and biotic defence responses

The grape and wine industries are heavily reliant on sulphite preservatives. However, the view that sulphites act directly on bacterial and fungal pathogens may be simplistic. Mechanisms of sulphur-enhanced defences are largely unknown; many sulphur-rich compounds enhance plant defences and sulphite can also have oxidative consequences via production of H(2)O(2) or sulphitolysis. To investigate the effects of sulphur dioxide (SO(2) ) on fresh table grapes (Vitis vinifera L. 'Crimson Seedless'), transcriptome analysis was carried out on berries treated with SO(2) under commercial conditions for 21 d. We found a broad perturbation of metabolic processes, consistent with a large-scale stress response. Transcripts encoding putative sulphur-metabolizing enzymes indicated that sulphite was directed towards chelation and conjugation, and away from oxidation to sulphate. The results indicated that redox poise was altered dramatically by SO(2) treatment, evidenced by alterations in plastid and mitochondrial alternative electron transfer pathways, up-regulation of fermentation transcripts and numerous glutathione S-transferases, along with a down-regulation of components involved in redox homeostasis. Features of biotic stress were up-regulated, notably signalling via auxin, ethylene and jasmonates. Taken together, this inventory of transcriptional responses is consistent with a long-term cellular response to oxidative stress, similar to the effects of reactive oxygen species.

Alternative Mitochondrial Electron Transfer as a Novel Strategy for Neuroprotection

Neuroprotective strategies, including free radical scavengers, ion channel modulators, and anti-inflammatory agents, have been extensively explored in the last 2 decades for the treatment of neurological diseases. Unfortunately, none of the neuroprotectants has been proved effective in clinical trails. In the current study, we demonstrated that methylene blue (MB) functions as an alternative electron carrier, which accepts electrons from NADH and transfers them to cytochrome c and bypasses complex I/III blockage. A de novo synthesized MB derivative, with the redox center disabled by N-acetylation, had no effect on mitochondrial complex activities. MB increases cellular oxygen consumption rates and reduces anaerobic glycolysis in cultured neuronal cells. MB is protective against various insults in vitro at low nanomolar concentrations. Our data indicate that MB has a unique mechanism and is fundamentally different from traditional antioxidants. We examined the effects of MB in two animal models of neurological diseases. MB dramatically attenuates behavioral, neurochemical, and neuropathological impairment in a Parkinson disease model. Rotenone caused severe dopamine depletion in the striatum, which was almost completely rescued by MB. MB rescued the effects of rotenone on mitochondrial complex I-III inhibition and free radical overproduction. Rotenone induced a severe loss of nigral dopaminergic neurons, which was dramatically attenuated by MB. In addition, MB significantly reduced cerebral ischemia reperfusion damage in a transient focal cerebral ischemia model. The present study indicates that rerouting mitochondrial electron transfer by MB or similar molecules provides a novel strategy for neuroprotection against both chronic and acute neurological diseases involving mitochondrial dysfunction.

Removal of H2O2 and generation of superoxide radical: Role of cytochrome c and NADH

In cells, mitochondria, endoplasmic reticulum, and peroxisomes are the major sources of reactive oxygen species (ROS) under physiological and pathophysiological conditions. Cytochrome c (cyt c) is known to participate in mitochondrial electron transport and has antioxidant and peroxidase activities. Under oxidative or nitrative stress, the peroxidase activity of Fe3+cyt c is increased. The level of NADH is also increased under pathophysiological conditions such as ischemia and diabetes and a concurrent increase in hydrogen peroxide (H2O2) production occurs. Studies were performed to understand the related mechanisms of radical generation and NADH oxidation by Fe3+cyt c in the presence of H2O2. Electron paramagnetic resonance (EPR) spin trapping studies using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were performed with NADH, Fe3+cyt c, and H2O2 in the presence of methyl-β-cyclodextrin. An EPR spectrum corresponding to the superoxide radical adduct of DMPO encapsulated in methyl-β-cyclodextrin was obtained. This EPR signal was quenched by the addition of the superoxide scavenging enzyme Cu,Zn-superoxide dismutase (SOD1). The amount of superoxide radical adduct formed from the oxidation of NADH by the peroxidase activity of Fe3+cyt c increased with NADH and H2O2 concentration. From these results, we propose a mechanism in which the peroxidase activity of Fe3+cyt c oxidizes NADH to NAD•, which in turn donates an electron to O2, resulting in superoxide radical formation. A UV–visible spectroscopic study shows that Fe3+cyt c is reduced in the presence of both NADH and H2O2. Our results suggest that Fe3+cyt c could have a novel role in the deleterious effects of ischemia/reperfusion and diabetes due to increased production of superoxide radical. In addition, Fe3+cyt c may play a key role in the mitochondrial “ROS-induced ROS-release” signaling and in mitochondrial and cellular injury/death. The increased oxidation of NADH and generation of superoxide radical by this mechanism may have implications for the regulation of apoptotic cell death, endothelial dysfunction, and neurological diseases. We also propose an alternative electron transfer pathway, which may protect mitochondria and mitochondrial proteins from oxidative damage.

Partial Functional Replacement of CymA by SirCD in Shewanella oneidensis MR-1

The gammaproteobacterium Shewanella oneidensis MR-1 utilizes a complex electron transfer network composed primarily of c-type cytochromes to respire under anoxic conditions a variety of compounds, including fumarate, nitrate, and dimethyl sulfoxide (DMSO), in addition to the minerals Fe(III) and Mn(IV). Central to several respiratory pathways is CymA, a cytoplasmic membrane-bound tetraheme c-type cytochrome that functions as the major hydroquinone dehydrogenase. To investigate functional redundancy and plasticity in S. oneidensis MR-1 electron transport, we isolated ΔcymA suppressor mutants and characterized one biochemically and genetically. Interestingly, in the characterized ΔcymA suppressor mutant, respiration of fumarate, ferric citrate, and DMSO was restored but that of nitrate was not. The suppression was found to be due to transcriptional activation of sirC and sirD, encoding a periplasmic iron sulfur protein and an integral membrane hydroquinone dehydrogenase, respectively. Biochemical in vitro reconstitution experiments confirmed electron transport between formate and fumarate via fumarate reductase by suppressor membrane fractions. The suppression was found to be caused by insertion of an ISSod1 element upstream of the sirCD transcriptional start site, generating a novel, constitutively active hybrid promoter. This work revealed that adaptation of an alternative electron transfer pathway from quinol to terminal oxidoreductases independent of CymA occurs rapidly in S. oneidensis MR-1.

A photophysical approach to investigate the photooxidation mechanism of pesticides: Hydroxyl radical versus electron transfer

The role of hydroxyl radical in different solar photocatalytic processes, namely photo-Fenton, titanium dioxide-based oxidation and organic photocatalysis (triphenylpyrylium) has been investigated. Using a methodology based on the flash photolytic generation of OH from N-hydroxypyridine-2(1H)-thione, followed by t-stilbene trapping, the reaction rate constants of different pesticides (dimethoate, methidathion, alachlor and pyrimethanyl) with this reactive oxygen species in deaerated acetonitrile have been determined. The results obtained under photo-Fenton conditions are in reasonable agreement with the estimated rate constants; hence the assumption that the photo-Fenton reaction mainly involves participation of the hydroxyl radical seems plausible. The oxidation pattern using titanium dioxide was also investigated; however, under these conditions no clear correlation could be found due to participation of an alternative electron transfer mechanism.Finally, pyrimethanil was the most reactive pesticide when photodegradation was carried out in the presence of TPP, in spite of its low reactivity towards hydroxyl radical. Laser flash photolysis experiments evidenced that the reaction occurs via photoinduced electron transfer within a ground state complex; this is in good correlation with the relative amount of pyranyl radical formation.

Structural and Biochemical Characterization of the Cytochrome P450 CypX (CYP134A1) from Bacillus subtilis: A Cyclo-l-leucyl-l-leucyl Dipeptide Oxidase

Cytochrome P450 CypX (CYP134A1), isolated from Bacillus subtilis, has previously been implicated in the three-step oxidative transformation of the diketopiperazine cyclo-l-leucyl-l-leucyl into pulcherriminic acid, a precursor of the extracellular iron chelate pulcherrimin. In this study, we present the first experimental data relating to CYP134A1, where we show that CYP134A1 binds cyclo-l-leucyl-l-leucyl with an affinity of 24.5 +/- 0.5 muM. Structurally related diketopiperazines sharing similar alkyl side chains to cyclo-l-leucyl-l-leucyl also bind to CYP134A1 with comparable affinity. CYP134A1 is capable of catalyzing the in vitro oxidation of diketopiperazine substrates when supported with several alternate electron transfer partner systems. Products containing one additional oxygen atom and which are intermediate products of the expected pulcherriminic acid were identified by GCMS. The oxidation of related diketopiperazines reveals that different oxidative pathways exist for CYP134A1-catalyzed diketopiperazine oxidation. The crystal structure of CYP134A1 has been determined to 2.7 A resolution in the absence of substrate and in the presence of bound phenylimidazole ligands to 3.1 and 3.2 A resolution. The active site is dominated by hydrophobic residues and contains an unusual proline residue in place of the normally conserved alcohol residue that typically plays an important role in oxygen activation. The B-B(2) substrate recognition loop, which forms part of the active site, shows considerable flexibility and was found in both open and closed conformations in different structures. These results represent the first insights into the structural and biochemical basis underlying the multistep oxidation catalyzed by CYP134A1.

Identification and characterization of PshBII, a second FA/FB-containing polypeptide in the photosynthetic reaction center of Heliobacterium modesticaldum

All known Type I photosynthetic reaction centers harbor three [4Fe-4S] clusters named F(X), F(A) and F(B) that function as terminal electron acceptors. We reported earlier that F(A) and F(B) in the homodimeric Type I reaction center from Heliobacterium modesticaldum reside on a loosely bound 54 amino acid protein named PshB. Time-resolved optical spectroscopy and low temperature EPR spectroscopy showed that on illumination, electrons were transferred from F(X) (-) to F(A) and F(B) at both cryogenic and room temperatures. Interestingly, the gene that codes for PshB, HM1_1462, is part of a predicted dicistronic operon that contains a second gene, named HM1_1461, which codes for a second ferredoxin-like protein with high sequence homology to PshB, including the two traditional [4Fe-4S] cluster binding motifs. RT-PCR results confirm that both genes are transcribed as a single transcript. We have cloned the HM1_1461 gene through PCR amplification of the H. modesticaldum chromosomal DNA and overexpressed the apoprotein in Escherichia coli. Reconstitution studies with inorganic reagents have shown that the holoprotein harbors ~8 iron and ~8 sulfide atoms in the form of two [4Fe-4S] clusters. Incubation of the reconstituted holoprotein with heliobacterial reaction center cores results in a charge-separated state characteristic of electron transfer past the F(X) cluster to the terminal [4Fe-4S] clusters F(A) and F(B). These results suggest that the HM1_1461 product, which we have named PshBII, is capable of functioning in lieu of PshB (renamed PshBI) as an alternative terminal electron transfer protein. Thus, unlike PS I, to which PsaC is tightly bound, two loosely bound ferredoxins, PshBI and PshBII, are capable of interacting with the heliobacterial reaction center. The presence of two, loosely bound F(A)/F(B) proteins represents a significant shift in our understanding of structure-function relationships in Type I reaction centers.

Engineering of an alternative electron transfer path in photosystem II

The initial steps of oxygenic photosynthetic electron transfer occur within photosystem II, an intricate pigment/protein transmembrane complex. Light-driven electron transfer occurs within a multistep pathway that is efficiently insulated from competing electron transfer pathways. The heart of the electron transfer system, composed of six linearly coupled redox active cofactors that enable electron transfer from water to the secondary quinone acceptor Q(B), is mainly embedded within two proteins called D1 and D2. We have identified a site in silico, poised in the vicinity of the Q(A) intermediate quinone acceptor, which could serve as a potential binding site for redox active proteins. Here we show that modification of Lysine 238 of the D1 protein to glutamic acid (Glu) in the cyanobacterium Synechocystis sp. PCC 6803, results in a strain that grows photautotrophically. The Glu thylakoid membranes are able to perform light-dependent reduction of exogenous cytochrome c with water as the electron donor. Cytochrome c photoreduction by the Glu mutant was also shown to significantly protect the D1 protein from photodamage when isolated thylakoid membranes were illuminated. We have therefore engineered a novel electron transfer pathway from water to a soluble protein electron carrier without harming the normal function of photosystem II.

Extending the models for iron and sulfur oxidation in the extreme Acidophile Acidithiobacillus ferrooxidans

BackgroundAcidithiobacillus ferrooxidans gains energy from the oxidation of ferrous iron and various reduced inorganic sulfur compounds at very acidic pH. Although an initial model for the electron pathways involved in iron oxidation has been developed, much less is known about the sulfur oxidation in this microorganism. In addition, what has been reported for both iron and sulfur oxidation has been derived from different A. ferrooxidans strains, some of which have not been phylogenetically characterized and some have been shown to be mixed cultures. It is necessary to provide models of iron and sulfur oxidation pathways within one strain of A. ferrooxidans in order to comprehend the full metabolic potential of the pangenome of the genus.ResultsBioinformatic-based metabolic reconstruction supported by microarray transcript profiling and quantitative RT-PCR analysis predicts the involvement of a number of novel genes involved in iron and sulfur oxidation in A. ferrooxidans ATCC23270. These include for iron oxidation: cup (copper oxidase-like), ctaABT (heme biogenesis and insertion), nuoI and nuoK (NADH complex subunits), sdrA1 (a NADH complex accessory protein) and atpB and atpE (ATP synthetase F0 subunits). The following new genes are predicted to be involved in reduced inorganic sulfur compounds oxidation: a gene cluster (rhd, tusA, dsrE, hdrC, hdrB, hdrA, orf2, hdrC, hdrB) encoding three sulfurtransferases and a heterodisulfide reductase complex, sat potentially encoding an ATP sulfurylase and sdrA2 (an accessory NADH complex subunit). Two different regulatory components are predicted to be involved in the regulation of alternate electron transfer pathways: 1) a gene cluster (ctaRUS) that contains a predicted iron responsive regulator of the Rrf2 family that is hypothesized to regulate cytochrome aa3 oxidase biogenesis and 2) a two component sensor-regulator of the RegB-RegA family that may respond to the redox state of the quinone pool.ConclusionBioinformatic analysis coupled with gene transcript profiling extends our understanding of the iron and reduced inorganic sulfur compounds oxidation pathways in A. ferrooxidans and suggests mechanisms for their regulation. The models provide unified and coherent descriptions of these processes within the type strain, eliminating previous ambiguity caused by models built from analyses of multiple and divergent strains of this microorganism.

The Electron Transfer System of Syntrophically Grown Desulfovibrio vulgaris

Interspecies hydrogen transfer between organisms producing and consuming hydrogen promotes the decomposition of organic matter in most anoxic environments. Although syntrophic coupling between hydrogen producers and consumers is a major feature of the carbon cycle, mechanisms for energy recovery at the extremely low free energies of reactions typical of these anaerobic communities have not been established. In this study, comparative transcriptional analysis of a model sulfate-reducing microbe, Desulfovibrio vulgaris Hildenborough, suggested the use of alternative electron transfer systems dependent on growth modality. During syntrophic growth on lactate with a hydrogenotrophic methanogen, numerous genes involved in electron transfer and energy generation were upregulated in D. vulgaris compared with their expression in sulfate-limited monocultures. In particular, genes coding for the putative membrane-bound Coo hydrogenase, two periplasmic hydrogenases (Hyd and Hyn), and the well-characterized high-molecular-weight cytochrome (Hmc) were among the most highly expressed and upregulated genes. Additionally, a predicted operon containing genes involved in lactate transport and oxidation exhibited upregulation, further suggesting an alternative pathway for electrons derived from lactate oxidation during syntrophic growth. Mutations in a subset of genes coding for Coo, Hmc, Hyd, and Hyn impaired or severely limited syntrophic growth but had little effect on growth via sulfate respiration. These results demonstrate that syntrophic growth and sulfate respiration use largely independent energy generation pathways and imply that to understand microbial processes that sustain nutrient cycling, lifestyles not captured in pure culture must be considered.

Microbial Reduction of Intragrain U(VI) in Contaminated Sediment

The accessibility of precipitated, intragrain U(VI) in a contaminated sediment to microbial reduction was investigated to ascertain geochemical and microscopic transport phenomena controlling U(VI) bioavailability. The sediment was collected from the U.S. Department of Energy Hanford site, and contained uranyl precipitates within the mm-sized granitic lithic fragments in the sediment. Bioreduction was investigated in a culture of a dissimilatory metal-reducing bacterium, Shewanellea oneidensis strain MR-1. Measurements of uranium concentration, speciation, and valence in aqueous and solid phases indicated that microbial reduction of intragrain U(VI) proceeded by two mechanisms: (1) sequentially coupled dissolution of intragrain uranyl precipitates, diffusion of dissolved U(VI) from intragrain regions, and microbial reduction of dissolved U(VI); and (2) U(VI) reduction in the intragrain regions by soluble, diffusible biogenic reductants. The bioreduction rate in the first pathway was over 3 orders of magnitude slowerthan that in comparable aqueous solutions containing aqueous U(VI) only. The slower bioreduction rate was attributed to (1) the release of calcium from the desorption/dissolution of calcium-containing minerals in the sediment, which subsequently altered U(VI) aqueous speciation and slowed U(VI) bioreducton and (2) alternative electron transfer pathways that reduced U(VI) in the intragrain regions and changed its dissolution and solubility behavior. The results implied that the overall rate of bioreduction of intragrain U(VI) will be influenced by the reactive mass transfer of U(VI) and biogenic reductants within intragrain regions, and geochemical reactions controlling major ion concentrations.

Sulfur oxidation in mutants of the photosynthetic green sulfur bacterium Chlorobium tepidum devoid of cytochrome c-554 and SoxB

A mutant devoid of cytochrome c-554 (CT0075) in Chlorobium tepidum (syn. Chlorobaculum tepidum) exhibited a decreased growth rate but normal growth yield when compared to the wild type. From quantitative determinations of sulfur compounds in media, the mutant was found to oxidize thiosulfate more slowly than the wild type but completely to sulfate as the wild type. This indicates that cytochrome c-554 would increase the rate of thiosulfate oxidation by serving as an efficient electron carrier but is not indispensable for thiosulfate oxidation itself. On the other hand, mutants in which a portion of the soxB gene (CT1021) was replaced with the aacC1 cassette did not grow at all in a medium containing only thiosulfate as an electron source. They exhibited partial growth yields in media containing only sulfide when compared to the wild type. This indicates that SoxB is not only essential for thiosulfate oxidation but also responsible for sulfide oxidation. An alternative electron carrier or electron transfer path would thus be operating between the Sox system and the reaction center in the mutant devoid of cytochrome c-554. Cytochrome c-554 might function in any other pathway(s) as well as the thiosulfate oxidation one, since even green sulfur bacteria that cannot oxidize thiosulfate contain a cycA gene encoding this electron carrier.