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School of Biological Sciences

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search.filters.applied.f.advisor: DiChristina, Thomas J. × End date: 2017 × Start date: 2013 × Item Type: Publication × search.filters.applied.f.isOrgUnitOfPublicationTitle: School of Biological Sciences ×

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Novel microbial platform for degradation of hazardous organic contaminants and production of sustainable bioplastics

2016-04-15 , Sekar, Ramanan

Improper disposal of 1,4-dioxane and the chlorinated organic solvents trichloroethylene (TCE) and tetrachloroethylene (PCE) has resulted in widespread contamination of soil and groundwater. In the present study, a novel microbially-driven Fenton reaction system was designed to generate hydroxyl (HO) radicals for simultaneous degradation of source zone levels of single, binary, and ternary mixtures of TCE, PCE, and 1,4-dioxane. The new Fenton reaction system was driven by the Fe(III)-reducing facultative anaerobe Shewanella oneidensis amended with lactate, Fe(III), and contaminant mixtures and exposed to alternating anaerobic and aerobic conditions. The novel microbially-driven Fenton reaction system successfully degraded TCE, PCE, and 1,4-dioxane either as single contaminants or as binary and ternary mixtures. Degradation of lignocellulosic biomass was also demonstrated through the novel microbially driven fenton reaction by S. oneidensis. In this study, we have developed a new method that combines both pretreatment and saccharification of cellulose and xylan in a microbially driven fenton reaction. The combined pretreatment and saccharification method for cellulose and xylan developed did not involve the addition of acid, alkali compounds or the use of hydrolyzing enzymes thus being an economically feasible process to directly produce simple fermentable sugars from cellulose and xylan. Microbial Fe(III) reduction is a dominant anaerobic respiratory process in soil and sediments, which suggests that the microbially driven fenton reaction may play an important role in the degradation of decaying plant and woody materials in the natural environment. The expansion of metabolic capability to convert D-xylose to a useful product such as PHB can be beneficial in biotechnological applications to couple multiple carbon sources such as glucose, glycerol and D-xylose by S.oneidensis to improve efficiency of electricity generation, biofuel production and bioremediation of toxic contaminants.

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Bacterial iron and manganese reduction driven by organic sulfur electron shuttles

2015-04-08 , Cooper, Rebecca Elizabeth

Dissimilatory metal-reducing bacteria (DMRB) play an important role in the biogeochemical cycling of metals. DMRB are unique in that they possess the ability to couple metal reduction with their metabolism. Microbial Fe(III) respiration is a central component of a variety of environmentally important processes, including the biogeochemical cycling of iron and carbon in redox stratified water and sediments, the bioremediation of radionuclide-contaminated water, the degradation of toxic hazardous pollutants, and the generation of electricity in microbial fuel cells. Despite this environmental and evolutionary importance, the molecular mechanism of microbial Fe(III) respiration is poorly understood. Current models of the molecular mechanism of microbial metal respiration are based on direct enzymatic, Fe(III) solubilization, and electron shuttling pathways. Fe(III) oxides are solid at circumneutral pH and therefore unable to come into direct contact with the microbial inner membrane, these bacteria must utilize an alternative strategy for iron reduction. Reduced organic compounds such as thiols are prominent in natural environments where DMRB are found. These thiol compounds are redox reactive and are capable of abiotically reducing Fe(III) oxides at high rates S. oneidensis wild-type and ΔluxS anaerobic biofilm formation phenotypes were examined under a variety of electron donor-electron acceptor pairs, including lactate or formate as the electron donor and fumarate, thiosulfate, or Fe(III) oxide-coated silica surfaces as the terminal electron acceptor. The rates of biofilm formation under the aforementioned growth conditions as well as in the presence of exogenous thiol compounds indicate that ∆luxS formed biofilms at rates only 5-10% of the wild-type strain and ∆luxS biofilm formation rates were restored to wild-type levels by addition of a variety of exogenous compounds including cysteine, glutathione, homocysteine, methionine, serine, and homoserine. Cell adsorption isotherm analyses results indicate that wild-type is can attach to the surface of hematite particles attachment , but ΔluxS is unable to attach the hematite surfaces. These results indicate that biofilm formation is not required for Fe(III) oxide reduction by S. oneidensis ∆luxS anaerobic biofilm formation rates were restored to wild-type levels by addition of exogenous auntoinducer-2 (AI-2), a by-product of homocysteine production in the Activated Methyl Cycle. This discovery led to subsequent experiments performed to detect the production and utilization of AI-2 by wild-type and ∆luxS strains under aerobic and anaerobic conditions. AI-2 production experiments showed wild-type, but not ΔluxS, was capable of producing AI-2. The addition of exogenous S. oneidensis and Vibrio harveyi-produced AI-2 to wild-type and ∆luxS resulted in the swift depletion of AI-2 from the media. These results provide evidence that S. oneidensis can produce AI-2 and subsequently utilize its’ own AI-2 as well as AI-2 produced by other bacteria as a carbon and electron source in the absence of preferred carbon sources. S. oneidensis produces and secretes a suite of extracellular thiols under anaerobic Fe(III)-reducing and Mn(III) and Mn(IV)-reducing conditions including cysteine, homocysteine, glutathione, and cyteamine. Exogenous thiols produced by S. oneidensis are intermediates of the Activated Methyl Cycle (AMC) and Transulfurylation Pathway (TSP). Reduced and oxidized thiols were detected, indicating that the thiols are in a constant state of flux between the reduced and oxidized forms and that the concentration of reduced thiols to its’ oxidized counterpart is indicative of the state of metal reduction by the microorganisms. Respiratory phenotypes Based on Fe(III) and Mn(IV) respiratory phenotypes observed in the AMC and TSP pathway mutants (∆luxS, ∆metB, ∆metC and ∆metY) we can infer that cysteine, glutathione, and cysteamine contribute to metal reduction by serving as efficient electron shuttling molecules, while homocysteine is critical for maintenance of the AMC, propagation of thiol biosynthesis, and maintenance of cellular metabolism via the AMC intermediate SAM. Furthermore, these findings suggest that all metal-reducing bacteria require thiol formation to reduce solid metal oxides. Direct contact mechanism is not the dominant means through electrons are transferred and metals are reduced, instead electron shuttles are the maid reduction mechanism.

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Novel pathway for microbial FE(III) reduction: electron shuttling through naturally occurring thiols

2014-02-17 , Wee, Seng Kew

The g-proteobacterium Shewanella oneidensis MR-1 reduces a wide range of terminal electron acceptors, including solid Fe(III) oxides. Pathways for Fe(III) oxide reduction by S. oneidensis include non-reductive (organic ligand-promoted) solubilization reactions, and either direct enzymatic, or indirect electron shuttling pathways. Results of the present study expand the spectrum of electron acceptors reduced by S. oneidensis to include the naturally occurring disulfide compounds cystine, oxidized glutathione, dithiodiglycolate, dithoidiproponiate and cystamine. Subsequent electron shuttling experiments demonstrated that S. oneidensis employs the reduced (thiol) form of the disulfide compounds (cysteine, reduced glutathione, mercaptoacetate, mercaptopropionate, and 2-nitro-5-thiobenzoate, cystamine) as electron shuttles to transfer electrons to extracellular Fe(III) oxides. The results of the present study indicate that microbial disulfide reduction may represent an important electron-shuttling pathway for electron transfer to Fe(III) oxides in anaerobic marine and freshwater environments.

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