Bacterial iron and manganese reduction driven by organic sulfur electron shuttles

Cooper, Rebecca Elizabeth

Title:

Bacterial iron and manganese reduction driven by organic sulfur electron shuttles

dc.contributor.advisor	DiChristina, Thomas J.
dc.contributor.author	Cooper, Rebecca Elizabeth
dc.contributor.committeeMember	Hammer, Brian
dc.contributor.committeeMember	Stewart, Frank
dc.contributor.committeeMember	Taillefert, Martial
dc.contributor.committeeMember	Spain, Jim C.
dc.contributor.department	Biology
dc.date.accessioned	2016-05-27T13:10:39Z
dc.date.available	2016-05-27T13:10:39Z
dc.date.created	2015-05
dc.date.issued	2015-04-08
dc.date.submitted	May 2015
dc.date.updated	2016-05-27T13:10:39Z
dc.description.abstract	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.
dc.description.degree	Ph.D.
dc.format.mimetype	application/pdf
dc.identifier.uri	http://hdl.handle.net/1853/54894
dc.language.iso	en_US
dc.publisher	Georgia Institute of Technology
dc.subject	Shewanella oneidensis
dc.subject	Metal reduction
dc.subject	Organic sulfur
dc.subject	Electron shuttles
dc.title	Bacterial iron and manganese reduction driven by organic sulfur electron shuttles
dc.type	Text
dc.type.genre	Dissertation
dspace.entity.type	Publication
local.contributor.advisor	DiChristina, Thomas J.
local.contributor.corporatename	College of Sciences
local.contributor.corporatename	School of Biological Sciences
relation.isAdvisorOfPublication	cb6665c2-bcd0-4164-b448-e7ff33dd279f
relation.isOrgUnitOfPublication	85042be6-2d68-4e07-b384-e1f908fae48a
relation.isOrgUnitOfPublication	c8b3bd08-9989-40d3-afe3-e0ad8d5c72b5
thesis.degree.level	Doctoral