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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Microbiology

Year Degree Awarded

Spring 2015

First Advisor

Derek R. Lovley

Second Advisor

James F. Holden

Third Advisor

Steven J. Sandler

Fourth Advisor

Dawn E. Holmes

Subject Categories

Environmental Microbiology and Microbial Ecology | Microbial Physiology

Abstract

Understanding the mechanisms for microbial extracellular electron exchange are of interest because these processes play an important role in the biogeochemical cycles of both modern and ancient environments, development of bioenergy strategies, as well as for bioremediation applications. Only a handful of microorganisms are capable of extracellular electron exchange, one of the most thoroughly studied being the Geobacter species. Geobacter species are often the predominant Fe(III) reducing microorganisms in many soils and sediments, can exchange electrons directly via interspecies electron transfer, and can both donate or accept electrons with a wide variety of extracellular substrates including the electrode of a microbial fuel cell.

This dissertation describes three research projects that aim to further understand these mechanisms and identify novel components involved in extracellular electron exchange by Geobacter species. The first uncovers components involved in extracellular electron transfer to insoluble Fe(III) oxides by Geobacter metallireducens. This project identified six c-type cytochromes, a NHL-repeat containing protein, and a gene potentially involved in pili glycosylation that were essential for reduction of insoluble Fe(III) oxide, but not for soluble Fe(III) citrate.

The second research project serves to reveal and examine PilA-pili independent mechanisms for extracellular electron transfer to Fe(III) oxides by Geobacter sulfurreducens. During the course of this study a pilA-deficient strain of G. sulfurreducens adapted to reduce Fe(III) oxide via production of the c-type cytochrome PgcA, which was released into the culture medium, and was required for the newly adapted mechanism of Fe(III) oxide reduction.

The third research project investigates the mechanism(s) utilized by G. sulfurreducens for extracellular electron exchange into the cell via the oxidation of the humic substance analog anthrahydroquinone-2,6-disulfonate (AHQDS) in cocultures with G. metallireducens. Cocultures initiated with strains of G. sulfurreducens deficient in genes for proteins previously identified to be important in extracellular electron exchange grew as well as the wild type strain, suggesting that mechanisms for exchanging electrons with extracellular electron donors are substantially different than for reduction of extracellular electron acceptors.

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