Environmental Functions And Electrochemical Regulation Of Bacterial Transmembrane Electron Transfer

The transmembrane electron conduits are the molecular basis of Electrochemically-Active Bacteria (EAB) involving in the redox chemistry in the environment, and also the core in cell-to-acceptor electron transfer. Current knowledges about the electron transfer mechanism are collected with molecular genetic and spectroelectrochemical techniques, which would not fully explain the electrochemical activity of EAB in environment and clarify the composition of electron conduits. For more in-depth understanding of the transmembrane electron transfer and environmental functions of the EAB, study on physiological activity of the redox components at environmental interfaces and the strategic adjustment for adaptation to the surrounding conditions by EAB cells are very important. In this thesis, we focused on the key issues at the interface between EAB cells and the electron acceptors and explored the electrochemical performance of EAB during transmembrane electron transfer process from several perspectives. The results will help revealing the flexibility and versatileness of EAB in mediating environmental redox processes. The findings in this study also provide theoretical support for the technical application aspects of bioenergy. Main contents and results in this thesis are as follows:1. To evaluate the reduction capacity of EAB to toxic acceptors with the diverse respiratory complexes and the corresponding molecular mechanism, Shewanella oneidensis MR-1was used for SeO32" reduction and Se sequestration. Mutants deficient in synthesizing respiratory complexes were tested for their activities in SeO32-reduction. Results shows that S. oneidensis MR-1reduced SeO32-rapidly to Se (0). The Se (0) particles were deposited first intracellularly and then on the outer membrane. Different with the general detoxification reaction driven by reduced thiols within the cytoplasm, S. oneidensis MR-1uses the electron conduits for anaerobic respiration to perform SeO32-reduction. A flavoprotein in periplasm, fumarate reductase FccA, worked as the direct reducer for SeO32-.Dependence of SeO32-reduction on CymA and the inhibition of SeO32-reduction by fumarate support this finding and suggest the global electron conduit to SeO32’. Nitrate reductase, nitrite reductase, and the Mtr complex in the outer membrane were hardly involved in the SeO32-reduction.2. To explore the feasibility of electron transfer for respiration from EAB cells to the vacancies at semiconductive acceptor surface under light excitation, electrochemical interface between Geobacter sulfurreducens cells and α-Fe2O3was constructed. Results shows that under suitable potentials (higher than-0.25V) the light excitation accelerated the electron transfer from G. sulfurreducens cells to α-Fe2O3. The illumination significantly improved the internal conductivity of α-Fe2O3, making the charges transfer quickly through α-Fe2O3and avoiding the reductive dissolving of Fe3+. The coupling of cellular respiration and charge separation in α-Fe2O3under illumination was evidenced. Electrochemical characterizations and physiological inhibition show the dependence of photocurrent production on the discharging of cells on α-Fe2O3surface. Light-induced electron transfer on the cell-α-Fe2O3interface correlated linearly with the rates of microbial respiration and substrate consumption at any stage of biofilm development. Furthermore, rapid electron transfer and moderate water-oxidizing potential enable the cells to maintain excellent electrochemical activity on the excited α-Fe2O3surface.3. In order to clarify the effect of polarizing potentials to the extracellular electron transfer of EAB, the dynamics of spontaneously adhesion to the electrodes, respiration, discharging, and biofilm formation by G. sulfurreducens cells at-0.1V～+0.6V were monitored. Results show that the G. sulfurreducens cells on electrodes set at+0.0V～+0.2V conducted more the most rapid cell adhesion and discharging, and eventually formed more active and thicker surface biofilm, compared with at higher potentials. The efficiency of discharging by single cell on the electrode surface was highest at+0.1V and much weaker at other potentials. Although all the biofilms were proven to conduct by electron tunneling between the adjoining electrochemical active centers, the biofilms developed at low potentials synthesized more extracellular hemeproteins and less extracellular polysaccharides, while exhibited a significantly lower electron transfer resistance. It was also found that for all the biofilms the major redox species on electrode surface were consistent with each other, but they were more abundant for biofilms developed at+0.2V～+0.4V.4. To seek the basic models and conditions for implementing a bacterially-catalyzed cathodic reaction, four species of physiologically-relevant to electron intake were individually attached to the surface of an inert graphite electrode. The electrochemical interplay of these cells with the electrode was characterized. The results showed that in the presence of electron acceptor, T. denitrificans and G. sulfurreducens cells on electrode surface immediately evolved the cathodic electro-catalytic activity, individually resulting in maximum current of-7.1μA/cm2and-4.2μA/cm2. While either A. thiooxidans or G. metallireducens showed considerable delay before a significant cathodic current occurred, which suggested some physiological adjustment. Acclimating the cells on the electrode surface for the polarization potentials was proved to be very important to the electrochemical activity of them, and cells acclimated at a cathodic potential of-0.3V were more active than in the anodic potentials. With G. sulfurreducens cells on the electrode surface we showed that the electron acceptor was required for significant cathodic current generation. Furthermore, cells attached to the electrode in both ways gave similar electrochemical activity, with multiple redox-active species presented at the interface.