Investigations of succinate dehydrogenase and fumarate reductase in whole cells ofShewanella putrefaciens (strains MR-1 and MR-7) using electron spin resonance spectroscopy

Using ESR spectroscopy we were able to detect succinate dehydrogenase (SDH) in the whole cells ofShewanella putrefaciens grown aerobically. ESR analysis of cells grown under anaerobic conditions in the presence of fumarate as electron acceptor revealed signals with the same parameters as SDH. However the temperature dependence was quite different. Furthermore, the addition of sodium azide led to disappearance of this ESR signal in anaerobic cultures, while the same treatment of aerobic cultures had no effect on the ESR signal of SDH. Based on these results, we postulate that the ESR signal from the anaerobic cells is that of fumarate reductase, similar in its properties to the fumarate reductase fromEscherichia coli.     

The use of electrochemical impedance spectroscopy (EIS) in the evaluation of the electrochemical properties of a microbial fuel cell

Electrochemical impedance spectroscopy (EIS) has been used to determine several electrochemical properties of the anode and cathode of a mediator-less microbial fuel cell (MFC) under different operational conditions. These operational conditions included a system with and without the bacterial catalyst and EIS measurements at the open-circuit potential of the anode and the cathode or at an applied cell voltage. In all cases the impedance spectra followed a simple one-time-constant model (OTCM) in which the solution resistance is in series with a parallel combination of the polarization resistance and the electrode capacitance. Analysis of the impedance spectra showed that addition of Shewanella oneidensis MR-1 to a solution of buffer and lactate greatly increased the rate of the lactate oxidation at the anode under open-circuit conditions. The large decrease of open-circuit potential of the anode increased the cell voltage of the MFC and its power output. Measurements of impedance spectra for the MFC at different cell voltages resulted in determining the internal resistance (R(int)) of the MFC and it was found that R(int) is a function of cell voltage. Additionally, R(int) was equal to R(ext) at the cell voltage corresponding to maximum power, where R(ext) is the external resistance that must be applied across the circuit to obtain the maximum power output.     

Flavin Redox Bifurcation as a Mechanism for Controlling the Direction of Electron Flow during Extracellular Electron Transfer

The iron‐reducing bacterium Shewanella oneidensis MR‐1 has a dual directional electronic conduit involving 40 heme redox centers in flavin‐binding outer‐membrane c‐type cytochromes (OM c‐Cyts). While the mechanism for electron export from the OM c‐Cyts to an anode is well understood, how the redox centers in OM c‐Cyts take electrons from a cathode has not been elucidated at the molecular level. Electrochemical analysis of live cells during switching from anodic to cathodic conditions showed that altering the direction of electron flow does not require gene expression or protein synthesis, but simply redox potential shift about 300 mV for a flavin cofactor interacting with the OM c‐Cyts. That is, the redox bifurcation of the riboflavin cofactor in OM c‐Cyts switches the direction of electron conduction in the biological conduit at the cell–electrode interface to drive bacterial metabolism as either anode or cathode catalysts.     

Studies of the iron-sulphur centers of the bacteriaShewanella putrefaciens (MR-1)

Shewanella putrefaciens (MR-1) can be grown using any one of several different terminal acceptors. This respiratory versatility stimulated our studies of the respiratory chain of MR-1 using whole cell analysis by ESR spectroscopy. Initial analyses of aerobically grown cells showed the presence of the succinate dehydrogenase complex. Also, it was shown that inS. putrefaciens which were grown anaerobically with iron as the only electron acceptor the ESR signal atg=2.021 appeared at low temperatures. This ESR signal resembled the ESR signal of [Fe-S] center of succinate dehydrogenase in oxidized state. However this new ESR signal had a different temperature dependence. It can be seen at temperature as high as 40 K. The line shape andg-factor of the new center are very similar to those of [3Fe-4S] centers. As this ESR signal can be seen only in the anaerobic samples grown with iron as acceptor we suggest that this center is located in the terminal part of the respiratory chain and is associated with the reduction of iron or another electron acceptor.