Using metabolic charge production in the tricarboxylic acid cycle (Q) to evaluate the extracellular-electron-transfer performances of Shewanella spp.

Using an electrochemical cell equipped with carbon felt electrodes (poised at +0.63 V vs. SHE), the current production capabilities of two Shewanella strains-NTOU1 and KR-12-were examined under various conditions with lactate as an electron donor. The metabolic charge produced in the tricarboxylic acid cycle (Q) was calculated by mass-balance. The data showed a linear relation between the electric coulomb production (Q) and Q with an R of 0.65. In addition, a large amount of pyruvate accumulation was observed at pH = 6, rendering Q negative. The results indicate an occurrence of an undesired cataplerotic reaction. It was also found that Q provides important information showing the oxygen-boosting TCA cycle and anodic-current generation of Shewanella spp. Linear dependence of the change in charge for biomass growth (4.52FΔn) on Q was also found as expressed by 4.52FΔn = 1.0428 Q + 0.0442, indicating that these two charge quantities are inherently identical under most of the experimental conditions. In the mediator-spiked experiments, the external addition of the mediators (ferricyanide, anthraquinone-2, 6-disulfonate, and riboflavin) beyond certain concentrations inhibited the activity of the TCA cycle, indicating that the oxidative phosphorylation is deactivated by excessive amounts of mediators, yet Shewanella spp. are constrained with regard to carrying out the substrate-level phosphorylation.

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