Effects of oxygen on Shewanella decolorationis NTOU1 electron transfer to carbon-felt electrodes

To clarify the major factor caused by oxygen-enhancing charge production of Shewanella decolorationis NTOU1 towards a polarized anode, a series of experimental runs (i.e., with/without ambient air flushing and with/without ammonia addition as nitrogen source) were conducted in this study. Within 6-day of operation at +0.4 V vs. Ag|AgCl and starting with 35 mM of lactate, consistently the electrical charge production under the aerobic condition was higher than that under the anaerobic condition. In all the experimental runs, the values of nicotinamide adenine dinucleotide (NADH) production were found to be correlated positively and significantly with the charge production, but the highest Coulombic efficiency of 18% was observed under the anaerobic conditions without ammonia addition while the lowest charge production occurred. Those results indicate that NADH production enhanced by oxygen is the leading cause of the increase of the charge production, but the biomass production and the oxygen reduction would both consume NADH electrons and lead to lower electron recoveries. In addition, whether under constant aerobic or anaerobic, or alternating aerobic/anaerobic conditions, chronoamperometric results made it possible to rule out other factors, like lactate uptake rate or cell growth, which might increase the charge production under aerobic conditions. By using high performance liquid chromatography, some diffusive flavins (e.g., 0.5 μM of riboflavin) were found under the aerobic condition, but were not found under the anaerobic one. However, from results of cyclic voltammetry (CV), the signals of flavins were found to be approximately the same under both conditions. Although it is inferred that oxygen renders the flavins secreted extracellularly, that is not the major effect of oxygen for boosting the charge production. Furthermore, bound flavins under anaerobic condition were found to be effectively electrocatalytic according to sigmoidal CV result.