Microbial electrosynthesis (MES) is a process where bacteria acquire electrons from a cathode to convert CO2 into multicarbon compounds or methane. In MES with Sporomusa ovata as the microbial catalyst, cathode potential has often been used as a benchmark to determine whether electron uptake is hydrogen-dependent. In this study, H2 was detected by a microsensor in proximity to the cathode. With a sterile fresh medium, H2 was produced at a potential of −700 mV versus Ag/AgCl, whereas H2 was detected at −500 mV versus Ag/AgCl with cell-free spent medium from a S. ovata culture. Furthermore, H2 evolution rates were increased with potentials lower than −500 mV in the presence of cell-free spent medium in the cathode chamber. Nickel and cobalt were detected at the cathode surface after exposure to the spent medium, suggesting a possible participation of these catalytic metals in the observed faster hydrogen evolution. The results presented here show that S. ovata-induced alterations of the cathodic electrolytes of a MES reactor reduced the electrical energy required for hydrogen evolution. These observations also indicated that, even at higher cathode potentials, at least a part of the electrons coming from the electrode are transferred to S. ovata via H2 during MES.

BACKGROUND Microbial electrosynthesis (MES), a process by which microorganisms reduce carbon dioxide to multi‐carbon compounds using electrical current as energy source, has so far been demonstrated at temperatures ranging from 25 °C to 37 °C. Elevated operating temperatures, however, could improve overall performance and product recovery. Here the effect of temperature on MES by the acetogenic thermophiles Moorella thermoacetica and Moorella thermoautotrophica is investigated. RESULTS Experiments were performed at operating temperatures ranging from 25 °C to 70 °C to determine the optimum operating temperature for MES. Optimal performance was observed to be close to the optimum growth temperatures reported for these strains. Production rate and activation energy of acetate at 60 °C was 6.9 ± 0.6 mM m−2 d−1 and 45.1 ± 3.8 kJ mol−1 for M. thermoacetica and 11.6 ± 0.9 mM m−2 d−1 and 58.9 ± 2.5 kJ mol−1 for M. thermoautotrophica with columbic efficiencies (CE) of 79 ± 15% and 72 ± 4%, respectively. CONCLUSION Considering CE and acetate production rate during MES, M. thermoautotrophica outperformed M. thermoacetica over a wide range of operating temperatures. Current‐dependent reduction of CO2 also occurred below the minimum growth temperature of these strains, suggesting that MES is non‐growth associated. © 2016 Society of Chemical Industry