The power supply from renewable energy sources like wind and solar is variable and sometimes unpredictable. Energy storage systems are required to overcome these challenges in the transition to a more sustainable future. Additionally, the development of technologies to capture, store, and convert CO2 is a priority for governments and research groups, since different factors have accelerated the CO2 accumulation in the atmosphere, causing global warming and environmental damage.
Microbial electrosynthesis (MES) is a biotechnology that combines the upgrading of CO2 into organic compounds, such as biofuels, and the storage of electrical energy using microorganisms as a biocatalyst. Thereby, Acetogenic bacteria are interesting for MES, since some acetogenic bacteria have the capacity to use cathodes as electron donors to reduce CO2 into acetate or other organic compounds. The electroactivity of acetogens has been the subject of several research efforts, but it is still unclear what makes some acetogens more efficient in the current consumption from cathodes than others. Different mechanisms have been proposed to explain the electron uptake by the bacteria from the cathodes. These include direct electron transfer and H2-mediated indirect electron transfer, as described in Chapter 1. So far, H2-mediated indirect electron transfer is the most likely mechanism for electron uptake, since most acetogens can use H2 as an electron donor and evidence for a direct electron transfer mechanism is still lacking. We, therefore, hypothesize that the capacity of acetogens to take up cathodic electrons depends on their H2 consumption characteristics, including their H2 threshold and H2 consumption kinetics.
The differences in the H2 consumption between acetogens were investigated. We experimentally determined the lowest H2 partial pressure at which acetogenesis halts (H2 threshold), for eight acetogenic strains as described in Chapter 2. We found significant differences of three-order of magnitude between the strain with the highest (Clostridium autoethanogenum) and the lowest (Sporomusa ovata 2662) H2 threshold. The H2 thresholds most likely reflect differences in the energy conservation mechanisms of these acetogens. We further evaluate in Chapter 3 the H2 consumption rates at low H2 partial pressures of three acetogens with a high, intermediate and low H2 threshold, respectively: Clostridium ljungdahlii, Acetobacterium woodii and Sporomusa ovata 2662. Sporomusa ovata 2662 presented the highest H2 uptake first-order rate constant, followed by Acetobacterium woodii. Clostridium ljungdahlii was at least ten times slower in consuming H2 than Sporomusa ovata 2662 at low concentrations. In Chapter 4, we investigate whether the differences in H2 consumption characteristics of the bacteria have implications for their performance in MES systems. H-cell-type reactors with a cathode poised at -605 mV vs. SHE were inoculated with the three strains. Sporomusa ovata 2662 and Acetobacterium woodii produced acetate with high efficiencies, while low H2 concentrations were constant in the system. Clostridium ljungdahlii did not perform acetogenesis in the MES reactors and H2 accumulated over time. Overall, we observed an apparent correlation between the strain-specific characteristics of H2 uptake and their performance in the bioelectrochemical system. The kinetic consumption parameters and H2 thresholds show that Sporomusa ovata 2662 is best suited to consume H2 at low levels, while Clostridium ljunghdahlii is the least well adapted for such conditions. Our findings suggest that the ability to metabolize H2 efficiently at low partial pressures represents an advantage in performing acetogenesis in MES, as is discussed in Chapter 5.