Alfred M. Spormann, Departments of Chemical Engineering, and of Civil and Environmental Engineering, Stanford University

Objective

This research program explores the opportunities and bottlenecks of electrosynthesis, a technology that uses microorganisms to produce transportation fuel from electrical energy and atmospheric CO2. The fundamental science encompasses microbial communities, their interaction with electrodes and the processes that allow efficient electron uptake, transfer and synthesis of fuels and fuel precursors such as methane, acetic acid and hydrogen. Understanding the interactions of microbes, and how their metabolic interactions are controlled, is an emerging field that is relevant for bioenergy as well as bioremediation, human health and ecosystem management.

Background

Petroleum and other fossil hydrocarbons are primarily used as energy sources for liquid (transportation) fuels and as raw materials to produce commercially valuable chemicals.  These uses represent the largest anthropogenic contribution to atmospheric CO2 and global warming. To reduce or eliminate this net release of CO2, new approaches are urgently needed that connect electrical energy to the infrastructure advantages of hydrocarbon fuels and chemical precursors.

In nature, some microorganisms are capable of transferring cellular electrons to insoluble extracellular compounds, in particular to iron-oxide mineral surfaces, such as in hematite or goethite. This mechanism is a key feature of microbial fuel cells (MFCs), where the anode serves as the electron acceptor, and the cathode is oxidized typically by molecular oxygen.  Microbes can also uptake low-potential electrons directly or indirectly from a cathode (Figure 1) to drive catabolic processes.

This project focuses on the microbial synthesis of methane, acetate and other hydrocarbons that can be easily separated from a bioreactor and used as carbon-neutral fuels or precursors.

. Figure 1: Excess electrons from carbon-neutral energy sources can be used by microbes to drive the reduction of atmospheric CO2 and synthesize biofuels, such as methane.

Specific reduction-oxidation (redox) enzymes, including hydrogenases and diaphorases, play an important role in mediating reductive reactions at the cathode (Figure 2). A cathodic fermentation process will be developed that feeds molecular hydrogen (H2) to microorganisms for CO2 fixation. The fixed CO2 will be used to produce precursors of biofuels, such as acetate. This research will advance our fundamental understanding of low-potential redox enzymes, including hydrogenases, and the molecular pathways of electron transfer within the enzymes. Moreover, the research will lead to the development of novel ways to engineer microbial communities for the production of biofuels.

. Figure 2: Examples of enzymes (e.g., hydrogenase and diaphorase) and redox processes involved in microbial electrosynthesis. Key: e- (electrons), H (hydrogen), Fe-S (iron sulfide), Ni-Fe (nickel-iron), FMN (biomolecule), NAD(P)H (coenzyme).

Approach

The initial research has focused on specific bacteria, such as Cupriavidus necator H16 and Shewanella oneidensis, which are known to metabolize electrons.

Using C. necator H16, the research team demonstrated for the first time the direct uptake and metabolism of electrons from the surface of a cathode. The data showed that C. necator H16 uses cathodic electrons during the metabolism of oxygen and nitrate. This important finding suggests that C. necator can be used as a platform for microbial electrosynthesis.  Previous studies have shown that the hydrogenase enzyme of C. necator H16 can be used to coat an electrode to consume hydrogen and produce a current. If the electrons from the cathode can be channeled into respiration, then the energy that is generated could be used to drive other cellular processes, such as carbon dioxide fixation.  Data obtained in the experiments with S. oneidensis indicate that this microbe might also be a very useful and effective.

Resources

• Lovley, D.R., et al., Dissimilatory Fe(III) and Mn(IV) reduction. Adv MicrobPhysiol, 49, 219-286 (2004).
• Logan, B.E., et al., Microbial fuel cells: methodology and technology. Environ Sci Technol, 40(17) 5181-5192 (2006).
• Schroder, U., Anodic electron transfer mechanisms in microbial fuel cells and their energy efficiency. Phys Chem Chem Phys, 9(21) 2619-2629 (2007).
• Burgdorf, T., et al., [NiFe]-hydrogenases of Ralstonia eutropha H16: modular enzymes for oxygen-tolerant biological hydrogen oxidation. Journal of Molecular Microbiology and Biotechnology, 10(2-4) 181-196 (2005).