Microbial electrosynthesis for sustainable bioproduction

Arpita Bose, PhD, Associate Professor at Washington University illustrates how microbes may prove essential for developing sustainable technologies as we strive for greener economies

Photosynthesis is believed to have evolved 3.5 billion years ago. Some of the earliest phototrophs, called photoferrotrophs, used dissolved iron as an electron source to drive photosynthetic carbon dioxide (CO2) fixation. Billions of years later, we have learned a great deal about these microbes and the vital role they play in ancient and modern environments.

Electricity-eating microbes

Photoferrotrophs capture electrons from reduced iron via extracellular electron uptake (EEU) and use these electrons to drive essential cellular processes (e.g., photosynthesis). Using the model phototroph Rhodopseudomonas palustris TIE-1 (TIE-1) we characterised the Phototrophic Iron Oxidation (pio) gene cluster, which is essential for EEU, and the electron-capturing proteins it encodes. (1) Additional work connected EEU to photosynthetic CO2 fixation in TIE-1. (2)

Though we continue to deepen our understanding of EEU in TIE-1, other EEU-capable bacteria await discovery. We recently demonstrated that EEU is prevalent in marine anoxygenic phototrophic bacteria, (3) and ongoing work in our laboratory seeks to identify more EEU-capable microbes and characterise the molecular mechanisms responsible for this process. We suspect that these “electrotrophs” are ubiquitous in nature and that novel electron uptake mechanisms await discovery. (4)

EEU: A catalyst for microbial electrosynthesis

EEU is not limited to photoferrotrophy; diverse bacteria can access electrons from hydrogen gas, hydrogen sulfide, iron minerals, and poised electrodes. (4) EEU can be leveraged for bio-commodity production via microbial electrosynthesis (MES). (5) During MES, CO2 is converted into organic carbon compounds using electrotrophs as biocatalysts. This occurs in a bioelectrochemical system comprised of a reactor with an anode and cathode in an electrically conductive bacterial growth medium. In the case of photosynthetic EEU, MES uses renewable inputs: light, CO2, and electricity. Consequently, photosynthetic electrotrophs like TIE-1 are promising biocatalysts for sustainable MES. Moreover, TIE-1’s metabolic plasticity and its genetic tractability make it a promising organism for both basic research and industrial applications. (5)

How is MES used?

To date, six main products are formed via MES:

• Acetic acid.
• Ethanol.
• n-butyric acid.
• n-butanol.
• Hexanoic acid.
• n-hexanol. (6)

Our lab has used TIE-1 to make biofuel (n-butanol) (7) and bioplastic (polyhydroxybutyrate) via MES. (8) We achieved the former by introducing the n-butanol biosynthesis pathway into TIE-1, and further improved production by deleting the electron-consuming nitrogen fixation pathway. Coupled with a bioelectrochemical platform that used solar panel-generated electricity, we achieved efficient biofuel production. This provides a foundation for carbon-neutral n-butanol synthesis using sustainable resources. We also investigated TIE-1’s ability to produce bioplastic (polyhydroxybutyrate, or PHB) which acts as an intracellular carbon and energy reserve for bacteria.

PHBs offer a promising alternative to petroleum-based plastics; they are thermoresistant, mouldable, biocompatible, and biodegradable polyesters that have been used in fields including agriculture, aerospace, biomedicine, infrastructure, and electrical engineering. Importantly, via MES using TIE-1, PHB production is based on renewable resources rather than fossil fuels. (8)

MES has utility beyond bio-commodities, including bioremediation, water desalinisation and other areas yet to be considered. Research focused on perfecting these applications is a crucial prerequisite to industrial applications.

Improving MES

Academic research has focused on cathode modifications, the biology of EEU, and isolating electrotrophs from the environment. Our laboratory demonstrated that modifying electrodes with iron-based composites can increase EEU. Using an immobilised iron-based redox mediator called Prussian Blue, we achieved a 3.8-fold increase in cathodic current uptake. (9) We also synthesised a composite of magnetite nanoparticles and reduced graphene oxide, which we electrodeposited onto a carbon felt cathode. (10)

This resulted in 5-fold higher EEU and 4.2-fold higher PHB production relative to unmodified carbon felt – 20 times higher than unmodified graphite. From a biological perspective, genetically modified strains will be a powerful tool. “Designer” strains lacking resource-consuming pathways, overexpressing commodity biosynthesis pathways, or expressing different EEU proteins from other organisms may further increase yields and efficiency. Finally, bioprospecting for novel strains capable of EEU will further expand our biological toolbox.

What challenges remain?

The primary question is one of scalability: How do we transfer this from laboratory to industry while striking a balance between costs and output? One bottleneck is EEU efficiency; low electron uptake means low product formation, and current densities above 50 – 100 mA cm-2 may be required for most MES applications. (6)

Biofilms also play an important role in achieving higher current densities; advancements in 3D-printed biofilms can maximise EEU efficiency by considering parameters like biofilm thickness, density, and spatial organisation. Mathematical modelling of MES is currently lacking and may clarify the electrochemical and biological dynamics, leading to improved reactor designs. Additionally, bioprospecting, genetic engineering and synthetic biology will yield novel strains with enhanced EEU capabilities and resilience to diverse conditions. The latter is especially important, as temperature, salinity, pressure, and pH all play important roles in dictating MES efficiencies. Changes in bioreactor design will necessitate strains that tolerate these conditions.

Working toward a circular economy and a sustainable future

The latest report from the Intergovernmental Panel on Climate Change is frank: Climate change is “widespread, rapid, and intensifying” and mitigating global warming requires “limiting cumulative CO2 emissions, reaching at least net zero CO2 emissions,” and reductions in other greenhouse gases. (11) Plastic waste poses a similar challenge, with global plastic volume reaching ~6.3 billion megatons in 2015 and expected to reach 12 billion megatons by 2050. (12) Innovations that push us closer to a circular economy can address these challenges.

The goal of a circular economy is to minimise negative externalities and waste using a systems-level approach to economic organisation that accounts for the flow of renewable and non-renewable materials. This framework forces us to consider the broader, systems-level impacts of MES technologies (e.g., does implementing an MES platform at industrial scales simply shift emissions from plastic production to feedstock or water use?) While MES should play a role in decarbonisation, implementing it without careful consideration of externalities will not yield truly sustainable solutions. Nevertheless, MES should be part of our toolbox as we rethink existing manufacturing pipelines.

Eric Conners, a PhD candidate in the Bose Laboratory, wrote this feature.

References

1. Gupta, D. et al. Photoferrotrophs Produce a PioAB Electron Conduit for Extracellular Electron Uptake. mBio 10 (2019).
2. Guzman, M. S. et al. Phototrophic extracellular electron uptake is linked to carbon dioxide fixation in the bacterium Rhodopseudomonas palustris. Nature communications 10, 1-13 (2019).
3. Gupta, D. et al. Photoferrotrophy and phototrophic extracellular electron uptake is common in the marine anoxygenic phototroph Rhodovulum sulfidophilum. The ISME Journal, 1-15 (2021).
4. Gupta, D., Guzman, M. S. & Bose, A. Extracellular electron uptake by autotrophic microbes: physiological, ecological, and evolutionary implications. Journal of Industrial Microbiology & Biotechnology: Official Journal of the Society for Industrial Microbiology and Biotechnology 47, 863-876 (2020).
5. Karthikeyan, R., Singh, R. & Bose, A. Microbial electron uptake in microbial electrosynthesis: a mini-review. Journal of Industrial Microbiology and Biotechnology 46, 1419-1426 (2019).
6. Jourdin, L. & Burdyny, T. Microbial electrosynthesis: where do we go from here? Trends in Biotechnology (2020).
7. Bai, W., Ranaivoarisoa, T. O., Singh, R., Rengasamy, K. & Bose, A. n-butanol production by Rhodopseudomonas palustris TIE-1. Communications Biology (Accepted), DOI- 10.1038/s42003-021-02781-z.
8. Ranaivoarisoa, T. O., Singh, R., Rengasamy, K., Guzman, M. S. & Bose, A. Towards sustainable bioplastic production using the photoautotrophic bacterium Rhodopseudomonas palustris TIE-1. Journal of Industrial Microbiology and Biotechnology 46, 1401-1417 (2019).
9. Rengasamy, K., Ranaivoarisoa, T., Singh, R. & Bose, A. An insoluble iron complex coated cathode enhances direct electron uptake by Rhodopseudomonas palustris TIE-1.
10. Rengasamy, K., Ranaivoarisoa, T., Bai, W. & Bose, A. Magnetite nanoparticle anchored graphene cathode enhances microbial electrosynthesis of polyhydroxybutyrate by Rhodopseudomonas palustris TIE-1. Nanotechnology 32, 035103, doi:10.1088/1361-6528/abbe58 (2020).
11. IPCC. Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press (2021).
12. Statista. Plastic Waste Worldwide – statistics & facts, <https://www.statista.com/topics/5401/global-plastic-waste/> (2021).

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© 2019. This work is licensed under a CC BY 4.0 license.