Microbial electrochemical techniques describe a variety of emerging technologies that use electrode–bacteria interactions for biotechnology applications including the production of electricity, waste and wastewater treatment, bioremediation and the production of valuable products. Central in each application is the ability of the microbial catalyst to interact with external electron acceptors and/or donors and its metabolic properties that enable the combination of electron transport and carbon metabolism. And here also lies the key challenge. A wide range of microbes has been discovered to be able to exchange electrons with solid surfaces or mediators but only a few have been studied in depth. Especially electron transfer mechanisms from cathodes towards the microbial organism are poorly understood but are essential for many applications such as microbial electrosynthesis. We analyze the different electron transport chains that nature offers for organisms such as metal respiring bacteria and acetogens, but also standard biotechnological organisms currently used in bio-production. Special focus lies on the essential connection of redox and energy metabolism, which is often ignored when studying bioelectrochemical systems. The possibility of extracellular electron exchange at different points in each organism is discussed regarding required redox potentials and effect on cellular redox and energy levels. Key compounds such as electron carriers (e.g., cytochromes, ferredoxin, quinones, flavins) are identified and analyzed regarding their possible role in electrode–microbe interactions. This work summarizes our current knowledge on electron transport processes and uses a theoretical approach to predict the impact of different modes of transfer on the energy metabolism. As such it adds an important piece of fundamental understanding of microbial electron transport possibilities to the research community and will help to optimize and advance bioelectrochemical techniques.

Microbial electrosynthesis is potentially a sustainable biotechnology for the conversion of the greenhouse gas CO2 into carboxylic acids, thus far mostly limited to acetic acid (C2). Despite the environmental benefits of recycling CO2 emissions to counter global warming, bioelectrochemical production of acetate is not very attractive from an economic point of view. Conversely, carboxylates and corresponding alcohols with longer C content not only have a higher economical value as compared to acetate, but they are also relevant platform chemicals and fuels used on a diverse array of industrial applications. Here, we report on a specific mixed reactor microbiome capable of producing a mixture of C4 and C6 carboxylic acids (isobutyric, n-butyric, and n-caproic acids) and their corresponding alcohols (isobutanol, n-butanol, and n-hexanol) using CO2 as the sole carbon source and reducing power provided by an electrode. Metagenomic analysis supports the hypothesis of a sequential carbon chain elongation process comprised of acetogenesis, solventogenesis, and reverse β-oxidation, and that isobutyric acid is derived from the isomerization of n-butyric acid.

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Microbial electrochemical technologies (MET) are promising to drive metabolic processes for the production of chemicals of interest. They provide microorganisms with an electrode as an electron sink or an electron source to stabilize their redox and/or energy state. Here, we applied an anode as additional electron sink to enhance the anoxic metabolism of the industrial bacterium Corynebacterium glutamicum through an anodic electro-fermentation. In using ferricyanide as extracellular electron carrier, anaerobic growth was enabled and the feedback-deregulated mutant Corynebacterium glutamicum lysC further accumulated L-lysine. Under such oxidizing conditions we achieved L-lysine titers of 2.9 mM at rates of 0.2 mmol/L/hr. That titer is comparable to recently reported L-lysine concentrations achieved by anaerobic production under reductive conditions (cathodic electro-fermentation). However unlike other studies, our oxidative conditions allowed anaerobic cell growth, indicating an improved cellular energy supply during anodic electro-fermentation. In that light, we propose anodic electro-fermentation as the right choice to support C. glutamicum stabilizing its redox and energy state and empower a stable anaerobic production of L-lysine.

A microbial electrosynthesis cell comprising two biological cathode chambers sharing the same anode compartment is used to promote the production of C2–C4 carboxylic acids and alcohols from carbon dioxide.