Researchers discover how bacteria use hydrogen to produce alcohol in microbial electrosynthesis

Researchers at the Leibniz Institute for Natural Product Research and Infection Biology (Leibniz-HKI) have made a significant advancement in the field of microbial electrosynthesis. This technology utilizes microorganisms, CO2, and electricity to produce alcohol and other organic compounds. However, the biological mechanisms underlying this process have remained largely unknown until now.

The team at Leibniz-HKI successfully confirmed experimentally, for the first time, that bacteria involved in microbial electrosynthesis utilize electrons from hydrogen. This discovery challenges previous assumptions and sheds light on the intricate workings of the process. Their findings, which have been published in the journal Green Chemistry, have provided crucial insights into the biological aspects of microbial electrosynthesis.

Microbial electrosynthesis holds immense promise in addressing climate change and facilitating the transition to sustainable energy sources. This technology not only captures carbon dioxide but also generates ethanol and other useful compounds that can be utilized as fuel. Furthermore, it enables the storage of excess electricity, adding to its potential benefits. Despite being recognized for over a decade, microbial electrosynthesis has faced hurdles in commercialization.

According to Miriam Rosenbaum, the head of the Bio Pilot Plant at Leibniz-HKI and Chair of Synthetic Biotechnology at Friedrich Schiller University in Jena, the lack of understanding regarding the underlying biology has impeded significant progress in the field. Rosenbaum and her team have long been devoted to unraveling the mysteries surrounding microbial electrosynthesis.

Through their groundbreaking research, the team has demonstrated that bacteria do not directly absorb the electrons supplied by the electric current. Instead, they employ hydrogen as a means to transfer these electrons. While this hypothesis had been speculated upon, no experimental evidence had been provided until now. Additionally, the researchers have discovered that the process can generate a wider range of valuable chemicals than previously known. They have also optimized the process to achieve the highest possible yields.

This breakthrough in understanding the biological mechanisms of microbial electrosynthesis paves the way for further advancements and commercialization of this promising technology. The newfound knowledge opens up possibilities for enhanced carbon capture, renewable fuel production, and efficient electricity storage, offering potential solutions in the face of climate change and the global energy transition.

Controlled conditions

Microbial electrosynthesis (MES) involves the application of electricity to an aqueous nutrient solution containing microorganisms, while simultaneously introducing carbon dioxide. The microorganisms harness the electrical energy and carbon to synthesize organic compounds like ethanol or acetate. However, the precise mechanism by which they utilize the supplied electrons has remained elusive until now.

Electron microscope image of the bacterium Clostridium ljungdahlii. Credit: Sara Al Sbei/Leibniz-HKI and Martin Westermann/ EMZ Jena

According to Miriam Rosenbaum, a study previously suggested that microbes in microbial electrosynthesis (MES) directly utilized the supplied electrons. However, this hypothesis lacked experimental evidence. Rosenbaum, on the other hand, believed that the microbes were more likely using hydrogen for their biosynthesis. This reasoning stemmed from the observation that when electricity and carbon dioxide were applied, water was split into hydrogen and oxygen, akin to classical electrolysis.

Santiago Boto, the lead author of the study, recognized the need to directly measure hydrogen in the system to confirm the involvement of microbes. To achieve this, he meticulously controlled various parameters in the MES reactor setup. By using a pure culture of the bacterium Clostridium ljungdahlii at different concentrations, he could precisely regulate the flow of electric current. Additionally, he employed microsensors to measure the hydrogen produced at the electrode and the hydrogen escaping from the liquid.

“With our experimental design, we gathered multiple pieces of evidence supporting the utilization of hydrogen by the bacteria,” Boto explained. The researchers observed that when the concentration of bacteria in the nutrient medium was such that they formed a biofilm on the cathode and little hydrogen was detectable in the electrode environment, the activity of the bacteria significantly decreased. This decrease in activity was also observed when the voltage was insufficient for electrolysis. However, when hydrogen was freely available to planktonic (free-swimming) bacteria from the electrode, they exhibited high levels of activity.

These findings provide compelling confirmation that the bacteria involved in MES utilize hydrogen rather than directly absorbing the supplied electrons. The ability to measure hydrogen directly in the system and control various parameters has enabled a deeper understanding of the biological processes underlying microbial electrosynthesis.

New biosynthetic pathways uncovered

Through their research, the team successfully optimized the voltage and bacterial concentration in microbial electrosynthesis (MES) to achieve the highest yields of acetate to date using a pure culture of bacteria. Santiago Boto expressed their achievement, stating that they obtained the highest acetate values ever reported for a pure bacterial culture. In an unexpected outcome, the team also discovered the formation of amino compounds that are not typically produced by the bacteria. The researchers, in collaboration with Falk Harnisch from the Environmental Research Center in Leipzig, also uncovered previously undescribed reactions between the nutrient medium and the cathode, which seemed to enhance the synthesis process.

Building on these findings, the team intends to further optimize the MES processes and delve deeper into the implications of their previous discoveries. Boto emphasized the significance of the amino compounds, noting their potential value to the chemical industry. Moreover, since the bacteria used in the study are already utilized industrially, the researchers may have stumbled upon a novel production method for these chemicals.

The overall impact of these results is expected to contribute to the commercial viability of MES. Miriam Rosenbaum anticipates a significant advancement in this technology in the coming years, emphasizing the need to focus on the biological aspects. The collaboration between the Bio Pilot Plant and process engineers aims to develop larger reactors for MES, facilitating its practical implementation and further progress in the field.

Source: Leibniz-Institut für Naturstoff-Forschung und Infektionsbiologie – Hans-Knöll-Institut (Leibniz-HKI)

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