(73d) Fermentation of Carbon Dioxide to Ethanol

Many fermentation processes have been used to convert biomass to various fuels. Syngas, which is a mixture of CO, CO₂, and H₂, is one process being studied for the production of ethanol.  Per mole of ethanol produced, 12 electrons are consumed. H₂ and CO are the electron sources in syngas fermentation. One issue involved in syngas fermentation is the production of waste CO₂.  In this study, using CO₂ as sole feed gas, an external electron source is utilized to create additional ethanol. Electrons were created by an external power source using a Bio Electrical Anaerobic Reactor (BEAR). An electron mediator, such as methyl viologen (MV) used in this study, is also a requirement since cells cannot direct use electrons from the power source efficiently. The characteristics of the three MV oxidation states are important issues in CO₂ fermentation. These characteristics include oxidation state distribution under different induced potentials, toxicity to cell growth, and the ability to transfer electrons. These characteristics are important not only because they affect the activity of enzymes involved in the metabolic pathway, but also because they influence the synthesis of intermediates and product. Therefore, it is critical to learn how MV can function for cell growth and product formation. Results from experimentation of these important characteristics of MV and a discussion of its application as a suitable external electron source are presented.