Redox manipulation to improve butyric acid and butanol production by clostridia strains

There has been increased interest in biological production of biofuels and chemicals in response to the depletion of petroleum oil and environmental issues. Butanol is an attractive alternative fuel offering several advantages over ethanol for gasoline-alcohol blending owing to its high energy content, low miscibility with water, and low volatility. Butyric acid is a useful chemical applicable in various fields such as food, animal feed additive, pharmaceutical, cosmetics, and precursor of butanol.

Strict anaerobic Clostridium species have been investigated for efficient production of butanol and butyric acid. A critical concern in biological production of butanol and butyric acid is the need to balance the demand and supply of redox intermediates such as NAD(P)H. Genetic manipulation of metabolic pathways provides means to increase NAD(P)H available to the cell. However, genetic modification of clostridia strains has been difficult because of the limited genetic toolbox. Here, I will show that the enhancement of butanol and butyric acid production can be achieved by manipulating the redox balance using both electrons from cathode and substrate as electron donors. Alternatively, supplementing a reduced substrate such as glycerol has been employed as a rational approach to enhance the production of NADH-requiring butyric acid and butanol by increasing NADH availability; however, a butyric acid-producing Clostridium tyrobutyricum cannot utilize glycerol as either a sole carbon source or a co-substrate. In this talk, I will present the essential role of acetate for redox balance during glucose-glycerol mixture fermentation by C. tyrobutyricum to selectively produce butyric acid. These results provide valuable insights on manipulating redox balance towards efficient biofuel/chemical production.