(693a) Solvent Production Using a CO2 - Fixing, Synthetic, and Syntrophic Clostridium Co-Culture

The demand for energy is steadily growing, and despite the current low oil prices, the stockpiles of petroleum reserves are quickly being depleted. As a result, new technologies capable of utilizing low value feedstocks and waste gases, such as H₂, CO and CO₂, must be developed to meet our growing need for liquid fuels. One interesting solution is to utilize the capabilities of the organisms found in nature. As such, solventogenic Clostridium acetobutylicum can utilize a wide variety of monosaccharides (glucose, fructose, xylose and mannose) and polysaccharides (starch, hemicellulose, and molasses) to produce acetone, ethanol and butanol in a process known as the ABE fermentation. During the solvent production, Cac releases excess electrons as H₂ gas, and approximately one third of the sugar substrate as CO₂ waste. In comparison, acetogenic C. ljungdahlii is limited in its ability to consume sugars (notably glucose) and instead can efficiently consume CO₂ gas in the presence of electron donors, such as H₂ or CO, using the Wood-Ljungdahl pathway. In the process C. ljungdahlii converts two molecules of CO₂ into acetyl-CoA, which is then converted into acetate and ethanol. Many previous studies focused on genetic engineering, and optimization of a single organism to carry out multiple reactions and produce commodity chemicals. In nature, most organisms live in microbial consortia, where the metabolic burden is divided among multiple organisms to produce molecules necessary for survival. Therefore, this study examined a synthetic co-culture of C. acetobutylicum and C. ljungdahlii. In the co-culture C. acetobutylicum converts the sugar substrate into solvents, while releasing CO₂ and H₂ gases as waste. At the same time C. ljungdahlii, which is unable to consume the sugar substrate, feeds on gaseous waste released by C. acetobutylicum to survive, and produce additional acetate. The extra acetate is then utilized by C. acetobutylicum to produce additional solvents. In the process, C. ljungdahlii improves the carbon balance of the process by recapturing the waste released by C. acetobutylicum. The synthetic co-culture discussed here was found to produce high titers of 3-C isopropanol which cannot be produced by either organism individually, and 2,3-butanediol, which can be produced by C. ljungdahlii only at trace levels. Here we present data from our efforts to engineer a synthetic Clostridium co-culture capable of utilizing glucose as a substrate together with inorganic waste gases in order to improve the carbon balance of the fermentation process, and maximize the solvent production.

SUPPORTED by the National Science Foundation through the NSF grant (Award No. CBET-1511660) and the IGERT fellowship (Award No. 1144726).