ABSTRACT
Few bio-based conversions of CO2 rely on energy sources other than light energy from the sun for reducing power for carbon fixation. Wind, solar, hydroelectric, tidal, and geothermal energy sources are all renewable energy sources available for the production of electricity. Some are investigating ways to efficiently convert electric power to liquid fuels for enhanced energy storage and transportability. One scheme currently under investigation involves using electricity from renewable sources to directly reduce CO2 to a microbial feedstock for biofuel production (Conrado et al., 2013). This production method would allow for diverse, renewable energy sources only capable of generating electricity able to transfer and store this energy in infrastructure-friendly chemical bonds.Biofuel fermentation generally consists of applying microorganisms to one of three groups of renewable feedstocks: biomass consisting of large portions of readily fermentable simple sugars such as sugar cane, biomass with more complex saccharides like corn or potatoes, or cellulosic biomass. These microorganisms consume the simple sugars and convert them to metabolic products. Common natural products like ethanol can be readily produced while more complex fuel molecules may require more complex synthetic metabolic pathways for production (Keasling, 2010). Feedstocks rich in simple sugars allow yeast or other microorganisms to quickly and efficiently produce biofuel in high titers. However, many of these sorts of feedstocks tend to grow in the mildest of climates. Complex starches are still useful for biofuel production and readily fermented by a variety of microorganisms, though may require additional processing and time for highest production efficiency. Cellulosic feedstocks are among the most abundant and lowest cost sugar-based feedstock available for biofuel production, yet the processing to make the sugars accessible to microorganisms is cost and energy intensive (Carroll and Somerville, 2009). Much investigation is centered on alleviating the costs associated with extracting sugars from cellulosic sources (Alvira et al., 2010).Other investigation in this field revolves around the novel production of fuels and chemicals from biological reactions (McEwen and Atsumi, 2012). The few natural metabolites that are able to be used as fuels have native pathways that may be highly productive and reliably processed; however, the fuel molecules that are
most desirable are not synthesized to any significant degree in microorganisms that are well characterized for chemical production. Thus, much of the current progress in this field centers on the metabolic engineering of “user-friendly” microorganisms to increase the chemical production of natural metabolites or synthetically introducing new chemical pathways into these microbes for the production of novel chemicals (Rabinovitch-Deere et al., 2013). 6.2 Fermentative AlcoholsEthanol is a native metabolite shown to be highly produced in a variety of organisms, and yeast fermentation has been extensively used for industrial and culinary production (Solomon et al., 2007). Ethanol is a two-carbon alcohol, and relatively less energy is stored in ethanol than the larger molecules that make up gasoline and other transportation fuels. Ethanol is one possible end product of glycolysis. Glucose, or any other sugar that can feed into glycolysis, is broken down to pyruvate. Pyruvate decarboxylase converts pyruvate into acetaldehyde and an alcohol dehydrogenase will catalyze the reaction to produce ethanol. This metabolic pathway resulting in ethanol is well conserved in microorganism fermentation.Ethanol production is highly optimized and is produced in titers of 150-175 g/L with productivity in excess of 3 g/L/h (Alfenore et al., 2002; Bafrncová et al., 1999). Some countries have embraced ethanol production as a large part of the fuel economy, with Brazil being one of the most successful. There extensive cane sugar production and subsidies from the government have largely ushered in far reaching modification to the fuel infrastructure to more fully accommodate ethanol (Andrietta et al., 2011; Basso et al., 2011). Ethanol is currently used in the United States as a fuel additive for all gasoline vehicles up to 15% v/v. This mix is the current maximum approved by the EPA and approaches the maximum that the current fuel infrastructure can safely handle without major and costly modifications. Additionally, due to the lower energy density of ethanol (20.8 MJ/L compared to gasoline 32.0 MJ/L), such mixing adversely affects gasoline mileage, leading to a less desirable fuel alternative. Other biologically derived, energy-dense alcohols that are more infrastructurally compatible are being investigated.
