Economics and Environmental Impact
The US demand for gasoline is about 103 billion gal per year (Holtberg et al 2009), and approximately 14 billion gallons per year of ethanol is being produced from corn (RFA 2012). However, the producing fuels from food crops places pressure on the price and availability of food, and the likelihood of conflict between food and fuel will increase as the world’s population grows.
Lignocellulosic plant biomass represents the largest source of renewable carbon on earth van Zyl et. al 2011), doesn’t compete with food sources, and is extremely cheap (corn stover is available at $83 per tonne - Kazi et. al 2010). Thus it is an ideal substrate for the sustainable production of commodities such as biofuel and industrial chemical precursors. It is estimated that it is possible to grow enough lignocellulosic biomass – in an economically feasible and environmentally sustainable manner – to produce more than 50 billion gallons (almost 50% of the US demand for gasoline) of biofuels annually (Kazi 2010).The main barrier that impedes the widespread use of this feedstock is the lack of low-cost technologies to break it down (van Zyl 2011).
The most commonly used method for bioprocessing is currently simultaneous saccharification and fermentation (SSCF) where enzymes hydrolyse the lignocellulose and microbes such as yeast ferment the sugars into products in the same reactor. However, this process is not economically viable due to the externally added enzymes required to hydrolyse the lignocellulose, which is estimated to be about $1 per gallon (Klein-Marcuschamer et al 2011). The yield is also low due to the optimum temperature for the enzymes differing from the optimum growth temperatures of the microbes.
Using consolidated bioprocessing (CBP) where microbes produce the cellulase enzymes themselves, would remove these costs. CBP is not viable either yet because to date no ideal organism has been developed for CBP conversion of biomass. Bacteria generally have high growth rates but lack robustness. Yeasts are sufficiently robust, but can’t be grown on many different substrates and it has been difficult to engineer them to degrade cellulose. Fungi often have a wide substrate range, but grow too slowly and have low productivity (van Zyl 2011). Our project, combining two different bacteria, is a step towards making CBP an economically viable reality.
The US is currently failing to meet the quotas set by the Environmental Protection Agency in production of renewable fuels (Bloomberg Politics 2015). Lignocellulose represents the most widespread and abundant source of carbon in nature and is the only source that could provide a sufficient amount of feedstock to satisfy the world’s energy and chemicals needs in a renewable manner (Hill et al., 2006; Van Zyl 2011). A huge amount of lignocellulose is produced as a by-product to crops but most of it is not utilized.
Biofuels offer a renewable solution to energy needs. Biodiesel in particular requires fewer engine or fuel modifications and releases fewer Greenhouse-gases compared to ethanol, other biofuels, and current petroleum diesel.
General Applicability of Our Approach
Here, we demonstrate a robust, environmentally friendly, and economically effective system for production of biodiesel, but our system can be applied to the production of many different products. One could replace the biodiesel genes we have chosen for E. coli with genes of their choice to generate a desired product. In addition, our method of creating synthetic communication pathways to stabilize our synthetic microbial consortia is an extremely important contribution to the field of synthetic biology. One could use this approach to stabilize different co-cultures with bacteria of varying phenotypes and metabolisms, or be utilized modularly so that population ratio can be modified via inducible signal.
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Willem H. van Zyl, Riaan den Haan and Daniel C. la Grange (2011). Developing Organisms for Consolidated Bioprocessing of Biomass to Ethanol, Biofuel Production-Recent Developments and Prospects, Dr. Marco Aurelio Dos Santos Bernardes (Ed.), ISBN: 978-953-307-478-8,
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Daniel Klein-Marcuschamer,1,2 Piotr Oleskowicz-Popiel,1,2 Blake A. Simmons,1,2,3 Harvey W. Blanch The Challenge of Enzyme Cost in the Production of Lignocellulosic Biofuels, Published online in Wiley Online Library DOI 10.1002/bit.24370
Hill, J.; Nelson, E.; Tilman, D.; Polasky, S. & Tiffany, D. (2006). Environmental, economic, and energetic costs and benefits of biodiesel and ethanol biofuels. Proceedings of the National Academy of Sciences USA, Vol.103, No.30, (July 2006), pp. 11206-11210
Mark Drajem (2015) EPA Relents on Ethanol Mandate With Overdue Renewables Quota http://www.bloomberg.com/politics/articles/2015-05-29/epa-cuts-mandates-for-corn-ethanol-with-overdue-renewables-quota-ia9q06do
Searle and Malins 2013 Availability of cellulosic residues and wastes in the EU