Developing Organisms for Consolidated Bioprocessing of Biomass to Ethanol

Abstract

Lignocellulosic biomass is an abundant, renewable feedstock for sustainable production of biofuels and chemicals. The main technological barrier that impedes widespread utilization of this resource for production of fuels and other commodity products is the lack of low-cost technologies to overcome the recalcitrance of lignocellulose. Organisms that hydrolyse the cellulose and hemicelluloses in biomass and produce a commodity product such as ethanol at a high rate and titre would significantly reduce the costs of biomass conversion. This would allow steps that are currently accomplished in different reactors, often by different organisms, to be combined in a consolidated bioprocess (CBP). While there is still no ideal organism to use in one-step biomass conversion, several candidates have been identified that are in various stages of development for establishment of a cellulolytic system and/or improvement of product-forming attributes. This chapter assesses the status quo for CBP organismal development either by enabling non-cellulolytic organisms to grow on cellulosic substrates or by improving product forming abilities of native cellulose utilizing organisms. The authors also discuss feedstocks that are available for the production of biofuels using CBP and assess how process integrations can make CBP economically feasible in the near future. The increasing demand for oil coupled to the premium many governments place on greater energy security and environmental concerns have led to the development of an active biofuels industry (Van Zyl et al., 2011). First generation biofuels such as ethanol from starch or sugar already contribute considerable amounts of liquid fuels in several countries. However these technologies suffer from a shortage in the availability of feedstock in order to displace a more significant amount of petroleum based fuels. 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 et al., 2011). Second generation biofuels such as ethanol form cellulosic biomass therefore seeks to overcome the problem of feedstock supply shortage by utilizing the energy contained in total plant biomass. Current technologies for conversion of biomass to ethanol commences with a pretreatment step during which physical and/or chemical processes are used to render the polymeric sugar fractions more accessible to conversion by enzymatic processes (Stephanopoulos, 2007). The type of feedstock will predetermine the optimal type of pretreatment which in turn defines the optimal enzyme mixture to be used in subsequent hydrolysis steps and the composition of the hydrolysis products. Four biologically mediated events occur during conversion of