Genomics of cellulosic biofuels
Edward M. Rubin1,2
The development of alternatives to fossil fuels as an energy source is an urgent global priority. Cellulosic biomass has the potential to contribute to meeting the demand for liquid fuel, but land-use requirements and process inefficiencies represent hurdles for large-scale deployment of biomass-to-biofuel technologies. Genomic information gathered from across the biosphere, including potential energy crops and microorganisms able to break down biomass, will be vital for improving the prospects of significant cellulosic biofuel production. he capture of solar energy through photosynthesis is a process that enables the storage of energy in the form of cell wall polymers (that is, cellulose, hemicellulose and lignin). The energy stored in these polymers can be accessed in a variety of ways, ranging from simple burning to complex bioconversion processes. The high energy content and portability of biologically derived fuels, and their significant compatibility with existing petroleum-based transportation infrastructure, helps to explain their attractiveness as a fuel source. Despite the increasing use of biofuels such as biodiesel and sugar- or starch-based ethanol, evidence suggests that transportation fuels based on lignocellulosic biomass represent the most scalable alternative fuel source1. Lignocellulosic biomass in the form of plant materials (for example, grasses, wood and crop residues) offers the possibility of a renewable, geographically distributed and relatively greenhouse-gas-favourable source of sugars that can be converted to ethanol and other liquid fuels. Calculations of the productivity of lignocellulosic feedstocks, in part based on their ability to grow on marginal agricultural land, indicates that they can probably have a large impact on transportation needs without significantly compromising the land needed for food crop production2. Lignocellulosic biofuel production involves collection of biomass, deconstruction of cell wall polymers into component sugars (pretreatment and saccharification), and conversion of the sugars to biofuels (fermentation) (Fig. 1). Partially because of the historically low demand for biologically based transportation fuels, each step in this process is in the early stages of optimization for efficiency and throughput. The crops from which biomass is currently derived have not been domesticated for this particular purpose and the present methods for saccharification and fermentation are inefficient and
expensive. However, the recent and pressing desire to develop alternatives to fossil fuels has made the rapid improvement of biofuel production a high priority, in which biologically derived energy (‘bioenergy’)-relevant genomic insights and resources will have an important role (Table 1).
From the perspective of transportation fuels, plants can be viewed as solar energy collectors and thermochemical energy storage systems. It is the storage of energy in a form that can later be accessed via thermochemical or enzymatic conversion that distinguishes biomass from other renewable energy sources. Cellulosic biomass, sometimes referred to as lignocellulosic biomass, is an abundant renewable resource that can be used for the production of alternative transportation fuels3. The three main components of lignocellulose are cellulose, hemicellulose and lignin (Fig. 2), with the relative proportions of the three dependent on the material source4. Cellulose, the main structural component of plant cell walls, is a long chain of glucose molecules, linked to one another primarily by glycosidic bonds5. Hemicellulose, the second most abundant constituent of lignocellulosic biomass, is not a chemically well defined compound but rather a family of polysaccharides, composed of different 5- and 6-carbon monosaccharide units, that links cellulose fibres into microfibrils and...