Basic Research Needs for Solar Energy Utilization - Office of ...

of high-value chemicals. Modern molecular techniques will enable improvements in existing
biocatalysts. Research is needed to identify and maximize the function of genes and gene
products identified through initiatives such as the Genomes to Life Program. Through metabolic
modeling and genetic engineering, it will be possible to predict how to engineer the microbial
metabolism, in particular that of photoautotrophs, for dramatic improvements in biofuels
production (e.g., to enhance reductant delivery for biohydrogen production). With regard to
gaseous biofuel production, the production of hydrogen in the biosphere is a very common
phenomenon.
NEW SCIENTIFIC OPPORTUNITIES
Plant Productivity and Biofuel Production
To maximize efficient biofuel production, we need a deeper understanding of the control of
carbon assimilatory processes at the biochemical, genetic, and molecular levels in plants and
microbes. Maximum CO2 fixation efficiency is directly linked to the energetics of the cell, and
recent findings indicate that carbon assimilatory processes in bacteria are tied to control of the
central pathways of nitrogen fixation, hydrogen production, and energy transduction. The
photosynthetic efficiency of plants in converting solar energy into biofuel feedstocks is
controlled not only by the intrinsic efficiency of photosynthesis but also by intricate genetic
controls that determine plant form, growth rate, organic composition, and ultimate size. Thus,
while the primary solar energy conversion efficiency of photosynthesis is as high as 5–10%
under optimal conditions, the overall rate of photosynthetic CO2 fixation is constrained by “sink
limitations” — biological control mechanisms that limit the conversion of energy from
photosynthetic electron transport into chemical storage. To improve the efficiency of solar
energy conversion into biofuel feedstocks, it is critical to develop an in-depth understanding of
the genetic controls of sink capacity and plant growth. Detailed knowledge of these mechanisms
will be required to optimize solar interception, increase plant size, sustain storage capacity
throughout the biofuel crop life cycle, and tailor the composition of biofuels for specific
purposes.
CELL WALL BIOSYNTHESIS AND BIOFUEL PRODUCTION
Lignocellulose can be utilized for energy production in a variety of ways ranging from
combustion to fermentation-based alcohol production. We need to understand how the chemical
composition of cell walls impacts the efficiency of the various conversion technologies. In
particular, there is a promising opportunity to modify the cell walls of biomass crops for
production of liquid fuels by replacement of poorly utilized components, such as lignin, with
structural polysaccharides. There are also important opportunities to improve the properties of
the enzymes that degrade cell walls to fermentable sugars. Most fungi and some bacteria secrete
a battery of enzymes that degrade polysaccharides and lignin to monomers that can be utilized as
substrates for microbial growth. Additionally, cellulolytic microflora found in the rumen utilize a
“cellulosomal” enzyme system comprised of complex scaffolds of structural proteins, which
assemble outside of the cell and organize enzymatic subunits capable of hydrolyzing cellulose,
hemicellulose, and other cell wall polysaccharides with high efficiency. Substantial progress has
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