BASIC SCIENCE CHALLENGES, OPPORTUNITIES, AND RESEARCH NEEDS IN
SOLAR FUELS PRODUCTION
Biomass-derived Fuels
Photosynthetic light-driven biological processes have enormous capacity for sustainable, carbon-
neutral, solar-powered replacement of fossil fuels by fixing more than 100 Gtons of carbon
annually, which is roughly equivalent to 100 TW of energy. However, this fixation rate is
currently in balance with respiration and other facets of the global carbon cycle, so adding
another 10 TW of fixation would require enormous land areas at present. Primary products of
photosynthesis include cell wall components such as cellulose and lignin, as well as storage
molecules, starch, sugars, lipids, etc. There are also many intermediate metabolites that could
lead to a wide range of other potentially useful organic molecules. These in turn, can be
bioconverted to a wide range of fuels and value-added chemicals. Through understanding and
discovery, it is possible to increase solar-energy-dependent biofuels production by plants and
microbes. Challenges associated with achieving this goal include the following: (1) mining
biological diversity to discover improved catalysts for biofuels production; (2) capturing the high
efficiency of the early steps of photosynthesis to produce high-value chemicals and fuels;
(3) understanding and modifying the bioprocesses that constrain biofuels production due to
photosynthetic sink limitations, inefficient reductant use, and environmental factors;
(4) elucidating plant cell wall structure and understanding how it can be modified and efficiently
deconstructed by protein assemblies; (5) extending nitrogen fixation to biofuel crops to reduce
dependence on fossil fuel nitrogen fertilizer; and (6) developing an overall deeper understanding
of the biological processes needed to improve plants and microbes to increase solar-energy-
dependent biofuels production.
Natural Photosynthetic Systems
Natural photosynthesis has achieved the ideal of solar-initiated water splitting coupled to
chemical energy storage using abundant, renewable, self-assembling, “soft” matter. The
resolution of fundamental structural design principles in natural photosynthesis provides a means
to accelerate the discovery of synthetic architectures that embody mechanistic principles used in
biology. These principles can be used to realize robust, scalable supramolecular architectures
amenable to global energy applications. Two important challenges are (1) the discovery of
design principles to maximize the efficiencies of solar energy capture, conversion, and storage;
and (2) realization of these enabling principles in advanced biomimetic assemblies where both
the supramolecular structures and surrounding supramolecular scaffolds exploit biological
designs for function.
Meeting these challenges will require the following: (1) understanding and controlling the weak
intermolecular forces governing molecular assembly in natural photosynthesis; (2) understanding
the biological machinery for cofactor insertion into proteins and protein subunit assemblies;
(3) adapting combinatorial, directed-evolution, and high-throughput screening methods to
enhance natural photosynthetic systems to increase the efficiency of solar fuels production;
(4) characterizing the structural and mechanistic features of new, natural photosynthetic
complexes to identify desirable design motifs for artificial photosynthetic systems; and