Leveraging Photosynthesis for Sustainable Solar Production of Biofuels
Photosynthesis in plants provides a clear proof-of-concept of the ability to form fuels from
sunlight. Use of the best-known plants would, however, require covering essentially all of the
arable land on Earth to meet current global energy needs. Modification of the biochemistry of
plants and bacteria, either genetically or through breeding, along with an understanding of the
mechanisms by which natural systems produce fuel, is needed to improve the efficiency of such
systems by a factor of 5–10 and to provide a convenient fuel for end use.
EXECUTIVE SUMMARY
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 materials, 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 useful organic molecules. These in turn can be bio-converted to a
wide range of fuels and value-added chemicals. Through understanding and discovery, it is
possible to increase solar energy-dependent biofuels production using plants and microbes.
Challenges associated with achieving this goal include: (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 bioprocesses that constrain biofuels production due to photosynthetic sink limitations
(i.e., biological control mechanisms that limit the conversion of energy from photosynthetic
electron transport into chemical storage); (4) elucidating plant cell wall structure and
understanding how it can be modified and efficiently deconstructed by protein assemblies; and
(5) extending nitrogen fixation to biofuel crops to reduce dependence on fossil fuel nitrogen
fertilizer.
SUMMARY OF RESEARCH DIRECTION
Photosynthesis provides >90% of the net input of energy into the biosphere. It produces the
oxygen we breathe and drives the biogeochemical cycles. Photosynthesis generates reducing
equivalents to convert inorganic materials to an organic form, produces cellular energy reserves
(starch, cellulose and other polysaccharides, oils, polyhydroxybutyrate, etc.), and results in
transmembrane gradients to drive bioenergetic pathways. The primary reactions of
photosynthesis can operate at near-perfect quantum efficiency. The goal is to link these primary
processes to produce useful chemical products and fuels. For example, the reduced ferredoxin
produced as the major product of solar energy capture from Photosystem I can be used to drive
the production of H 2 and methane production from CO 2.